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hagtech wrote:

I run the driver tube at 170V @20mA.  I don't think you need to run this tube at high current.  It is very linear with wide signal swings at this bias.

Good operation points for use it as a driver. You might what to jump voltage to 200-210V and drop current a bit as it will give you higher bias and further up the input stage from a danger of A2.

*** I can get roughly 70Vrms out of it.  Output stage clips first, driver continues with perfect waveforms well into clipping of output. 

Is it the output driver that clips of the output transformer? Looking at the very soft clipping it looks like it was your DTH. Dima taught me that in a properly designed amplifier a transformer should clip before the active stages.

hagtech wrote:

Oh, using transistor constant current source on driver plate.  Output is push/pull parallel SE.  I can set the ratio between single-ended and differential modes via a resistor value.  Suprisingly, best performance was with almost all SE parallel.  I figured more of the differential mode would be needed, as least per WE 93A schematics (their idea). I run the driver through cap to SE/PP interstage tranny.  The tranny is required for my new topology, which has CCS on both output tube cathodes, hence allowing good operation even without matched tubes.  No bias adjustments are necessary, both 2A3 always run at 50mA.  If both working, then voltage across 2A3 is about 235V.  I don't get much power, but this is more of a sonic sweet spot.  Better to give up a watt or two.  Output stage continues to run even if one of the 2A3 is pulled.

Jim, I certainly have no qualifications to be critical in what you do from a technical perspective but from point of a common sense, that I definitely possess, I am very skeptical about what you are trying to do. I am critical not about YOU do but about the entire objectives of the type amps that you are trying to build. Let presume that sound of amp is coming from the gain/driver stage (you might degree but it is my debatable position). If so, then the output stage is necessary primary to develop current. People do not use high efficiency load but what to use low power output stages and in order to rectify it they parallel the output stages, defeating the entire idea of SET purity. With a prober driver and properly used output stage it is very possible that output stage does not matter. What matters is a correspondence of the output stage to the load demands. If so, then why instead of going for push/pull, phase splitter and other unnecessary inventions juts do not take more capable current driver, or in other word to get more powerful tube? The 2A3 might be a nice tube but what are you planning to drive with 3W? Some kind MF channel with sensitivity over 106dB?

hagtech wrote:

All heaters are AC (I think DC is bad for a DHT).  Output noise is 0.16mV A-weighted.  Measurements are with the cheap Sovteks.

… and I wish that everyone do the same. The DC on heaters must not be used for power amps. I was pissed when I saw Lamm ML3 used DC on heaters… Talking about the smelling roses in gas masks…

hagtech wrote:

The main drawback of this design is a low damping factor.  Output impedance from the 9 ohm tap is 3.5 ohms.  That's pretty dang high.  It is because the rp of 2A3 at this bias is about 1k ohms.  It is not the most powerful output tube.  More elegant than robust.

The damping factor? What  do you what: you use a low power tube with huge plate impedance. This thing, from my point of view, should not drive anything under 1000Hz, unless you listen it in a closet with over 105dB sensitivity.

hagtech wrote:

At the moment, my listening setup is in total disarray.  I sold everything I had and am starting over…

Too bad, if you have a stable playback then I'll propose you to use a simplified Milq biasing model and then to pay attention to what happened to sound.

hagtech wrote:

Oh, bandwidth is 6Hz to 25.5kHz.  Not very high, and this is purely a limitation of my interstage tranny.

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