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In Reply to: RE: Dynaco mod posted by Tube747 on May 08, 2017 at 10:48:18
It could make sense.
Mid-range tube amp usually have decent output transformers with relatively low resonant frequency and high leakage inductance (leading to considerable phase shift as signal frequency increases).
Thus, if you measure THD/IMD across entire audio spectrum, you may notice that with 20 dB GNFB & high output power THD rise from 0.5%/1 KHz to as much as 5%/20 KHz (IMD raises proportionally).
In order to reduce side effect, decreasing GNFB may help.
Capacitor you mention one should select manually with minimum overshoot at square wave around 2 KHz. Use variable capacitor from old radio in series with 0.01 - 0.05uF film cap and oscilloscope to do so.
Follow Ups:
The article also mentioned the change of cathode resistor of el34 from 15.6 ohm to 300 ohm for pure class A operation rather than class AB plus 50uf 50v cathode capacitor?
The output power would drop from 35W to 28w.
I use fixed bias on output stage, so can't comment on this change, prefer to stay with it without any cathode resistors and capacitors. IMHO pure class A for push-pull doesn't make sense.
PS. My previous post backed up by real experiments and measurements with spectrum analyzer.
"pure class A for push-pull doesn't make sense."
Class A push-pull offers the significant advantage of reducing signal current through the power supply filter caps to near zero. That takes those components out of the sonic equation.
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