Your journey with lower-watt tube amps -- Can a kit be good enough?

Thanks for the Dynaco ST-70 et al. suggestions. Since I am already running a Quicksilver Mono 60 with KT-77 tubes, I'm not sure this amp is really going to sound all that different. If I stick with the vacation analogy, I want to emphasize that I want to go somewhere really different and not have a "staycation."

I've had really good experience & sound with Mellow Tone Kits. Easy builds & their build instructions are posted on YouTube videos. They also give you all the specs & measurements for their equipment. Now I've only built 2 of their preamps..one in full kit form & the other just using the PCB boards they sell separately for a good price..though they did go up in price this year. Their boards are very versatile so you don't have to stick to the schematic they share. 

I've listened to their GU50 8watt monoblocks recently & was very impressed.. especially for 8watts. I thought it sounded great.  It's like $2k for the full kit though. I will add that I really like these guys at Mellow Tone/ValvesnMore. Super cool. 

I've built the original ST-70 & find it ok.. good amp for the money. When you part them out all new & spend under $1k it's just a good deal for a tube amp. A buddy has the Tubes4Hifi 12au7 ST-70 & it seems solid & an upgrade to the original..just a little different. I built their most recent SP13 preamp which is good, I prefer the MTK's but they have limited inputs & no remote in comparison. I wanted to try the SP14 & their ST70 but my experience with them wasn't stellar so I've kinda put them on the back burner. Skunkie designs just did a build video for the Tubes4hifi amp that kinda got me interested again. I have some chassis & transformers from the ST70 laying around. 

I really like the Van Alstine Ultimate PCB ST70. I use a sub so I  cut/roll off the bass on the Alstine amp around 50hz or so. I've built a couple of them at this point. Finishing another one up right now with transformers from Heyboer & Pacific Transformer. The OPT's from Pacific are cheap on price so I'm very curious how they will sound when complete. The owner insists that I'll be happy..we will see. First time I've used non-Dynaco iron. 

Pete Millet has some amp designs I'd like to try. He has some 50watt monoblocks that I just seen posted recently..though I'm not sure if he sells the PCB's for them on his eBay page. He has some amp projects on Wauwatosa Tube Factory WTFamps.com..i believe he still sells PCB's for some of those projects. 

Edcor Amps/Kits has links to DIY designs. 

Transcendent Sound sells full kits

Analog Ethos has very nice looking low powered kits. 

Specimen Products. Chicago based has an expensive SET & a couple other amp kits 

No personal dealings with those I listed..I just have them saved in my phone from when I was probably searching for kits/designs. 

PCB's & schematic designs are not kits though obviously. Like others have mentioned there are not a lot of full kits out there for sale. I'm hoping that if anyone  knows of any that exists to please mention them in this thread. Skunkie Designs has done a few low powered amp builds on her channel & posts all the parts needed. I wish I had the tools & skills to build the amp chassis/enclosure. If anyone knows of a place or person that will fabricate/cut out top plates for a fair price please mention it. 

At the moment I'm using a solid state class D Hypex Nilai DIY amp. High power extremely easy build. No soldering involved. I've paired it with a non-kit Icon Audio LA4 mkiii tube preamp that I did the Signature upgrade on. In my scenario I find the Nilai kit to perform & sound fantastic for the price. 

I forgot to add. To answer your original question I don't have any experience with low powered kits. But I'm interested in them. The one 8watt unit I heard was definitely good enough on speakers over 92db..4 & 8 ohm

@mattw73 Thanks for your detailed response. Very generous of you! The GU50 looks interesting and I like those guys too

@hilde45 no problem...I was just babbling on about whatever was popping into my head. Your thread is a topic I've been thinking more about recently though. I always just assumed one needed speakers with just a high sensitivity like 98db plus to be able to use an oldskool sound SET amp. Trying my Fritz speakers with a 25watt tube hybrid amp made me think that I might be able to get away with a low watt SET amp. 

I didn't have my Fritz when I listened to the GU50. I sold this guy an old pair of speakers & drove almost 2 hours to hear it & some other things he had. All of his speakers were older & mostly diy stuff. I just took his word for it that the couple speakers we listened to were 92-93db sensitivity when using the low powered amps. Everything sounded natural, big dynamic swings, clarity, just realistic. But I couldn't honestly say if my speakers would sound or could be handled the same way with low power. 

So I debate that if I'm really going to invest in a true SET amp that I should first build or own some speakers that where designed to really thrive with that type of amplification. Though I would take a chance on a good budget priced kit. Maybe it's all in my head but I do think there is something different yet enjoyable to a tube rectified single ended triode preamp & amp. 

I do think that going forward I will be more interested in diy & kit stuff..speakers included. I see now that I'm better off buying quality drivers using crossovers someone else designed & then implement quality parts. I've only done budget type preamp & tube amp builds but the outcome so far has exceeded my expectations. Low noise simple circuit tube gear sounds pretty good. 

I guess it just depends on where one is in their audio journey. If one is still out there striving to find the best sound possible then a kit is probably the very last thing on your mind. If you've gone that route, spent that cash, seen what you really get every time you up your budget & want to explore something really different..try some DIY. Get your feet wet with a kit that has great build instructions. Buy a soldering iron. It's kind of a cool feeling when someone comments that the stereo sounds good & I can say, thank you..I built that. My wife thinks it's amazing. I have yet to tell her that I'm just following someone else's instructions. 

Just my two cents, but having owned multiple ST70s and ST35s, I like the ST35 much better.

@hilde45 This is a comment worth exploring! The ST35 uses a triode Voltage amplifier while the ST70 uses a pentode. In addition, the smaller output transformers of the ST35 have wider bandwidth.

The input of the ST35 uses a 7247 tube, which is a 12AX7 section and a 12AU7 section in one envelope and is in current production. The 12AX7 section is the Voltage amplifier and the 12AU7 the driver. Since the 12AX7 is far more linear than a pentode (and has the most gain of any miniature triode) when feedback is applied, the tube distorts the feedback less (than a pentode) so the feedback is more able to do its job properly. This results in less higher ordered harmonic generation, so this is a much more interesting and relaxed amplifier to listen to and will fulfill your lower powered amplifier interest.

If one of these is properly built/refurbished it will give any SET a run for the money! It will play bass better, be more revealing (less distortion) and more neutral (less distortion).

And there’s a kit! Or there was. Their website says out of stock.

Go with the metal film option. Carbon resistors are really terrible.

Another source for pre-built all things Dynaco is: GetDynaco GETDYNACO.  My understanding is that Kenny Russell is the builder.  I have no connection to his products, but learned about them from the You Tube channel drD E Audio who uses a Kenny Russell ST-70 and Pas-3 preamp.  The combination sounds stunning with his Klipsch speakers.

Wow! Thank you @jetter and @atmasphere for the additional comments. The thread is really becoming a resource for others, as well as for me.

I built the Analog Ethos 2A3 Legendarium kit.

Perry is very helpful and the instructions are easy to follow.

Along with the instructions is a section explaining how it works.

Good parts, I didn’t feel the need to upgrade anything but the coupling caps.

You can download the instructions on his site.

I have to say though, a James Burgess 2A3 amp sounded a bit better for just a bit more money.

Have built several Audio Note Kits and thought they were very good for the money. Customers were very happy.

I have plenty of dynamics, no roll off at frequency extremes with my Tron 211 amplifier used with my Mhorn speakers rated at 93db. I easily get sound in the 90db+ in my 15x18 listening area.  It’s all about speaker matching and how well the amplifier is built in reference to power supplies.  The amplifier needs to have power reserves when needed. I have use 45, 2a3, 300b and 211 tubes in amplifiers I have owned over the years.  I agree with most on the 45 and 2a3 power output that you need 98db efficiency plus speakers(I prefer 100db +).  I used both of these tubes in Yamamoto and Triode Lab amplifiers with Klipsch Forted IV and that was in a larger room and was satisfied with the sound.  The 45 tube is the best sounding to me but your only going to get so much volume out of it and that’s a tube that I think needs 102db or better to get the most out of it unless your in a smaller room or near field setting.  PM me if you want to chat more about it.

Good luck

Don

@cfarrow Thanks for relating all that. Really interesting.

FWIW, last night I did some listening comparisons comparing the Sun Valley 845 vs. QS Mono 60 w/ KT77 tubes. I used my solid state amp (very neutral) and my Holo Spring DAC. All of my descriptions relate to how things sound in *my* room, of course.

Main impressions: Sun Valley 845's tonality was a bit softer, more detailed, further back in the soundstage, and slightly narrower soundstage (but very good); the bass was weaker, sometimes too weak. It was a bit less snappy in transients but still quite good—not sloppy at all. Instruments and voices had a bit more texture. It was very easy to listen to; never strident or approaching strident.

QS had better transients and dynamics by a bit; it had much better bass, and instruments were more forward in the soundstage. Voices and instruments were detailed but not as much, a bit less warm and intimate.

Overall: hard to say which is better. They were different, though both obviously tube amps. I suspect that a tube amp like the Sun Valley with a bit more grip on the speakers which keeps the intimate/texture/smoothness and improves the bass might beat them both. 

Not sure if any of this translates for folks, but sharing just in case.

 Sun Valley 845 doesn't use an inter-stage transformer between driver and 845 tube.

845 is impossible to drive without IT! You get more driver tube distortions than 845 output stage distortions. You also can't get a good deep and fast bass.

The same I can say about 300B. But 845 is much more difficult to drive than 300B!

For PX25, 2a3, 45 output tube or if kt88/kt66/el34/6L6/6v6 used as SE output in triode mode you can receive a good sound without an inter-stage transformer.

I use a 2016 Dennis Had Firebottle HO (high output, whatever THAT means...max 15 watts pc or something? Likely less with some tubes but, meh...) that sounds astonishingly good with my 99db horns (mid horn damped titanium mid and tweeter drivers Heresy IIIs), and swap that amp with a Pass XA-25 when the mood strikes. 2 REL subs resulting in absolutely no lack of bass, tons of headroom with either amp, and joy in my brain. These amps sound somewhat different but both are so good and similarly mirth inducing that I simply don't need anything else. 

To @wolf_garcia, which output tubes did you land on for your DH Firebottle HO amp?

I'm currently using Gold Lion KT66s, a NOS Amperex 6SN7GTB (looks like a re-branded GE), and a "Tubestore Preferred Series" 274B rectifier. Note I have a drawer full of tubes including more GLs (88s, 77s, 90s, 120s, etc.) and other stuff. The 66s are pretty great, but I'll likely get bored and swap em out at some point...for now they stay.

@alexberger Here's what my tech friend and amp builder said about your comment, in case you're interested:

"It doesn't [have an interstage transformer], but then again most single-ended amps don't have an interstage transformer, nor most PP amps for that matter, they are expensive. The 845 is hard to drive and since the SV uses a 12BH7 to drive it, there is less power than if driven with a larger tube or an interstage transformer. Clearly it is not "impossible" as the amplifier operates."

I don't want to interpret for someone else, particularly someone with vastly more hands on equipment building and technical knowledge than Alex Berger, but, I don't think "impossible" meant it cannot be done--its more like it cannot be done optimally.  Clearly Sun Valley is doing it; the question is whether this is a good approach.  I think that there are lots of suboptimal designs that are done to lower cost, so it is helpful that Alex Berger is pointing this out.  I too have my doubts about the plethora of low cost 300B SET amps with 12AX7 drivers and no interstage transformer--this might be less than ideal, so the question is whether this is the right cost-quality tradeoff.  I am not saying it is the wrong tradeoff, but, it helps for buyers to be aware of the issue.

845 is impossible to drive without IT! You get more driver tube distortions than 845 output stage distortions. You also can't get a good deep and fast bass.

@alexberger This statement is entirely false (and is what IT manufacturers would like you to believe).

If the driver is a cathode follower and direct-coupled to the grid of the power tube you'll have no problem with drive and less distortion than you can get with an interstage transformer. SET designers don't do it because you have to have a B minus supply for the driver tube and they don't want to spend the money although a good interstage driver transformer will cost more. The other reason SETs designers don't do it is they didn't think of it!

You'll recall I mentioned this technique on another thread on which you were active on this website.

For the record we've been driving power triodes in exactly this manner for decades (47 years to be more exact). The driver has an iron grip on the grid of the power tube and can easily drive it into grid current with good linearity. It also prevents blocking distortion which is a problem for any tube amp which employs coupling capacitors between the driver and grid of the power tube so you get instantaneous overload recovery.

You might try it since you're into DIY. If you have any questions about how to go about it, email me.

Hi @hilde45 

The problem of 300B they need a driver with 150 volt peak to peak voltage swing. For 845 you need much more voltage swing.  

It is possible to drive 845 with RC coupling, but you get much more distortions vs IT drivers with the same tubes. With IT - the output tube can be driven when it is in class A2 and you get more power and better clipping recovery.

I have a DIY 300B amplifier and I used 6f6 in triode with RC coupling, then I changed to IT coupling. It was a huge difference! It sounded like a different amplifier. And I used super expensive V-Cap CuTF capacitors in RC coupling.

Then I changed the driver tube from 6f6 to 6v6 and to 5881 (6L6) and each time a more powerful driver sounded better, bigger soundstage with better instrument separation. 

A bit late to the party here, but it's a topic near and dear.  I've had Altec 604-8G in one of my systems for many years.  One day a friend and I did some trading, and I wound up with an Alan Eaton SE45 stereo amp.  It was quite pleasant and I enjoyed listening.  He had also brought over a 300B amp made by BEZ.  It was not impressive, very veiled, mushy, etc.  I kept the Eaton amp until I decided to make my own SE45 amp from vintage parts.  Output trannies are Triad HSM-79.  Borrowed and merged bits of circuit design from various sources, and the result was an amp that outperformed the Eaton amp, both on the bench (output at clipping and sq waveform) as well as sonically.  If you're wondering, I listen first, then measure.  I don't recommend scratch building unless you enjoy the journey, but the point of this comment is buy a kit with the best output transformers you can afford, as they are the heart of any single ended amplifier.  As for wattage concerns, 1.6 wpc does seem a bit lean, but if you're speakers are 95 db at 1 watt, they'll be 85 db at 1/10 watt.  Distance from speakers matters, room sound absorption matters, but unless you're looking for slam at disco levels, SE is a fun part of the audiophile journey.  Life is short, build the kit, savor the results.  I also have one of Dennis Had's KT88 Inspire amps, and it brings a little more punch, with up to 6x more wattage on tap.  All good fun.

The mention of a directly coupled cathode follower by atmasphere is interesting.  Today, I just happen to be looking at the schematic for an old Wyetech SET amp and noticed it had a directly coupled cathode follower to the grid of the output tube.  

Looking for stories about your low-watt amp journeys.

Using 100db Volti Rival for investigation tube sonics, chosen for its neutrality and affordability - I want to hear tube differences starting with flea at SET amplification.

Started with 300b because it’s the most notable SET. Purchased a used Coincedent Frankenstein 300b v2 because of its high performance/price sold directly.

Debating purchasing a TEKTRON TK TWO 2A3/50S-I VACUUM TUBE - 2a3, 45, 50, 300b - to investigate other flea watt SET tube types

Also debating DECWARE Model SE84UFO25 - long wait but great sonics and can tube roll

@alexberger 

The problem of 300B they need a driver with 150 volt peak to peak voltage swing. For 845 you need much more voltage swing.  

If you direct-couple with a CF that's no problem at all! Here's something to consider:

The grid of triode power tubes is the most capacitive of any kind of power tube. Triode power tubes are also very low mu (gain). For example the 300b is a bit over 2. The 6AS7Gs we drive in our OTLs is only 2.

So driving such a load from the plate of a driver tube is a task- hence the common use of an interstage transformer. But that part is expensive if its done right and it makes distortion and limits bandwidth.

When you use a cathode follower its input impedance is really high so its a lot easier to drive than a power tube and easier to drive than using an interstage transformer. So its a lot easier for the Voltage amplifier to drive it without distortion and the driver itself is inherently low distortion as well.

Since you are into DIY you might consider trying this. Its not that hard and like I said, the tube section to do it and the minus supply is usually less expensive than a good interstage transformer. As a general rule of thumb whatever the B+ Voltage is that you use now for the driver, if using a direct-coupled cathode follower you'll need a B- of equal absolute value. That will allow it to have the Voltage swing you need. A single 6SN7 section can easily swing 150V or more no worries. We've been doing it for decades. Usually you can obtain the negative supply if you use a bridge rectifier. That requires a power transformer with a center tapped secondary. 

I’m not a tube guy, but I am enjoying this thread a lot. Thanks @hilde45 for starting it, and thanks @atmasphere for your well thought out comments which, despite being nominally tube-oriented, hold a universal appeal especially regarding what we listen to, how we listen to it and what we seek to get from it.

My entire history with tube gear consists of a vague yearning for big-iron VTL monoblocks... someday... so I don’t have much to contribute to the subject other than, if you’re still considering the DIY route, this:

Unlike another poster earlier, I find PCB work easy. Insert component in its assigned spot, solder, done. Point to point on the other hand gives you a lot more latitude in how you structure your circuits in 3D space. Personally I find point to point more rewarding, but harder to keep my OCD impulses in check 😃

For what it’s worth, I would recommend going the DIY route, if only because the satisfaction of building something with your own hands is without peer.

PCB is easy to solder, except when it comes to repairs.  Usually that involves an extra step of having to unmount the board to get to the correct side, and it sometimes means having to bridge a trace or make other such repairs where you pulled a trace or pad while unsoldering and trying to replace a component.  

For looks, as well as the benefits of ease of repair, I like point-to-point using turret boards.

Speaking of good point-to-point wiring, I saw an amp built by a Russian builder that was so carefully laid out that there is hardly any wire in the amp, almost all connections being lead to lead (and short leads at that).  All of these connections were extremely tightly wound together before being soldered.  The dealer showed me an amp that had been running for years that this builder made while a bit drunk.  About a third of the connections weren't even soldered but they functioned perfectly.  Another repair professional charged extra to work on this Russian's amps because of the difficulty in removing components.  I owned one of these amps and I planned to replace the diodes in the bridge rectifier, but when I looked at what that entailed, I gave up.  The entire bridge rectifier was about the size of a thumbnail and was stuck in a tight space under other components that would have to be removed to get at it.  I could not even figure out how he got everything into such a tight ball.

@larryi Decades ago I put myself through engineering school by working as a repair technician at Allied Radio Shack. I saw equipment that clearly was not built with the ability to easily repair it in mind. Electronic components fail no matter how well you vet them or how well you rate them in the circuit. IMO/IME easy repair should always be high on the list when laying out a circuit. 

If you do it right and use good quality boards the repair time is about the same (if you do a neat job) whether circuit board or hand-wired.

Hi @atmasphere 

Thank you for your advice. I can try it if I build a new amplifier.

So. Do you think the issue is not voltage swing of driver but the current and power it should drive on capacitive load. And the power is function of the voltage swing in squire.

Do you think the issue is not voltage swing of driver but the current and power it should drive on capacitive load.

@alexberger Generally speaking, yes. Specifically you want to be able to swing the Voltage without distortion. To do that the source that does the swinging must be able to provide the current needed, if any. Power triodes tend to have a bit of grid current anyway even if you aren't driving them positive with respect to the cathode; some of this is due to grid capacitance.

The bottom line is a gutsy driver is needed when driving power triodes of any kind. That is why RC coupling is not the best solution- you use either an interstage transformer or direct-couple; the latter being lower distortion since its a much easier load to drive for the Voltage amplifier (cathode followers tend to have a very high input impedance).

Transformers make distortion of their own even if properly driven and loaded and they limit bandwidth. So if you want the best transparency you don't put them in the circuit. 

The real question in my mind is how many different low powered or SET amps does the average audiophile go through before they find the right amp that matches up well with their speakers to their satisfaction, or before giving up?

3-4 seems typical

4-8 not uncommon

10+ for some

..and then others changing speakers along the way 3-4 times too trying to get "there". The designer of my current amps gave up on SET amps after a few decades and told me he prefers a bit higher power ultralinear amp designs with more versatility anyhow. My journey down the SET path was short lived, but enjoyable along the way fwiw.

@decooney

Great perspective. 

The stock Dynaco 70 measures right up to 35wpc no clipping for 1% THD (good amount of NFB) and almost everyone loves the sweet sound. You do need output tubes that are in really good shape to get the 1% THD (they all need to be tested regardless of brand).

Mods allow lowering of the NFB which gives a bit more dynamic response at the cost of a bit more harshness in the treble with some recordings.

I had Spendor 2/3 speakers for around 5 year. A number of years I used the Plinius 8200 integrated to drive these speakers. But I felt something was wrong and I started looking for a good matched amplifier. I stopped on the McIntosh MC30 vintage monoblocks. It was a great match between amplifier and speakers. MC30 was better than Plinius in everything: more natural tone, better separation, better bass control, more musical. Then I bought an Altec 604E (102dB sensitivity by spec and around 98dB in real). Spendor 2/3 are 88dB sensitivity.

I used Altec with McIntosh first and then tried a 300B SET amplifier. The 300B DIY SET had a big advantage over the push-pull MacIntosh with highly sensitive Altec. Sound was so much more transparent, natural, musical and detailed. McIntosh had more powerful bass but SET bass sounded more natural. My current DIY SET is in a different league compared to my first DIY 300B SET.

One friend of mine moved from his modified Quad II to 300B SET. He builds DIY speakers. But he mostly used the big 15" JBL 4 way clone DIY speakers. These speakers are more difficult to drive than my Altec 604. And he also clearly prefer 300B SET over Quad II.

So, a good SET with matching high sensitive speakers will be a huge step over any vintage push-pull amplifier including Dynaco.

Nice discussion! Kits have added a lot to my hi-fi enjoyment and understanding

A major issue with tubes is the output transformer, especially in SE use.  In a single ended circuit the quiescent state plate current is going through the OPT, in a PP configuration the plate current is flowing in the opposite direction through each half.  In a PP setup the magnetic field goes through the zero point on each cycle, for SE the magnetic field varies but is "biased" above the zero point.    I am not sure if this is in SE's favor or PP's!  But it surely means that the SE OPT must be more substantial and designed to avoid saturation.  This is an interesting article  https://www.x3mhc.no/dokumenter/SE-v-PP-Part2.pdf written by an SE fan

I do have an Elekit 2A3/300B amp (basic) that I now use to drive the Harbeth P3ESR speakers in my BlueOS "Kitchen" system.  I had given my VTL85 to my daughter and had been using an NAD amp - OK for cooking by - but the Elekit, with 2A3 tubes makes for rather good listening.

Building that kit made me contemplate designing and building a fully balanced 65N7/300B amp.  Over a couple of months I did just that.  I'm using switch mode power supplies in a separate chassis (in rural VT power voltages are rather unsteady and the thoughts of designing and building regulated supplies put me off).  Also separating the PS from the Amp keeps any AC from the amp chassis and allows me to move it all - at 80 years old I have limitations.  When the amp is on and with no input signal the speakers are totally silent - ear 1" from driver silent.

Looking at load lines, and other designs, gave me values for the 6SN7 anode load resistor and their cathode bias resistors, similarly for the 300B cathode resistors.  Because the LA4 pre has balanced outputs there is no phase splitter, just a single 6SN7 amp/driver stage, with the cathodes of each "half channel" strapped.

The MeanWell supplies have very low ripple (the WORST is 7 millivolts RMS on the supplies for the 6Sn7s).  These supplies are adjustable +/- 10% so I was able to set the heater/cathode voltages to precisely 6.3 and 5 volts.  Similarly I could play with the B+ to get to a good place in the load line simulator for the 300Bs.

It is the simplest possible circuit, zero feedback.  No SS components in the amp.  This also means that there is no protective circuitry so I do not leave the amp on when I am not nearby!

I used rather good components throughout (Vishay metal film or wire-wound resistors and Vishay caps except for Jensen coupling caps - the only cap in the circuit path).  OPT's are Lundahl.  Tubes are WE 300B (these had been in my VAC 30/30) and TungSol 6SN7GTB.  Wiring is Kimber throughout - an old PBJ provided the connection from the Neutrik XLRs to the 6SN7 grids.  Heaters/direct cathodes Kimber twisted pairs, not really necessary as it's DC.

The chassis were made by Protocase - a fun design activity in its own right.  The physical component layout was done in Adobe Illustrator (from my Graphic Arts/paper engineering work for my wife, she designs holiday cards for MoMA.)

This amp is wonderful to listen to at moderate levels, note: I listen exclusively to "classical" music (from Josquin to Shostakovitch - and perhaps Gorecki).  In particular, solo piano recordings are REAL, from the impact of the hammer to the varying overtones of the decay it is as though I was there in the recital hall, every nuance of the performance is presented.  Similarly human voice, especially a capella, is magical, the Rachmaninov All Night Vigil is glorious and ethereal - I am in the chapel. The bass is solid, tuneful, controlled (though it does not hit you in the gut), the treble pure and the mid-range delicious.

If you do build your own, kit or otherwise, do invest in a good soldering kit (Weller e.g.), use 63/37 solder and have a "solder sucker" and solder wick available.  Jameco is a great source for that sort of kit.

@retiredaudioguy 

Great post, thank you! 

Another necessity for this type of work is flux, no-clean variety.

So, a good SET with matching high sensitive speakers will be a huge step over any vintage push-pull amplifier including Dynaco.

@alexberger I suspect your sample size is a bit small to be making that conclusion. For example I think the ST35 is more musical than the ST70. I also think a properly refurbished Citation 2 is one of the better amps from the classic vintage era. I’ve always felt the Mac stuff from back then (my exposure limited to refurbished amps, M30, M240 and M275) to be less articulate but that’s me and entirely anecdotal.

Fisher made some very interesting monoblocks that featured a damping control. A pair of those allow you to compare on older speakers like the Altecs by dialing the damping to be a bit lower, since a Mac or even an ST70 will have an output impedance that is too low to sound right on the classic Altecs. EV had a similar feature on some of their amps too.

@retiredaudioguy You might want to keep an eye on any coupling caps you have that are oil filled. I’ve found over time (we’ve tested a variety of them) that they can often develop a slight amount of electrical leakage across them (IOW not entirely blocking the DC Voltage they are supposed to block, sort of like having a resistance in parallel with the cap) which can throw off the operating point of a power tube! The exception to this is the ODAM parts from VHAudio.

I use a Triode TRV P:845SE / a single 845 per channel operated SET and good for about 22wpc.  This has been one of the amps in steady rotation in my system, for the last 6-7yrs.  Bias is easily adjusted and has been rock steady. It sounds as good today as it did when I purchased it.  I say all of this because SET amps and old tube amps in general have reputations for requiring periodic maintenance.   

Many recommendations for the venerable ST70 / AKA the Model T of tube amps !  They are ubiquitous and easily found in various degrees of operation and condition.  It should be required education for those new to tubes to listen to a fully restored ST70 (stock specification- no modifications) before moving on to more expensive amplifiers.   It sounds smooth and its sins are of omission.

As noted the ST35 is a superior amplifier; more liquid, incisive, detailed but with plenty of tube warmth.   The transformers are outstanding and among the best produced back in the day.  They other key to its sound is the use of 6bq5 output tubes.  This tube has a well earned reputation for sounding wonderful in many applications.  Some say it is the ideal tube for original QUAD ESL speakers, and it easily drives horn speakers into sonic bliss.  The ST35 is a great place to start. Others to consider are Eico HF14 / 86, Heath UA-1 and AAA-111, Fisher 20a and 30a monoblocks.  Simple circuits and easy to restore, but great output transformers !

Regarding SET amps, if the ST70 is the most common amplifier, a close 2nd would be a 300b SET amp from any 1 of several manufacturers.  The basic circuit is very simple thus price differences come down to transformer quality and design of the power supply.   If you want to investigate 300b amps, there are kits, but I would look for a Triode TRV, Line Magnetic or Cary 300b amp.  All sound great, are easy to use with many speakers, and are built to last a long time.

Good Luck.  

Just to let the thread know, the advice here from @atmasphere and others caused me to go find a newly built Dynaco ST-35. It should ship soon. I bought on Ebay. Here's the description:

"This beautiful, newly built amp is now for sale. I have used this amplifier for the past three months in my shop. It sounds so rich with the new matched set of Sovtek 6BQ5 power tubes. The pre-amp tubes are Sylvania nos 12DW7. The only items that are not new is the transformer set. These were happily donated from another Dynaco amplifier. Almost everything else came fr Kevin at Dynakitparts.com . All of the other parts, from the power cord to the protective cage are new!"

My local audio tech friend is interested, too. (He has the PBN speakers M2!5). When the amp comes in, I'll bring it down to him and we'll give it a good look over (and inside) and put it on his Audio Precision and other test equipment. Then we'll try it with his speakers.

So excited. Audiogon people help again!

Sorry to be a contrarian, but I did not find the VTA ST70 to be my cup of tea, this even with my own upgrades with resistors,caps internal wire, highest quality tubes. Keep in mind  this at the end of my journey with class a/b tubes. .

Part of the reason I gave up on a/b tubes was inability to get the kind of articulate bass I require. EL34 my fav tube for this class, EL34 mids I love, bass and lack of extension in highs are the downside. This with CJ, Cary, Dynaco, clone, modded Prima Luna. KT family not for me, doesn't have EL34 magic in mids.

So, this in comparison to 845, 2A3, 300B amps  I previously owned or have at present. 845 can drive loudspeakers 300B and lower power SET have no hope of driving, I know I've tried\. 845 successfully drove 90db with benign impedance curve, with 2A3 same speakers sounded like transistor radio. 300B superior to 845 with my extensively modified Klipschorns, while Klipschorns very efficient, 104db they do have an impedance dip to around 2.5ohms at 100hz, this with shallow slope crossover. 845 amps have more self generated noise vs the lower power SET, nearly 1000V plate voltages vs aprox. 400V with the lower power SET. Point is speakers/amp matching important for these SET.

If you have excessively resonant or one note bass with SET you have issues with speaker match and/or output transfomer/ power supply issues with amp. As others have mentioned output transformer quality is critical, power supply is important in that you need something that can supply current with possible lower impedances found in some speakers. I have absolutely no bass issues with my custom build 300B monoblocks or 845 amp with the Klipschorns. And this  with much experience and comparisons  with a/b SS, a/b tubes, class A SS with various speakers.

As to future with SET, at some point I'm thinking Audio Note interstage 300B monoblock kit, Aric Audio 300B or possible 2A3z, Whammerdyne 2A3. Emotive 300B, Triode Lab 2a3 or 300B amps. Again, looking for quality output transformers and power supply.

@sns To be clear, the Klipschorn rolls off sharply below 45 Hz.

The older Klipschorns are not designed to be driven by an amplifier that has a low output impedance like an ST70. You'll know if your speakers are like this (the Klipschorn has been in production since the 1950s which is a bit of a record) if there are level controls on the back for the midrange horn and tweeter.

SETs are a technology which I call the Power Paradigm. PP amps like the ST70 are Voltage Paradigm technology since the ST70 employs enough feedback to allow it to behave as a Voltage source. IOW this has to do with the output impedance of the amplifier; whether or not its low enough for the amplifier to behave as a Voltage source. If your Klipschorns have level controls, essentially they are Power Paradigm technology also and there will be tonal aberrations if you try to use them with an amp that is Voltage Paradigm.

For more on this topic see http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html

@hilde45 

congratulations. The first thing I would do is upgrade those footers. 

@hilde45 Nice! I'll be interested to hear your comments. I saw that amp on ebay as well and really had to think about buying it just so I could see how far I could push it.

A couple of friends I helped with on their Altec-Onken 102 db horn speaker creations use modified ST35s on rotation when taking their custom modified AudioNote 300B amps out of the loop. They seem to like them and use them quite a bit to save their WE300Bs. I demoed one once, it was different sound from other amps I've tried in my system.  Fairly plush sounding too.  

@decooney , @hilde45 , One weakness of that amp is seen in the schematic.

The 4 power tubes share a single resistor (bypassed by a capacitor) to set up the bias of the output tubes. There really should be a resistor and cap for each output tube pair. This was a space and cost saving move on Dynaco's part.

That's a pretty well known issue in this amp and the SCA35 which uses the same transformers. By separating the channels in this manner the amp performs and sounds better. 

One thing missing from this long and delightful discussion is a mention of another kit building option that offers tube like sound with solid state gear. I’m talking about the DIY audio store where many designs by Nelson Pass and others are available along with an associated forum of DIY minded people. I began my kit building journey with Nelson Passes Amp Camp Amp at this store and have gone on to build a Firstwatt F6. and a Waynes 2018 preamp. I can’t say enough good things about the people on the forums, the build guides, and the end products which I find compare very nicely with my tube amps for sound and listening pleasure. All kit builders or would-be kit builders should be aware of this resource.

Never ran the Dynaco clone with the Klipschorns, was running Merlin VSM-MM at the time. Didn't have any particular issue with the amp, just blah, lack of transparency, resolution. This compared to two 845 SET amps I was running in that time frame.

Didn't have any particular issue with the amp, just blah, lack of transparency, resolution. This compared to two 845 SET amps I was running in that time frame.

@sns  The Merlins are not a good match for any SET unless you are using them in a small room. They are 88 or 89dB 1 Watt/1 meter and 6 Ohms (so the 4 Ohm tap should have been used). You need a good 50-60 Watts for them in my experience.

The VTA ST70 is something different from the ST70; it didn't measure as well as the original FWIW. 

@atmasphere Found a video related to your comment. Just adding in in case it’s of interest.

TITLE: Dynaco ST-70 Original vs. VTA Bench & Listening Test Comparisons - Stereo 70 Tube Amp

VTA vs. Original

Hi @atmasphere ,

In parallel to the McIntosh MC30 I had Marantz 8b.

I used these amplifiers with Spendor 2/3 and Altec 604E with original n1500 crossovers.

This Marantz was burned by one of the previous owners.

So output transformers were fixed, and power transformer and choke was replaced by a local transformer store built. On another hand, I and one of my technician friends replaced resistors to RikenOhm, capacitors to Black Gate and signal capacitors to Audio Note tin and copper foil. The technician adjusted the feedback of the Marantz. So this Marantz 8B had pluses and minus versus the original Marantz 8B. I also used the original Mullard EL34 on this Marantz.

Yes this Marantz 8B sounded more clear then McIntosh MC30. But despite Marantz bass wasn’t bad, MC30 bass was on the other league. It was also a big difference in complex music reproduction. 8B was good on very simple music but the sound became muddy and congested with bigger number of instruments and voices. On the other hand, the complex music wasn’t an issue for MC30. The Plinius 8200 Mk1 integrated that I had then also had a problem with playing complex music, but it did it a little bit better than Marantz.