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

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