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RE: Got it. Yes, that is what Dennis says.

"A gentle roll-off IS NOT a sharp cut-off
filter! There is still plenty of energy
above 20 KHZ to listen to, but it IS slightly rolled-off,
as in SE amps in general."

Of course it's not a "sharp cut-off filter. It's a first order filter with a 6db per octave slope beyond the knee.

When you talked about the roll off starting, in your amplifiers, at 9kHz with one of two vintage 2a3s and at 11kHz with the other vintage 2a3 (I think those were RCA and Sylvania and I don't remember which one was which) and 15kHz with the current production tubes you were using at the time, those numbers line up perfectly with the math.

The listed inter-electrode capacitance will tell you (with a little math) the in circuit Miller capacitance and from there it's even simpler math to calculate (quite accurately) where the roll off will start.

What you reported was accurate.

The source impedance of your driver stage is too high to keep the roll off up out of the audio band. To keep the roll up high enough as to not affect 20kHz the knee of the low pass filter (-3db point) needs to be at 200kHz. Now I know a single ended series output transformer will not reach that but that is no reason to "cheat" with a low pass filter within the circuit. Even when the output transformer is limiting the high frequencies, when you make the circuit right you can still hear the benefit.

"Let the audio LISTENING experts tell you what is
actually happening."

Dennis, I am a audio listening expert. I worked as a professional recording, mixing and mastering engineer for 20 years (part of that time as a live sound engineer for a national act who is a member of the Rock and Roll Hall of Fame) and had many professionals consider me to be very good at it. People who have been successful in the business for years were convinced that I had "golden ears". I don't let that go to my head but why do you think that your the only person that can hear? I have years of experience and know what real instrument sound like. Much more so than the average listener.

I know what is actually happening.

Besides being able to hear the roll off that a circuit like yours will cause, that roll off can be measured and, more importantly, it can be fixed. And you don't have to resort to push pull to accomplish it.

I didn't even get into the fact that your driver stage can not deliver the needed current that the Miller capacitance requires if distortion is to be avoided. Maybe latter.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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