300b lovers


I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations.  I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is.  Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.  

Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence.  They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music.  They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding.  Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a  unique circuitry to mate with his 300b monos via XLR connections.  Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it. 

I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system.  300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm.  Of course, 300b amps favor efficient speakers so carefully component matching is paramount.

Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public.  Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC.  I will be there to hear what I expect to be an outstanding sonic presentation.  

To allay any questions about the cost of Don's 300b mono, I do not have an answer. 

 

 

whitestix

@lynn_olson 

Gaining entry into Japan is nearly impossible. It can take decades. Even Wal-Mart has barely cracked it, and think of the massive resources they can bring to bear. Tourists are always welcome, but good luck immigrating to Japan, or cracking the local market

Some could arguably  accuse Japan as being a closed homogeneous society but out of control illegal immigration will never be a concern for them. Absolutely no way they’d allow such a debacle.

As always good insight Lynn.

Charles

My father was a US diplomat and head of the Economic Section of the U.S. Consulate in Hong Kong, and held a similar position at the US Consulate in Osaka, Japan. His primary function was smoothing the way for US business overseas, and making sure they didn’t break local law and get arrested (not joking about this). It’s the job of the nearest US Consulate (or Embassy) to fish Americans out of jail when they unwittingly break local law. Most Americans are unaware they are subject to local law when they travel, with the only exception being US diplomats and members of the Armed Forces. They have separate passports ... I myself held a diplomatic passport until I was 18 and returned to the States.

Hong Kong was very unusual being a Free Port, with zero import duty on goods from anywhere in the world. Back then in the Sixties, the only prohibited goods were guns (of any kind), illegal drugs, and gold. Anything else got a perfunctory inspection and went into Hong Kong warehouses for re-shipment to anywhere else in the world.

Thanks to decades of trade agreements with most countries in the world, the USA is very nearly a duty-free zone, with only the lightest restrictions into our markets. The Chinese government greases the wheels even more by providing free shipping for Chinese manufacturers into the US market (yes, really). But shipping back to China is definitely not free, as anyone with a defective Chinese product will discover. That’s why you should always confirm there is a functioning, and fully staffed, repair agency in North America when buying a Chinese product.

Americans cheerfully assume the rest of the world is this way ... LOL, ha ha, no, not at all. Not even slightly. There’s free trade within the EU, but there’s a maze of mind-numbingly complex safety and technical regulations to gain entry into that market. The big Japanese firms have the resources, but they have full-time staffs doing just that. The EU is a beautiful place and delightful to live in, but in terms of trade, it is a walled garden. Entry is possible but the requirements are daunting ... and are designed that way.

Gaining entry into Japan is nearly impossible. It can take decades. Even Wal-Mart has barely cracked it, and think of the massive resources they can bring to bear. Tourists are always welcome, but good luck immigrating to Japan, or cracking the local market.

Every country in the world is different, with different sets of legal hoops to jump through. Some, like Japan, are essentially closed. China is nearly closed, with complex and difficult currency restrictions. Hong Kong and the USA are very much the exceptions, not the norm.

The USA took the political decision after World War II that it wanted to sponsor worldwide free trade, backed up and protected by the US Navy. (The US Navy has 13 nuclear-powered Carrier Battle Groups. Any other country has just one carrier, and they are not nuclear-powered. Think about that for a while.) The US wanted to dissolve the squabbling European empires that had caused two World Wars, and had the economic muscle to do so. This is still US policy.

@lynn_olson

The unusual thing about the US market is that it is really easy to sell into ... tariff rates are some of the lowest in the world, technical safety requirements are not too severe, and the market is huge and easy to serve. Markets everywhere else are different ... more fragmented, higher tariff barriers, multiple languages, many different technical standards, and other obstacles

What a very good synopsis of what manufacturers face if deciding to officially enter international markets. Good grief!! This is not a cavalier undertaking by any stretch of the imagination. A  multitude of serious issues  to contend with.

Charles

Pindac, I should mention when a private party flies into the USA, buys a product here, and returns with it to the UK or EU, that is technically a "grey market" sale. There is usually no legal problem with the new toy, assuming the local duties and VAT are paid ... but usually it is not legal to re-sell in that country. EU countries take a dim view of grey market sales ... while I have no idea how aggressive UK enforcement is. (In the US, the Federal government doesn’t care about grey market sales, but overseas warranties will not be honored by repair shops.)

And warranty support ceases once it is taken out of the North American market. The primary purpose of an overseas servicing center is to provide continuing warranty support, at a quality standard that matches the original manufacturing quality, with the same parts, service manuals, employee training, etc.

(A "black market" sale, of course, is something that is completely illegal to import into the host country, like an AR-15 into the UK, or something like that. Many laws are broken and arrest warrants are issued. "Grey market" means the product has not been tested by the relevant safety agencies, and is not certified for sale in that country. The most common outcome is that it is seized by Customs and is not returned.)

Hi there, Pindac!

The decision to export out of North America is largely up to Spatial Audio. Don and I are the technical advisors and consultants, but exporting is a business decision. We can suggest and advise, but we do not make the final decision.

As Don has mentioned, exports to Europe would have to meet rigorous EU safety standards, and an EU servicing center would be wise. The EU is a big place with many technical and legal requirements that are quite different from the North American market, which has one dominant language, safety standards, and electrical power. If you can sell it in Los Angeles, you can sell it in Toronto, and everywhere in between.

Sure, there are small audiophile manufacturers who import on a "grey market" basis into the EU and the UK. That’s fine until you get caught. Don, myself, and the team at Spatial intend to be on the straight and narrow when it comes to regulations ... we are not Tesla, Apple, or Microsoft, with armies of lawyers to smooth the path into new markets.

Back when I was at Audionics in the Seventies, we eventually surmounted the many EU technical regulations and sold our products into the European market. But it took several years, and we didn’t attempt it until we had significant sales volume in the domestic market. Sure, it’s easy to order a Monolith power transformer with multiple input voltages. That doesn’t make the finished product legal to sell in the UK or the EU.

The unusual thing about the US market is that it is really easy to sell into ... tariff rates are some of the lowest in the world, technical safety requirements are not too severe, and the market is huge and easy to serve. Markets everywhere else are different ... more fragmented, higher tariff barriers, multiple languages, many different technical standards, and other obstacles.

I have not inquired to the VAC Owners if they are using their Amp's with a Step Up Mains Tranx, or with any other US Imported Audio Ancillaries I have been demo'd.

I am a listener first and foremost.

The offer of the Prototype Amp's will be a great temptation to somebody, the Pedigree of the EE's and made known design intent behind this design alone, should be enough to convince a special experience is to be had. The reports of listening experiences to be found within this thread can only but reinforce, the idea of the purchase being very worthy of a consideration.

In the UK there are excellent Third Party support services for Electronics Support in both Valve and SS, these services are able to be found across the Country. The Services are not unreasonable in their costings, I have only ever seen the waiting time from certain services being the cause to search further afield. I do believe that the the notable trust that has been placed in these very services, along with the idea that receiving EE support is not being too much of a difficulty to achieve, is an instrumental factor in the making way for rarely seen items of Audio Equipment arriving in the UK from a variety of Countries where products have not got a UK support network.

As for overseas shipping, there are lots of Chinese Origin Audio Equipment arriving in the UK, especially Valve Amp's, of which some are requesting multiple £0000's to acquire. I am yet to have seen any horror stories about the Quality of the goods , resulting from their Transportation.

I myself have imported many items of Audio Equipment from Japan and over approx' 10 years, have experienced two incidents where goods arrived with a damage that was not as advertised.

Either a undeclared damage or transit damage was received ? 

I will keep looking in on this valuable thread.  

@pindac Would it require a 240 VAC version or have most of the ones you have heard been US market 120VAC ones that someone was using a step up transformer with on your side of the pond?   I will say that I have the pair of prototype amps that will go into production other than some cosmetic changes to the cases and panels.  The circuit and power supply will not change as they are done.  I abuse the amps regularly, putting them on the bench and tweaking something and turning them on and off 10 times in 30 minutes.  Nothing has fazed them in 3 or 4 months since their birth.  So I am pretty darn confident if they arrive undamaged they will simply just work and be very reliable.  I/we wouldn't sell them if they were not very reliable.  But shipping overseas is always dicey, even with good packing.

Hi Don, I have heard a VAC Amp' within the UK, one of the earlier models is owned by a HiFi Group Member who owns a 300B.

Another newer Model VAC is also owned by a Group Member, I am yet to be demo's this one.

I have heard Modwright as well other Brands that are not with a dealer distribution, it is not too strange for products to end up in the UK without the supporting networks behind them.

There is a fair amount of UK Audio Enthusiasts who are willing to look far and wide for their entertainment to be satisfied. 

  

@pindac that will be up to Spatial Audio Lab.  I am going there in late Nov to teach them the builds of amps and matching preamp.  There will be a review pair made that will spend some time in the USA next year.  After that we shall see.   It is not hard at all to spec power transformers with dual windings and make a 220-240 VAC input version and source a proper IEC connector.  While I expect both these pieces to be very reliable, the problem with supporting European and Asian markets is shipping cost, and if the customer has a problem of any sort, the shipping costs are enormous to help them out with any warranty issues.  If you make 100 units of anything, no matter how reliable, one will either be damaged by a reckless carrier, or some weird problem may arise.  The important thing in that case is to immediately take care of the customer, and that is expensive outside of N America.  So that will be Spatial's call.   It is much more expensive to support markets outside the USA and Canada.

There are now over the past few months arrived Three 300B Power Amp's within my Local HiFi Group.

Two are from the Designs done by TDP and Andy Groves for World Audio Designs, One of these Two has an upgrade which I believe has Interstage Transformers.

The other is a design from a 'Heart' of Dutch Origin.

I am now becoming re-familiarised with the 300B sonic.

With the recent experiences and the knowing there are certain designers early in their career knowledge been applied to the Amp's, it really does make the idea of listening to the Amp's being discussed in this thread, a experience to be had. 

Who knows a Pair may one day be on the UK to be demonstrated.   

Alex, remember, you cannot harm your amplifier if you completely remove the grid-load resistor. With a transformer, runaway from DC instability is impossible. There is always a DC path through the secondary ... as long as the secondary is intact.

If you have an oscillation lurking in there, at some high frequency like 5 to 20 MHz, that's a different story, and unrelated to the value of the grid resistor. 5 to 20 MHz oscillations, even at a very low level, will absolutely make the sound bright and unpleasant. If they are -40 dB down, you will never see them on a scope ... that's no more than a trace width. You need an RF spectrum analyzer to sniff out the little monsters. They look like little spikes rising out of the noise floor (which should be very smooth).

If you suspect this, you need a grid-stopper in series with the grid pin, like 100 to 500 ohms of carbon-comp resistor soldered no further than 1/2" from the grid pin. (NEVER use a wirewound for a grid-stopper.) That will kill self-oscillation.

I would try a grid-stopper first before futzing around any more. The only way you can solidly rule out self-oscillation is use an RF spectrum analyzer that's good to at least 20 MHz, preferably 100 MHz. These things aren't cheap, and only have one use, chasing out RF nasties. Low-level RF oscillations are surprisingly prevalent in high-end audio equipment, with poorly designed regulators as the usual culprit.

Try the grid-stopper first before anything else. After that, play around with various value of grid resistor, including nothing at all. It should not be sounding bright, unless something is wrong.

In my case, I compared the sound of my amplifier with different IT resistor load. 120K (Kiwame 2W) vs 56K (Alan Bradley 2W).
The sound with 120K resistor was too bright and rough, with 56K tonal balance was more natural and sound was more refined.
But with RC coupling the sound was even less bright vs IT with 56K resistor. I think the resistors material AB vs Kiwame can make a difference too.
I am afraid if I completely remove the load resistor the sound will be even brighter. So I want to try 39K AB 2W resistors (that I have in my stock) to make the sound a little bit warmer.

Try removing the 56K load resistor entirely and give it a listen. You might like it better. Remember, with conventional signal sources, there is almost no ultrasonic content, so the ringing in the transformer is never stimulated. And getting rid the load resistor has no effect on circuit stability or DC stability, since the grid sees the DCR of the transformer, which is a few K at most. All the grid resistor does is make square waves look pretty. It has no other function.

Its been my experience that allowing an audio coupling transformer to ring will result in brightness, since any signal presented to the transformer can cause ringing (this is easy to demonstrate- try it!). I'd not be surprised if some people mess with the damping to compensate for a weakness in the circuit elsewhere; IME/IMO you're far better off finding those weaknesses, sorting them out and making sure your transformers perform as good as they are able!

The overshoot, as these things go, isn't bad. You might be able to zero in on it a little bit more. The thing is, the more energy the overshoot has, the brighter/livelier the presentation so if you're going for a warmer sound this is something to avoid (you can see here how easily distortion can influence the tonality of the circuit). I would not reduce the loading resistance to the point it rounds the leading edge. A slight bit of overshoot is OK when trying to hit that critical damping value.

In a zero feedback circuit you have no correction to deal with this sort of thing, so you have to sort out details like this and get them right. The reward is greater detail since distortion and detail really don't go hand in hand. You can see by doing this sort of measurement how different power tubes and different speaker loads affect how well the output transformer can perform- and why people might have contradictory observations about how the same OPT and power tube sounds, because the way the transformer behaves changes depending on the speaker load.

 

Try removing the 56K load resistor entirely and give it a listen. You might like it better. Remember, with conventional signal sources, there is almost no ultrasonic content, so the ringing in the transformer is never stimulated. And getting rid the load resistor has no effect on circuit stability or DC stability, since the grid sees the DCR of the transformer, which is a few K at most. All the grid resistor does is make square waves look pretty. It has no other function.

Now, in a cap-coupled circuit, it is absolutely necessary, since grid current, as small as it is, has to go somewhere. With transformers, it just goes through the secondary, and the current is so minuscule there is no effect on the core.

Hi @donsachs and @lynn_olson ,

Here is square wave 1KHz on my amp output:

https://photos.app.goo.gl/jcdjn88Q71WaEvoR9

Output transformer and driver IT have much wider bandwidth compared to input to driver IT. During the measurement the 6sn7 current was 7.2mA. I just increased it to 8.75mA. Load resistor is 56KOhm.

The problem with two successive stages that are balanced and DC-coupled to each other is that DC drift is a big deal. A 1 volt shift on a 150 volt plate is normally inconsequential, but becomes a serious concern when the grids of the following stage have a 1 volt offset between them ... which is what DC coupling does.

In the post in which I suggested a differential topology, I also suggested several cures for this problem, which I pointed out in that post.

Hi @donsachs ,  

I changed a load resistor 56K Ohm (Alan Bradley 2W), in my SET amplifier. And still have a 2-3 times bigger overshoot vs Blackbird has. However the sound is more balanced compared to the 120K (Kiwame 2W) load resistor I had before. I'm still waiting for more break-in time.

So far IT compared to RC coupling between 6sn7 and 6f6 - IT sounds more fast, more precise rhythm and pace, faster transient, more clean in upper frequencies. But I don't hear a big difference in terms of low level resolution. The tonal balance is a little colder too. Bass is more fast and controlled but less deep.

Which resistor will you recommend for power supply? Now I use Ohmite gold 10w and Milles 12.

Scope pr0n. Same pix as earlier, just more zoomed in.

Don Sachs scope photo of Blackbird at 30% power (8 watts) at 1 kHz. Zero feedback, with no grid resistors to "trim" the response.

A quick note: Don and will not be making the part-Mullard or the suggestions made by others in this thread. The Raven and Blackbird are where our attention is, and that’s where it will stay. Our focus this year, and the next, is getting production moving smoothly, making sure the Raven and Blackbird are reliable as possible, and growing the customer base.

On my part, I’ll be completing the long-awaited "Beyond the Ariel" speaker project over on DIYaudio, with the assistance of Troy Crowe in Canada. I do my best work collaborating with others, and Gary Dahl, Bjorn Kolbrek, and Thom Mackris have made a real difference on that project. I’ve been meditating on an appropriate name for the speaker, and "Phoenix" feels right, considering how many times it has been re-born.

The problem with two successive stages that are balanced and DC-coupled to each other is that DC drift is a big deal. A 1 volt shift on a 150 volt plate is normally inconsequential, but becomes a serious concern when the grids of the following stage have a 1 volt offset between them ... which is what DC coupling does.

A Mullard sidesteps this by direct-connecting the plate of the SE input stage to ONE driver grid. The other grid (of the driver) is AC-connected to ground through a 0.1uF cap and DC-connected to the other grid via a 100K ~ 220K resistor. As a result, the two driver grids always DC-track each other.

By contrast, if the Mullard input section is replaced with a DC-connected balanced or diff stage, then DC balance and drifting of the first stage becomes critical, requiring a servo circuit to always keep the plates of the input tube exactly matched. No thanks.

The Blackbird is fully balanced, input, driver, and output, with DC balance issues resolved by using transformer coupling. Transformers are incapable of passing DC from primary to secondary, since the coupling is magnetic. Charge/discharge issues associated with capacitors, as well as potential coloration, are also avoided since cap coupling is not used anywhere in the forward path.

The hard part is getting transformers of high enough quality ... this is where working directly with the transformer designer, making them a part of the design team, is essential. These are not off-the-shelf parts.

A minor side benefit is avoiding turn-on pops and clicks, since the circuit remains balanced in all modes of operation, without relying on servos to maintain balance.

As mentioned above, a part-Mullard is great way to build a PP DHT amplifier. Not too complex, a well-known circuit that behaves predictably, and capable of scaling up the driver so it has enough power to motivate DHT grids.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

I suggested this earlier in this thread; and added to it that with the input circuit also being differential there is additional benefit. A fully differential circuit has harmonic cancellation at every stage of gain; not just at the output. This results in the 3rd harmonic being dominant; it is treated much like the 2nd by the human ear in that its innocuous. But compared to an amp that does not use this topology, the 3rd is at a lower amplitude, and succeeding harmonics fall off at a faster rate (than seen in an SET) on an exponential curve. So the 3rd is thus more effective at masking higher orders and since distortion is lower, the circuit can be smoother and more transparent.

There's no feedback, and there's gobs of 2nd-harmonic distortion. Parts coloration works for or against that 2nd-harmonic distortion. Also, because power-supply rejection is zero, power supply coloration is right in your face.

Driving DHTs takes two to three times the swing of pentodes or beam tetrodes. This exposes driver nonlinearity as well. Even worse, in some designs, driver and output distortion partially cancels ... but only at certain power levels. So the distortion signature is strongly level-dependent, which is very undesirable.

@lynn_olson

I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative,

Lynn,

What do you attribute to this wide spectrum of SET performance from superlative to terrible amongst them? Given their relatively simple circuit, is it primarily part quality (Output transformer?) or power supply quality and design?

Charles 

 

And of course a part-Mullard circuit is perfectly acceptable for a PP 300B amplifier. Unlike a PP pentode amplifier, though, you need about two to three times as much voltage swing in the driver, so a Dynaco circuit is definitely not the right choice.

A Mullard PP 300B works as follows: input tube direct-coupled to a long-tail pair (or CCS) of triode-connected 6V6 drivers. These in turn are connected to a PP interstage transformer with a modest step-up ratio, between 1:1.4 and 1:2. The interstage then drives the PP 300B grids. This would be a non-feedback amplifier, so good power supplies are required. I would imagine a number of the PP 300B amplifiers already on the market use this topology.

Back in 1993 when I was trying many different amplifiers on the newly completed Ariel speakers, I came to the conclusion that a stock ST70 was the minimum acceptable standard for hifi. Most transistor amps fell below this mark, and most tube amps exceeded it. Restored Mullard designs did very well, as anyone might expect.

What I did not expect was the performance of the Ongaku and the Herb Reichert Silver 300B. I expected all SETs to be terrible, but those two were the best sound ever on the Ariels (which are 92 dB/meter efficient). After hearing several other SETs, I was struck how variable they were. A few were superlative, almost otherworldly, but many others were pretty bad. One was so terrible that Karna and I just burst out laughing ... it sounded like a 1960 transistor radio left out in the sun too long. That’s how variable SETs are ... all over the place.

By contrast, a competently engineered Mullard with a decent power supply is almost guaranteed to sound pretty good. And the best ones are superb.

The octal boards for ST70 are direct fit and do not stress the power supply.  You can use the diode replacement for rectifiers, but it sounds different.  Basically, the amp is not bad, and a great deal for the money when restored and updated.  A decent preamp and some reasonably efficient speakers and a person can have a very pleasing stereo.  That said, an ST70 is nothing special because there really isn't room to make it special.  I think we agree on that.  You might as well start over and do it right without the compromises.

But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation?  

Feedback allows the amplifier rejection of that which is not the signal, so generally speaking, yes.

I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does.   It makes the amp considerably better. 

The problem with any mod that adds tubes to the circuit is the extra load on a power transformer that might already be 65 years old. Dynaco strikes me as being pretty precious about their transformer ratings- I don't like to take chances with them, especially in light of their age.

This is just me of course but if I'm going to modify a vintage piece I follow two simple rules. The first is don't add any extra load to the power transformer. The second is don't do anything that does not fit very easily into the existing chassis. Violate these and you're likely better off doing the whole thing from scratch.

The really glaring weakness in the ST70 is it should have been designed with dual rectifiers; as a result the 5AR4 is the most likely tube to fail in the amp. Triode Electronics of Chicago has a beefed up power transformer that is a drop in replacement that allows you to add a second 5AR4, thereby keeping the correct B+ operating point and so not stressing the output transformers as well. But you have to find room beneath the chassis for some 500V filter caps. It starts to get a bit ridiculous- at that point why not just do your own chassis so you can lay out things properly?

If an ST70 is properly refurbished but pretty well the stock circuit, it can be surprisingly good against a lot of modern PP and SET amps. Since it really does not have enough feedback, you have to help it along with good quality coupling caps and resistors in the voltage amplifier and driver circuit. The second thing to understand about this amp is because of its power supply weakness, you really should not push it hard (which is better for sound but also keeping that 5AR4 alive). CE Distribution in Arizona makes a drop in replacement filter can that features an 80uf section, which should be deployed after the choke, for the plates of the power tubes. That's about as much extra capacity as you can safely add to this amp without stressing the 5AR4.

Actually, the Kootenay, and the the Valhalla amps use a similar circuit for input and drivers (with judicious use of a CCS), and the power supplies are the basis for what we used in the Blackbird 300b amps.  We improved the power supply design for the Blackbird, but the same basic ideas are in the Kootenay KT88 power amp and Valhalla 6L6 integrated amps.   Long tailed pair with CCS and really good power supplies and iron.  As Lynn said, it makes for a really good tube amp.  Not the level of the 300b project, but very nice indeed.

The three-tube 6SN7 circuit board for the Dyna ST70 converts it to a Mullard circuit, with lower distortion and stronger drivers. Since nearly all the ST70 circuitry is on the single circuit board (for both channels), swapping that board basically gives you a new amplifier ... while retaining the power supply, chassis, and transformers. Lots of ST70 variants, since so many were made and are still kicking around. And the output transformers are pretty good.

Of course, if you are replacing the power transformer and upgrading the power supply, you might as well build on a new chassis, and have an all-new amplifier. Nothing wrong with a 6SN7 Mullard circuit and modern power supplies ... that will take you into the $3000 to $10,000 quality bracket right there.

@donsachs 

If you build a vintage amp with the sort of power supply you are using in your SET that vintage amp would sound much better.

+1 

Charles

@alexberger Fine is a relative term.  If you improve the power supply in a citation II the amp is greatly improved.  That is why the KT88 amps I used to build have FAR better power supplies than vintage amps.  If you build a vintage amp with the sort of power supply you are using in your SET that vintage amp would sound much better.

All classic vintage PP amps from 50-60 have very weak PS in terms of chokes, power transformers and especially capacitors values. If you use the same power supply for SET with no feedback with a such PS will be a disaster. But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation?  

Actually I have rebuilt 3 or 4 of the ST70 amps.   The best version is to skip the 7199 or 6GH8 types and to one install of the octal driver replacement boards.  I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does.   It makes the amp considerably better.  Really in every way.  That said, they still run out of steam under load because the power supply is only adequate and there is no room in that tiny chassis to install a better supply.  They are great amps for the money.  But they are still not particularly great.  However, there is little else you will find at that price point to touch it.

The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz

Actually the Dynaco has ~ 6Hz LF cutoff (-3dB) running open loop. Its distortion rivaled that of the Marantz 8B which was and is well respected, for a lot less money. You can reduce the distortion easily by obtaining a socket adapter off of eBay, which allows you to replace the 7199 driver tube (which is rare) with the much more common (and cheaper) 6GH8A. No other changes are required.

Another walk down Memory Lane. This time, we’ll go into the late Forties, when the Williamson burst on the scene. This English design wiped out all other designs in the USA until about 1955 or so, with the exception of the McIntosh and a few others.

How does it work? There’s an input tube, typically a triode like the 6SN7, direct-coupled to a split-load inverter, also called a "concertina" stage. This always has identical plate and cathode resistors, and gain a bit lower than unity. The plate output drives the upper half of the push-pull amplifier, while the cathode drives the lower half. Despite appearances, the voltages on top and bottom are equal and opposite ... provided the total loads match, as well.

The inverter is then cap-coupled to a separate push-pull driver stage, which is sometimes also set up as a differential stage, depending on the resistance presented to the common cathodes. High impedances move it towards a differential stage, with the limit being modern constant-current sources. 6SN7’s were typically used here, with later designs replacing them with 12AU7’s (which typically have more distortion).

The drivers are then RC cap-coupled to the output tubes in the usual way. The drawback of a classical Williamson are the two stages of cap coupling, which can introduce low-frequency instability unless the output transformer has extremely wide bandwidth. The Partridge transformer specified for the original design had one of the widest bandwidths of any output transformer ever made ... but lesser transformers introduced stability problems, sometimes "motorboating" at low frequencies, but more commonly long recovery times from overload.

The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz, the 20 dB of feedback nicely corrected it, since the input section used a high-gain pentode and there was plenty of "excess gain" to drive the feedback network.

The Dynaco had the advantage of being the cheapest of all to build; a combined pentode/triode, the 7199, took care of the entire front end, and all that was left were a pair of EL34 output tubes and an output transformer. In addition to Dynaco, many receivers used this approach as well. It was simple, saved money, and saved space, which was at a real premium in a low-profile AM/FM stereo receiver.

Receivers in the early Sixties (Fisher, Scott, Sherwood, Harman-Kardon, etc.) all had Bass and Treble tone controls, an AM and FM tuner with two different IF strips, an FM multiplex stereo decoder, a stereo power amp with at least 20 to 35 watts/channel, and last but not least, a stereo phono preamp. All with vacuum tubes, in a very crowded chassis, with marginal ventilation and caps of much lower quality than we have today.

We don’t see many Williamson amplifiers today. The dominant PP-pentode designs are Mullards and Dynacos, depending how price-sensitive the amplifier is. The monster tube amps with 4, 6, or 8 output tubes per channel typically throw in a dedicated cathode-follower section to drive all those grids ... sometimes one cathode follower to drive them all at once, or preferably, each output tube gets its own cathode follower. The RC coupling is then moved to the input side of the cathode follower, and the CF directly drives the grids of the output tube(s). This easily provides independent biasing of each of the output tubes, which is important when that many tubes are used.

I agree. This is a stable topology, taking full advantage of specialty transformers designed by two of the world’s top designers, and using vacuum tubes that are in current production as well as ample NOS stocks.

As mentioned earlier, it’s a very simple signal path, with only transformers and vacuum tubes, and fully balanced from input to output. Zero feedback, with the audio signal only propagating in the forward direction.

Well... Lynn has lots of wild ideas, but the thing is that they are very well thought out, and they are based on years of technical experience with Tektronix.  I learned many years ago in academia, and as part of my main career in forest ecology research, to listen to really smart people with wild ideas.  Many of them were simply ahead of the mainstream thinking.  The mainstream often ends up there.....eventually.  

Lynn suggests improvements and where practical, I try them.  We finally ended up with a larger chassis that could accommodate all the things we wished to try.  They are still under 19 inches so they fit any rack, since 19 inch is the traditional rack width.  To my ear, it worked beautifully....  The final touch was the addition of old school gas VR tubes to further isolate the input tube supply from the drivers, and of course the move to KT66/6L6 or KT88 drivers.  I think we are done finally.

And I am very much looking forward to fellow enthusiasts hearing the Raven preamp and Blackbird power amp. I’ve been a voice in the wilderness for about twenty-five years ... neither a member of the SET fraternity (well, maybe on the edge of it) nor mainstream Audio Research/Jadis/Conrad-Johnson push-pull pentode big-watt amplifiers dominating the hifi shows. A handful of people built the Karna amps, but many abandoned the difficult project halfway through.

Don was one of the very few who persevered through two years of building prototypes that were far off the beaten path of mainstream tube gear. He’s had plenty of hands-on experience with the fiendishly difficult Citation II, the most complex amplifier of the Golden Age, and his own designs, the Valhalla (6L6) and Kootenai (KT88).

Of all the people I know in the industry, Don is the most qualified to honestly tell me what is unrealistic and pie-in-the-sky, and what is practical and a good solution. He’s been there and done that. Oh, and he has good taste, too, which isn’t that common in the industry.

You might think I’m being snarky about the "good taste" but I am perfectly serious. The industry has plenty of competent engineers, and whole hifi shows filled with high-powered marketers, but good taste? It’s not all that common, and I’ve been in the industry since 1973.

@tinear123  No.  Production starts in late November when I go to Salt Lake and teach the guys at Spatial the builds.  I would expect a review by late spring and perhaps an audio show in the west somewhere next summer.  That is kind of going to be the schedule I think.   There will be a review pair that could potentially end up at a show in the east next year, but that would be Spatial Audio Lab's call.  I really wish that folks could hear what I am listening to in my living room.  It would be fun to have any of you over.  There will be a complete setup in Salt Lake City area by January and I am sure audition arrangements could be made for anyone who wants to hear it along with a top end Spatial Audio Lab speaker.  Of course this doesn't help any of you out East....     

It is definitely the plan to have a review setup of both preamp and amps and have it reviewed by legitimate reviewers next year as early as possible.  Perhaps sometime next year we can have them appear at a show in the east somewhere.

A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

Usually in a differential amplifier, the plate resistors are matched if both halves are driven. In the case circuit of the above description, only one side is driven. So if a CCS is not used, the plate resistors can't be matched; one side must have a slightly higher plate resistance to compensate for the mu (gain) of the tube of the un-driven side, so as to get equal outputs from each half.

A good CCS eliminates this problem (which is nice since in the real world you can't count on the mu of each section to be equal or matching that of the tube specs on paper). A good CCS is both inexpensive and reliable, allowing the tube to be removed from the circuit while active (hot plugged) without damage.

Further benefit can be had from placing a CCS in the output tube cathode circuit, if you control the output tube(s) bias using fixed grid bias. The cathodes are tied together and the CCS feeds them; thus improving the differential effect of the output section, which reduces distortion and makes it slightly easier to drive due to increased gain.

Of course, if you have the input tube be a differential amplifier too, it can accept a balanced or single-ended input and can have excellent performance if a CCS is used for this stage as well. But its not a good idea to direct couple both plates to the succeeding driver stage; its OK to do one but the other should be capacitively coupled so as to prevent DC offsets of the first stage of gain from causing distortion in the driver.

The original BAT VK-60 of the 1990s used a differential input direct coupled to a differential driver; to deal with the DC offsets a potentiometer in the cathode circuit of the input tube allowed the plate voltages to be equalized. I found this approach to be problematic (we had tried that back in the early 1980s; one obvious problem is that it requires the user to make this fairly critical adjustment...) and often causes more problems than it solves.

So in the quest to keep the number of coupling capacitors down but retain easy operation, we started using a differential cascode voltage amplifier. The advantage of this was that all the gain of the amplifier was in a single gain stage, consisting of three dual-section triode tubes, one for the input differential amplifier, one for the top of the cascode, being plate loads for the bottom tube sections, and finally a 2-stage Constant Current Source for the circuit, tied to a B- supply of equal potential to the B+ supply. The CCS prevented changes in the AC line voltage from affecting performance of the voltage amplifier from 107VAC to 126VAC the difference was only 17 parts per million. So you couldn't see any performance change on an oscilloscope over that range!

So that allowed for enough gain, low distortion (once the correct operating point was set up), and only one pair of matched coupling caps (of a small value, in our case only 0.1uf, further minimizing the sonic impact of the coupling caps). They drive a pair of cathode followers which are direct coupled to the output tubes. So the power tubes obtain their bias voltage from the driver; therefore the bias and DC Offset controls are in the grid circuit of the driver tube. This allows for instantaneous overload recovery and rock solid bias control of multiple high-capacitance triode grids, with low frequency response to 1 or 2Hz no problem at all.

If you use a coupling cap in the critical area of the grids of the output tubes, it must be large so as to get good bass response since the grid bias network must be of relatively low impedance to properly control the power tubes. This means that the driver tube has a difficult load to drive and the large coupling cap can cause blocking distortion and slow the overload recovery. While this really isn't much of a problem driving pentodes, using this topology to drive triodes is a bad idea IMO/IME.

I've been describing how our OTLs work but obviously this would work well with a 300b too. We've managed to get our OTLs to 0.5% THD which is pretty low distortion for a zero feedback circuit! SETs by contrast tend to be about 10% THD at clipping which might be only 7 Watts. Since the OTLs tend to be much higher power capacity, the tendency is, for any power level the SET might have, the OTL has distortion that might be 2 orders of magnitude lower or more at the same power level. This is why they tend to be so much more transparent than SETs.  I've no reason to think this cannot be applied to a 300b circuit with similar results; SETs have the distortion they do out of the topology rather than the power tube that is used. So a pair of 300bs could be used to much greater advantage!

For those that might want to see more about how our OTLs work (and how this might be a topology for a 300b amplifier), there is a DIYaudio.com thread from several years ago that has a schematic and discussion. A lot of this would work very nicely with a 300b; for example the Circlotron output can be transformer coupled of course and have all the advantages (such as zero DC saturation of the output transformer) it offers.

As a minor diversion, I should describe the "Golden Age" amplifiers I keep referring to. This aren’t just the amplifiers made in the Fifties and Sixties; it describes the majority of PP tube amps made since then, including today.

There were only a few basic Golden Age circuits, or topologies, as we like to call them. (Topologies omit circuit values, but are easily worked out once you know the tubes.) The first was the Williamson of 1948, but it had the drawback of marginal stability. Still, it dominated the US market until 1955 or so, when the much simpler Dynaco variant came in. (The Dynaco topology simply omits the driver stage of the Williamson and uses the phase splitter to drive the output tubes. More distortion but more stable.)

The Mullard became the prototype of many tube amps as the better-performing alternative to the Dynaco circuit, and is still widely used today. Let’s walk through it.

There’s a high-gain input tube, typically either a 12AX7 or a pentode like an EF86. This is direct-coupled to one half of a differential stage, with the other grid AC-coupled through a cap to ground. Because the grid of the diff stage is at 150 volts or so, the cathode is a little bit higher, maybe 155 volts. This requires a large value resistor that goes all the way to ground, so the diff stage is frequently called a "long-tailed pair". A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

The diff pair are a pretty good phase splitter, and unlike the split-load inverter of the Dynaco circuit, audio-frequency balance is not too sensitive to load. It also has more drive capability than the split-load inverter, and unlike the split-load inverter, it has some gain, too. So a win all around.

And we’re not talking about a lot of parts here: 3 triode sections, and the output pair. A Dynaco is even simpler, with 2 triode sections, and the output pair. The only coupling caps with either circuit are between the grids of the output pair and the preceding circuit, so not really complex, and simple enough that a stereo chassis, running off a single B+ supply, is quite practical.

The point of the high gain (in the input section) is to give feedback something to work with. Feedback requires "excess gain" to work its magic; you need 20 dB of excess gain to get 20 dB of feedback, which will reduce overall distortion tenfold. In a pentode or ultralinear connected amplifier, the output impedance is way too high to use with most speakers. The feedback really comes in handy here: 20 dB of feedback reduces output impedance tenfold.

What limits applicability of feedback is loss of stability if too much is used (I’m not going to get into Nyquist Stability Criteria here, nor phase margin, settling time, etc.) In other words, if we slap in another gain stage and try for 40 dB of feedback, it will just oscillate. At full power. And take out a tweeter before damaging itself and letting the smoke out.

A more clever approach is wrapping local feedback around the most distorted stages, like the output section, and then add overall global feedback on top of that. This was done in the McIntosh, Citation II, and a few other amplifiers. This really gets the distortion numbers down, but clipping can get ugly, and settling time from transients can be an issue. Multiple feedback amplifiers can be quite sensitive to operating conditions. It’s more often seen in modern transistor amps as "two-pole compensation", and is not trivial to design.

Note: To puzzle out a schematic, by convention, signal flow is left to right, just like you’re reading this. To see what a tube is doing, look what the grid (the dotted line) is connected to. Often, there will be a coupling cap, typically 0.1uF. If it is much smaller than that, like 30 mmF or 30 pF, it is bypassing RF or has something to do with stability. Larger caps are cathode bypasses or power supply. The plates (the flat-topped dingus) is the output of the tube and typically heads to the right side of the schematic.

You usually have to stare at a phase splitter quite a while before the function becomes obvious. One side is quite simple, coming directly from the input tube, but the other side can be pretty weird. A diff stage can be puzzling, because the DC connection is a high-value resistor going to the other grid, and the AC connection just goes to ground through a 0.1uF cap. The "other half" is actually driven from its cathode, not the grid.

What gives away a split-load inverter, or "concertina" stage, are the equal cathode and plate resistors. This is a dead giveaway you are looking at an inverter, since no other tube stage uses equal resistors ... for one thing, it’s kind of useless for anything else, since gain is a bit less than unity.

I leave the "floating paraphase" as an exercise for the reader. I kind of like them, actually, because current drive for the power tubes is pretty good, although balance is only so-so.

@atmasphere 

perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing

Different pathways to audio nirvana is something I’ve acknowledged long ago. It’s an undeniable individual journey with numerous successful outcomes. What I have found to be most pleasing and satisfying for me certainly may not be the choice for another.

I don’t believe my comments above contradict this perspective. I was merely comparing two earnest efforts to build amplifiers that are vastly different in concept, design and implementation.

Charles

@charles1dad My point was addressing a comment made earlier by Lynn about overshoot in amps employing feedback; simply that if you do it right its not a problem. The OP mentioned using Spatial Audio Triode Masters who were a dealer of ours and have used our OTLs and class D on their speakers. It didn’t seem that off topic, especially if we discuss the issues of signal coupling, operating points and the use or lack of use of feedback.

As I understand it, you are particularly enamored of SETs; perhaps this thread might have convinced you there is more than one way to reach audio Nirvana 😉 I’m sure the Blackbird is well worth hearing.

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

@alexberger Ringing contains higher ordered harmonics which can be heard as brightness and harshness. You also get lower orders which contribute to richness. Both are colorations and will obscure low level detail.

Hi @atmasphere ,

How does excessive transformer ringing can influence on sound?

Does it make it too sharp or bright?

 

@atmasphere 

our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; 

This GaN  class D balanced amplifier with copious utilization of NFB  is literally at the opposite end of the audio design spectrum from the DHT (300b) balanced class A  zero NFB Blackbird under discussion on this thread. Talk about traveling different roads toward the destination of Rome.

The in depth information presented here concerning the Black bird amplifier has me exceedingly curious to hopefully hear it one day. Its development is a fascinating story.

Charles

I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and 47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

@alexberger As you have noticed, if you are using a coupling (interstage) transformer, it will be needing proper loading to prevent ringing (distorting). You are nearly there with your technique so far; put a potentiometer across the output of the transformer, run a square wave through the active circuit prior (6SN7) and adjust the pot for minimum ringing (critical damping). IME its probably best if you leave a very slight amount of overshoot as opposed to rounding the square wave.

Its important that the driver to the transformer be active, since transformers transform impedance: Whatever impedance on the primary side, if it varies, will affect the critical damping value on the output side. Conversely, whatever is loading the output side will also load the input thru the ratio of the transformer. So you want to feed the squarewave to the active 6SN7 circuit so your loading value will be correct. Best to have the 6F6 running also for this very same reason, although the loading resistor will likely dominate that side of the transformer equation.

Once that is sorted out, you might find it interesting to measure the impedance at 1KHz on the primary side (you won’t need the 6SN7 in the circuit for that, but it would be a good idea to have the 6F6 in place and active) just to see what the load on the 6SN7 actually is. You may find that you have to adjust the operating point of the 6SN7 to obtain greater linearity (by adjusting the cathode resistor value if you have one); if you do that then you may have to readjust the loading value of the transformer since the source impedance of the 6SN7 varies a little with the operating point; in this way zeroing in on the optimal values.

Obviously you can stop at any point (call it ’good enough’), but in a zero feedback design I’ve found that the more you pay attention to refining things like this, the more it pays off in the end!

@lynn_olson I agree overshoot in a circuit using feedback is bad!

But if the feedback is applied properly you’ll get no overshoot at all. Our OTLs don’t exhibit any squarewave overshoot, being zero feedback and free of inductors in the signal path. The 10KHz waveforms are pretty good since they have wide bandwidth; our class D, which has less bandwidth owing to the output filter, nevertheless has a very similar 10KHz waveform, despite (well, actually because of) running 37dB of feedback; the difference being the residual sine waveform imposed by the switching frequency.

@alexberger Please report back on the sonic diff between the A-107 and RC coupling.  Completely different amps, but I much preferred all IT coupling.  Curious to hear your impression

Hi @donsachs ,

Yes Dave Geren at Cinemag do great job!

I use Hashimoto A-305 IT between 6f6 (driver) and 300B. The square wave was almost perfect at 10KHz, 20KHz, 30KHz. I use an 80K Ohm grid resistor on 300B. But I’m not sure this resistor is needed! The -3dB frequency response is from 6Hz to 95KHz. This IT is extraordinary on measurement and sound.

For 6sn7 I use Hashimoto A-107 that is not as perfect but has more primary inductance that is essential for higher input impedance 6sn7.

Hashimoto A-305 is designed specially for SET, but A-107 is universal IT that can be phase splitter or SET 1:1 or SET 1:2.

I just installed A-107 and it has zero break-in time. So I can’t comment how does it sound compared RC (V-Cap CuTF, AN Silver Tantalum) that were before. It need more break-in time.

Now’s a good time to discus the difference between overshoot in feedback vs non-feedback amplifiers. Despite similar appearance on the scope, they are caused by completely different mechanisms.

* Overshoot in a feedback amplifier is quite malign, since it indicates the onset of oscillation, something that can destroy the amplifier and the speaker it is connected to. It is caused by the amplifier running out of phase margin, possibly the result of a reactive load, but also the result of design oversights in the feedback loop.

* Overshoot in a non-feedback amplifier is quite different. Now, it might be the result of high-transconductance tubes self-oscillating, but this can prevented by grid-stopper resistors and good layout practices. Normally, though, it is merely transformer overshoot, the result of phase shift at the edge of the passband, and has nothing to do with stability. That is what we are seeing here.

Something to be considered about ultrasonic behavior in the time domain: if there is no spectral content in the frequency range of the overshoot, it will never be stimulated in the first place. It never happens.

This is the difference between overshoot in a feedback amplifier and a non-feedback amplifier: in a feedback amplifier, it is a warning sign, like the LOW OIL light in a car. You ignore it at your peril. In a non-feedback amplifier, it has nothing to do with stability, since there is no feedback loop to induce oscillation. It is simply the behavior of passive parts, in this case, the input and interstage transformers.

As you can see, Cinemag has done a very nice job here. (Same photo as above, just tidied up a little.)