The Amp Garage

Colossal wrote: ↑Fri Oct 16, 2026 9:05 pm My load line is 8k, plates at 343V, screens at 320V and cathode at 21.5V (79.6mA cathode current). No NFB. The amp was scoped with a 1kHz signal.

Are plate and screen voltage referenced to the cathode?

Colossal wrote: ↑Fri Oct 16, 2026 7:53 pm My power tubes are biased to 24.5W at idle. Close enough for government work.

So clean output must be less than 24.5W without exceeding class A.
How did you verify that the amp doesn't leave class A at large outpu?
See my edited post above.

That I believe, as I have my own Class A 2xEL34 push-pull amp and it is doing exactly that. So that's why my interest was piqued when I saw the 2xKT88 Vimana which said 54W. Clean class A PP output is always less than the plate dissipation per tube at idle. This follows from the general class A rul...

The frequency of the audible oscillation seems to be changing. And I am also a beginner with oscilloscope. There is so much happening in the screen and I don't know how to isolate things. To read the oscillation frequency you need a steady trace. Can you dial in the oscillation without any input si...

The theoretical maximum efficiency of a class A amp is 50% at full clean output. So if PDmax is 40W, you could get 20W in SE and 40W in PP. But that would require ideal power devices (tubes) that swing to 0V at the peak current. As no such tube exists, class A efficiency is always lower than 50%. Ad...

That 15k is the tail resistor of the LTPI. The voltage drop across the 15k determines the PI plate currents. Should be 15V to 18V (unfortunately the schematic voltages are not consistent). Power tube bias could also be adjusted (within limits) by changing the 220k/2M2 voltage divider before the grid...

The oscillation was audible through the speaker when both the Gain and Master Volume were set close to maximum. Engaging the Boost could trigger it, and lowering either control stopped it. I could also see oscillation on the oscilloscope, but I did not measure its exact frequency, so I cannot say w...

How did the oscillation manifest?

I find it so strange that the thump is quieter with the P.I. removed. That's a direct consequence of that strange PI input signal, which should not be there. As this signal gets amplified by the PI, producing opposite phase PI plate signals, it gets processed like a normal (guitar) signal without c...

The neon flash test works fine on the primary winding of a transformer, which is just an inductor when the secondary is open. I don't see a reason why it shouldn't work with a typical amp choke. The test is based on the flyback voltage across an inductor generated when the current is interrupted. Th...

Astronomicum wrote: ↑Tue Sep 29, 2026 11:10 pm Knowing that the turns ratio provided is just half the OT helps. Thanks.

Just guessing myself.

Does the signal at the PI input disappear when you disconnect the PI input coupling cap (1nF)?

An OT having an Raa of 4.3k means a tube load of Ra = Raa/4 = 1075 Ohm across each half-primary in class B.

Doubling load impedance lowers plate current but significantly increases screen dissipation.
So likely lowers tube life compared to optimum load.

For reference:
https://www.dropbox.com/scl/fi/vpijj3al ... 4&e=1&dl=0
(by courtesy of R.G. Keen)