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. 




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.

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.

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.

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 and @lynn_olson ,

Here is square wave 1KHz on my amp output:


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.

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.

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.


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.

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.

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.   

@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.

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 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.

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.  

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.

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.)


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.


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.


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.


Japan is interesting. Unlike the USA, it looks outward to the world (Americans by and large don’t know or care), but any kind of immigration is nearly impossible. Visit, yes, but don’t stay. This is a demographic problem with a rapidly declining population and no effective guest-worker program.

What makes the USA unique is that it only imports 12% of its economy, less than nearly any other nation, and half of that are imports from Mexico and Canada, which are effectively domestic markets. Get rid of those two and it is 6% or less.

It is self-sufficient in energy for the foreseeable future, and for the history of the country has always exported food to the rest of the world. The USA has never had a food famine in its entire history, which is unique as far as I know. So Americans are pretty insular compared to most other nations, and only the larger companies have any awareness of the ins and outs of selling to non-North American markets.

In the rest of the world, it is Export Or Die, and governments are tightly focused on this, every day. Because essential inputs, like food or oil, come from elsewhere, and are critical to the survival of the population. In the USA, there’s always more. It’s always been that way. Our economic problems are self-inflicted. The idea of HAVING to export is very foreign to Americans, but is no joke in the UK, the EU, China, Japan, and much of the rest of the world.

My son lives in Japan and has worked and lived there for the last 4 years. ?? 


but any kind of immigration is nearly impossible. Visit, yes, but don’t stay. This is a demographic problem with a rapidly declining population and no effective guest-worker program.

About 12,000 naturalized immigrants a year, it turns out, out of a population of 123 million. Not enough to offset a population loss of about a half-million to a million a year.

Returning to the original question, Asian companies are a lot more export-oriented than US companies. They have far more experience exporting to North America and the EU, and are expert in making "localized" versions.

Japan is also interesting because half the country is 50 Hz (Tokyo), and the other half (Kobe/Osaka) is 60 Hz. Asynchronous 50/60 Hz Grids. In a sense, even the internal market is an export market, since there are two power frequencies.

I saw Kurobe Dam while it was under construction by Kansai Electric. The underground power station, deep under the mountain, is like sometime out of a Bond movie set. Two of the generators were running while the third was suspended from a crane. The fourth was an immense concrete pit.

Very interesting discussion about gear sold on one side of the Atlantic and that sold on the other side.  Reading the British hifi magazines, I have always been curious why they and the Europeans fancy such different gear than we do in the US.  Naim and Hegel are two EU brands that have deeply penetrated the US audio market, as well as the venerable Linn TT's.  With Lynn's erudition, I now understand why that is the case. 

As I am sure Don can attest after his renovation of lots of vintage HK and Marantz gear, a boatload, literally, of these vintage tube amps made their way to Japanese audiophiles for decades.  Don't flame me for saying this, but the Japanese have a purist view of a lot of things, including architecture, gardening, city planning, vital mass transit, delicate and inventive cuisine, etc etc.  It comes as no surprise to me that the Japanese clamor for the best audio gear on this side of the pond.  Kondo Audio Note is a prime example of their aesthetic.  We all learn from one another.  


I was only 11 at the time, but I was dazzled by the private tour of Kurobe Dam. It’s size of Hoover Dam and freaking huge. The dam was nearly finished when we visited as guests of Kansai Electric, and we took this strange little Elves railway deep, deep into the mountain. We got out of our mini-train carriage into a brightly lit station with gleaming tiles, and directly opening into a modernist office area, with desks and office furniture.

Rather than office windows looking outside, there was a massive expanse of glass overlooking the turbine hall, from three stories up. I assume the control room was in the floor underneath us ... didn’t get to see that, but we did go to the main turbine hall. Two were up and running, the third Hitachi-made unit was on a crane, and there was a vast and deep concrete pit for #4, yet to arrive. All new and the walls all done in gleaming white tile, not bare concrete. Basically, an office and power station deep in a mountain, with a very quiet thrum as enormous quantities of water flowed through the penstocks into the turbines. You could just barely feel it.

We left by another route and saw the ultra high tension power lines coming out of the mountain, There was a faint glow around the wires from corona discharge and a sound like crickets from overhead.

Dad drew our attention, as we left on the micro train out of the very deep valley, that all this was being built 15 years after the most overwhelming defeat in history, with nearly every Japanese city in smoking ruins, and tens of millions on the verge of starvation. A scant 15 years later, a Japanese-made technological wonder that put Hoover Dam in the shade. It left quite an impression.

I have to give Dad credit. As Economic Attaché, he wangled some spectacular tours while we were overseas. Kurobe Dam. A Japanese steelworks at full blast. A visit to the 8-reactor carrier Enterprise. A visit to a factory assembling magnetic-core memory. An aerial tour of Hong Kong on a four-engine Super Constellation as the FAA aligned Kai Tak’s landing systems.

Although he was a diplomat with a Masters in Economics, he was a big fan of technology, the more spectacular, the better. If you want Big Tech, Asia’s the place.

As Whitestix mentions above, the Japanese have a "purist" esthetic that applies to audio. But it goes much deeper than that. The CEOs and top engineers of Japanese, Korean, Taiwanese, or Chinese companies are not going to be impressed by the latest mass-produced gadget. Hardly. That’s the stuff they make every day, and they make it to very high reliability standards.

What they appreciate is artisan audio made to the highest standards, by artisans of superb taste. Not 1" thick anodized-aluminum front panels with a giant meter in the middle. That only impresses naive Americans who have never worked in manufacturing,

What impresses them is genuine American aerospace-grade quality, if circuit boards are involved, or artisan hand-tuned point-to-point circuitry. And they are not impressed by magazine reviewers, since they have an insider’s view of the publishing industry, and are very aware of how American marketing works.


What they appreciate is artisan audio made to the highest standards, by artisans of superb taste. Not 1" thick anodized-aluminum front panels with a giant meter in the middle. That only impresses naive Americans who have never worked in manufacturing

I wonder how these Japanese engineers/designers view the products manufactured by Robert Koch (Robert Koda Audio). His approach seems to be a blend of Japanese aesthetic/philosophy and heirloom visual appeal for the North American and European markets. I don’t know how well received his components are in Japan.


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.

To gain the CE mark (which says that the product meets EU Directives) you can self-certify and no lawyers needed. Its not intended as a trade barrier.

If there are no digital or switching circuits involved, you have exemption from a good number of directives. Beyond that, if good practice for the AC wiring is observed (with a proper AC ground if a metal chassis is employed) and there are no exposed voltages, then you are 95% of the way there. It isn't required that you have a tube cage but if its not shipped with the equipment it must be available as an option. Finally the CE mark must appear somewhere: either on the shipping box, the associated owner's manual or the equipment itself.

Its a good idea to test the equipment for RFI generation, even if there are no RF sources such as switching technology. If you paid attention to Ps and Qs regarding layout and grid stoppers this is likely not an issue. 

RoHS must also be observed. For the most part this means lead free solder, but there's a percentage involved so it is possible to use leaded solder. You'll have to review the regulations on that.

a fascinating discussion… and remarkable in that blind American exceptionalism has yet to …intervene…much.

I spent much of my career working globally…for the largest exporter in the USA. 

Aerospace grade….. ;-)

Enjoy the music and i wish all well

Japan has a very good used audio stuff market with Tango, Tamura transformer; Kondo, Acrolink cables; used amplifiers and CD player,... Japanese people sell all this stuff on the internal market. A very few used stuff from Japan is sold internationally , for example, on eBay. And because Japanese people don’t use eBay, all used Japanese audio equipment sold by hucksters - prices on eBay are at least twice higher than in the local market.

Yeah, RoHS-approved solder might be a stumbling block, since it requires higher temperatures and a little different technique. We’ll cross that bridge when we get there. Our first goal is getting production under way in Salt Lake City, home of Spatial Audio.

Fortunately, both Salt Lake and Denver have a number of aerospace vendors that make various subassemblies for rockets and satellites.

I have a thorough dislike of working with lead-free solder.  Nothing flows in a point to point build like a good 2% silver solder.  Lately I have been using Wonder Solder for the past year or two and it is just wonderful to work with.  Of course, one could use lead-free solder, and pass all the hurdles and build things for shipping in large numbers to the EU.  First you get the product off the ground in N. America.  We have pretty much free-trade between Canada and the USA, and it is quite easy to make things in either country and ship to the other, duty and tariff-free.  My main point above was that it is very expensive to deal with customer support across the pond.  If you ship 100 things to 100 people, about 1% of the time something happens.   For example, UPS will admit that they have a 4% claims rate.  Things happen.   No matter how robust you make something, and how thoroughly you test it and the tube set before you ship it, things happen a very small percentage of the time.  Or a customer plugs a tube you didn't sell them in there and it grid shorts.  This gear is quite robust against serious damage when that happens because in all these years of building things, I have seen all sorts of failures.  When a bad tube kills something, I improve designs so that will only blow a fuse instead next time.   So these things are quite stable, but still, every now and then things happen.  When they happen in N. America it is far cheaper to sort out.  You always take care of the customer.  It is much more expensive to do so overseas.....

Which is why medium to large scale companies have regional service centers, which combine repair, refurbishing, and overall product support in one center. It’s equivalent to a low-yield manufacturing center with a marketing wing (in the local language) attached.

They support local retailers, or have direct customer support, and offer the kind of services manufacturers do, just on a smaller scale. Somebody to answer the phone. A parts warehouse. Several techs to repair or upgrade the product. Somebody to manage the facility and make sure things actually get done, along with inventory and personnel management. In short, at least three to ten people working full-time, along with leased space in an industrial park. It’s equivalent to a small factory, with equivalent monthly overhead costs, too.

Once your company passes the threshold of about five to fifty million dollars a year in sales, yes, opening overseas centers in regional markets makes economic sense. You can afford to hire three to ten new workers on another continent, and train them and point them in the right direction. Opening a regional service center requires a pretty steep investment in time and money.

Otherwise, you are supporting the overseas customers by remote control, with very expensive shipments back and forth, and a certain percentage of those returned products severely damaged in shipment because the customer threw away the original packaging and improvised their own packaging.

I'm not sure what to tell potential overseas customers to do. Sure, after you hear all the market buzz and read the first reviews, you might be tempted to get on a plane, fly right to Salt Lake City in the state of Utah and buy yourself three good-sized boxes ... a matching Raven, and a pair of brand-new, state-of-the-art Blackbirds. Be the first person in London or Berlin to own the full set. How cool is that?

But ... if anything goes wrong, or you just want some hand-holding, or want to ask what our favorites cables are ... you're kind of on your own. You're not in the same time zone as we are. We don't know anything about the ins and outs of UK and European power supplies, nor what favorite NOS tubes are in your area. Sure, with time, we'll find European affiliates that are a good match. With time, in Asia as well.

I have a thorough dislike of working with lead-free solder.  Nothing flows in a point to point build like a good 2% silver solder. 

@donsachs That is why its worth it to work out the math. The lead content in your product is a percentage allowable. Since you have a lot of transformer weight and otherwise the circuit is fairly simple, my surmise is you can use leaded solder with no worries. But you'll want to look at the regulations and do the math to know for sure.

We'll cross that bridge when we get to it. So far, we haven't ordered any multi-voltage power transformers. When we do, we'll dig into the applicable CE regulations ...

The gear could easily be wired for 220-240 VAC 50Hz.  Everything we use supports it except for ordering multiple primaries on the power transformers.  Again, the project must get off the ground in the USA/Canada.  If they are selling, and people from Europe are inquiring, then perhaps Spatial may be interested in that market.  Hard to say.  If demand is outstripping supply there is no reason to make your life difficult dealing with markets outside N America.  Down the road......maybe

I should mention additional heating from 50 Hz working is a non-issue. The design of the power transformer, rectifiers, and regulators are extremely conservative and run very cool. Don has measured the thermals all around the chassis, and we have plenty of headroom everywhere. So no issues there.

I ask again this technical question again. Which rectifier is better: full wave or bridge?

My driver stage power transformer has three taps: 250 0 250 and supports both - full wave and bridge. Full wave affords use x2 times bigger DC current. Why do many people prefer a bridge rectifier over full wave?

They’re different.

* Full-wave uses two rectifiers and only one half of the secondary at a time. Each active half-secondary switches back and forth 120 or 100 times a second.

* A bridge uses four rectifiers and the entire secondary all the time. The current through the full secondary switches direction 120 or 100 times a second.

That’s the most basic level. To a first approximation, full-wave transformers need twice the voltage for the full secondary, since only half the winding is used at a time.

In more depth, if there is a first cap following the rectifier(s), the rectifiers only conduct for very brief intervals (a few milliseconds) when the cap recharges. These brief, high-current pulses can shock-excite the half-secondary, causing ringing from the abrupt cutoff of the unused half-secondary. The shape of that shock is controlled by the on-off ramp of the diode, and if there are charge-storage effects from a solid-state diode.

The rest of the time, after the first cap is recharged, the diodes are switched off, and the amplifier is powered from the charge in the first capacitor. This cap is steadily discharged until the next pulse comes along and recharges the cap all over again. This charge-discharge cycle happens 120 or 100 times a second.

There's a tradeoff in sizing that first capacitor. If you double the size, the voltage sags half as much ... but the inrush current is also doubled, too. If you really overdo it, the inrush current will be so large it pulls the circuit breaker.

A "choke-fed" supply has the diodes directly feed a special inductor rated for very high voltages. The diodes remain "on" for most of the AC waveform. But ... the special inductor has to tolerate a significant voltage kickback when the diodes do cut off ... this in turn can create ringing and possible voltage breakdown of the windings in the choke.

All of these supplies create very large current pulses that radiate into the air and are are transmitted back down the power cord of the amplifier, which turns the cord into an antenna radiating a 120 or 100 Hz pulse train, with harmonics extending throughout the audio band,

A general rule-of-thumb in power supply design is to minimize the "loop area" of the most powerful transmitters, which are the loop created by the transformer secondary, the diode array, and the first filter element. Minimizing the wire length, tightly twisting these wires, and keeping them as far away from the input circuit as possible is highly desirable. This is why running AC power to a front-panel switch is undesirable from a noise perspective.

There’s a simple emulator called PSUD that lets you play with different power supply topologies and circuit values, and lets you scale the load to a real amplifier.

Which rectifier is better: full wave or bridge?

Full wave is preferred if you are planning a bipolar (+ and -) power supply. If only a single pole, a bridge does have the advantage of the power transformer not needing a center tap. That advantage is because a center tap is never truly centered, so the output from the transformer applied to a full wave is slightly different with each half of the AC waveform. This means the diodes are making slightly different current spikes as they commutate (turn on and off). 

That might not make much of a difference, since the best way to snub the circuit (to kill the swept resonance that occurs when the diodes shut off) is to snub the rectifier with a resistor and capacitor in series across the input to the diodes. In case its not clear, the source of 'diode noise' is really the power transformer inductance, interacting with the capacitance in the junctions of the diodes. For this reason, if you use a semiconductor rectifier setup, you really want to keep the leads from the power transformer as short as possible.

If you do that right, you can make the nasty silicon rectifiers perfectly silent; obviating any need for a tube rectifier (who's main advantage is low 'rectifier noise'). I put that last bit in quotes since the transformer is what is causing the noise, reacting to the rectifier.

In the old days it was common practice to put a 0.01uf cap in parallel with semiconductor rectifiers. This does nothing and might actually make the problem worse. Any diode junction, if you plan to snub the rectifier itself, must use a resistor in series with a small capacitance to properly snub the rectifier. But usually you can have great success with just a resistor and small capacitance at the input to the rectifiers instead, since snubbing the transformer is putting out the match rather than the forest fire.

Hi @lynn_olson ​​​​@atmasphere ,

Thank you a lot for detailed answers!

I have another question:

In balanced push-pull self bias output tubes is not a big issue because the cathode resistor and capacitor are out of the main signal path. But what to do in my case with a 300B SET. 

Now I use self bias with 50uF AN Kaisei NP bypassed by 4750uF Nichicon and two Ohmite Gold 10W resistors in parallel.

So the question is what is the best solution for SET? Fixed bias? Is fixed-bias reliable and safe? What is important to know for building a negative power supply for fixed bias?

To complete the current loop around the 300B (or any other power tube), there has to be a low-impedance path from the cathode to the ground side of the filter cap of the B+ supply.

This can be (A) fixed-bias operation with the cathode at ground, or using something like a 10-ohm current-sense resistor. This requires an adjustable low-noise -80 volt supply, and a meter across the current-sense resistor. Adjust the minus supply until you have 70 to 80 mA flowing through the 300B. The plate voltage also has to be reduced by 80 volts or so, since you’re not dropping 80 volts across a cathode resistor.

A potential source of instability for the fixed-bias circuit is having a regulated minus supply while the main B+ supply is unregulated. Small variations in AC power voltage can result in large changes in bias point. The solution is either have a tracking feature for the minus supply, or regulate the main B+ supply, which is not cheap. In other words, when both ends of the tube are controlled by plus and minus supplies, they should track each other, or be fully regulated.

The (B), self-bias option is dropping 80 volts across a 20-watt cathode resistor, and bypassing it with 200 uF of film capacitor. B+ will be around 425 to 480 volts at the top of the output transformer primary. Sonics will be strongly affected by the quality of the film capacitor since it is in the main current loop of the 300B. Adjustment of current flow is not necessary since the cathode resistor provides a degree of negative feedback for the DC current flow through the tube.

I continued tuning my amplifier for the Hashimoto A-107 interstage transformer (IT).

I changed the load resistor from 57K to 39K Ohm. The difference in square wave overshoot is very small, but frequency response peak is just +0.5dB instead of +1.3dB.


But the most important - the rough upper mid coloration was gone with 39K.

I also increased 6sn7 idle current from 7.5mA and 9mA. When it was RC coupling before the idle current was 6.2mA.

In general, sound became more refined and organic with much better low level details. Now in addition to advantages that IT gives versus RC that I talked about before (like faster transients, better more detailed rhythm reproduction and bass control). I got more low level details, much more realistic drums kick/punch reproduction, piano is more real, as well as timbales, better instrument separation (less congestion on complex music), and more real musicians presentation in my room. 

Next step moving from an old amplifier box + power supply external maquette to two box solution (stereo amplifier + power supply).


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I changed the load resistor from 57K to 39K Ohm. The difference in square wave overshoot is very small, but frequency response peak is just +0.5dB instead of +1.3dB.

You can also see that bandwidth has opened up a bit. Something to keep in mind: when you load down that interstage transformer, it changes the load presented to the tube driving it. You might be able to further optimize the circuit by making changes in the driver tube circuit. Also, if I  were you I'd be try to sort out why the FR drops off so dramatically at the bottom end (is it the transformer or something else). 

Hi @atmasphere ,

The bottom end is predictable. ~8K 6sn7 (260v, 9mA) internal impedance and 70-80H transformer primary inductance gives -3db around 20Hz. Actually I measured -2.37dB at 20Hz and -3.62 at 15Hz.

According to plate voltage-current characteristics, 6sn7 has very small distortion (0.5-0.7% 150 Volt peak to peak) with both 39K and 57K Ohm load. To drive 6f6 it needs 40 Volt peak to peak. I thought about trying a 27K load, but with 27K we get ~2% distortion.
But I use Linlai 6sn7 and it doesn’t meet the classical 6sn7 spec plate voltage-current characteristics. I have a bunch of classical vt231 40x productions. It will be interesting to make measurements with these tubes.

The bottom end is predictable. ~8K 6sn7 (260v, 9mA) internal impedance and 70-80H transformer primary inductance gives -3db around 20Hz. Actually I measured -2.37dB at 20Hz and -3.62 at 15Hz.

@alexberger Here's something to keep in mind. If you are not using feedback, then its likely at either end of the bandwidth of the amp that the FR will fall off on a 6dB slope. Filter theory tells us that a 6dB slope introduces phase shift to 10x or 1/10th the cutoff frequency. So a 20Hz cutoff will manifest phase shift to 200Hz since there is no feedback correction.

The ear perceives phase shift of a single frequency very poorly, but over a band of frequencies it interprets it as tonality. At high frequencies the ear also uses phase to sense the sound stage. A high frequency rolloff above the audio band can cause darkness in the presentation. At low frequencies a rolloff will be perceived as a loss of impact.

Transformers have bandwidth limits. The smaller you make the transformer, the wider its limits can be. So you have a chance using an interstage transformer to have one that has good LF response- 5Hz is a good place to be. Because you are designing an SET you're dealing with a low frequency bandwidth issue in the output already. All I'm saying here is the less phase shift you present to the output section, the less phase shift will be present in the output.

If you're planning to use a sub with this system this might not be much of a concern. But of you really want to use a sub properly, it should be cut off no higher than about 80Hz otherwise it will tend to draw attention to itself, requiring that it be in the same place as the main speakers. If a sub is used, probably best to drive it with a preamp signal rather than that of the amp due to this bandwidth issue.



Will you consider ultrapath capacitor as opposed to cathod bypass capacitor in self-bias?

It turned out the ultrapath capacitor wasn’t sonically that different than a conventional cathode bypass cap. In the context of a regulated supply with an output impedance of 3 milliohms, and equivalent to a 2000uF passive supply, the Zout of the power supply is effectively zero compared to the cathode bypass or ultrapath capacitor.

By comparison, a passive CLC supply with a Zout similar to a 100uF capacitor, a ultrapath bypass might be more appropriate, but then the exact value has to trimmed against the noise introduced by the passive CLC supply. By contrast, the active regulator has 130 dB of noise isolation, so there are no issues of noise introduced by the supply and getting into the cathode circuit.

So it all comes down to the power supply. The optimal solution for a passive CLC supply, with its distinctive noise profile, might not be optimal for an ultra quiet supply with a very low output impedance.

And then we get into the deeper waters of the sonics of the regulators themselves. Some are slow and noisy, and intermodulate with the music. Others are fast and silent. Regulators do not all sound the same, and passive CLC supplies can have a signature too, depending on the capacitors chosen. There is no one-size-fits-all solution.

I should mention the battle of active vs passive supplies has been going on for at least three decades, and is somewhat biased by prejudice against the low-quality active regulators available 30 years ago. High-voltage regulators now are far better, and far more reliable, than what we had then.