Clipped positive cycle wave form

[Axtman]

What would cause this sine wave to clip on the positive cycle? This is that 70's Silverfaced Princeton tube amp that I am still fooling with.

[Helmholtz]

Several possibilities.
- What are screen voltages?
- What are idle currents?
- How do the grid signals looks?
What is the scope input sensitivity ( V/div setting, what probe)?

[Justin Thomas]

Where in the signal path is the scope at?

[The Dude]

I'd first scope through the amp and see where clipping starts. It may be clipping before the output stage.

[Axtman]

The scope is connected to the speaker output of the amp.

I think the clipping starts at the phase inverter. At this point, I have checked all resistors and capacitors. They are all within spec.

Here is the schematic: PRINCETON_REVERB_bf_AB1270.pdf

[Chuck H]

I want resolute clarity about this... The above posted waveform is taken from the speaker output of the amp, as stated, correct?

[Axtman]

That is correct. When I crank the amp that is the output I get on the scope.

To me, it looks like a waveform one would get from a solid-state amp when the positive bumps up to the rail voltage. Tube amps usually don't distort with a hard clip.

[Chuck H]

Something is definitely wrong and there's more output to be had from the amp. The clip is just barely greater than the 0V line. I think another voltage check on power tube pins is in order. The small crossover notch in the waveform near the bias level would seem to indicate that both tubes are conducting but something is clamping one tubes positive swing to the 0V line. Seemingly through a small resistance since it does push slightly above 0V. Tomorrow I may build a model and experiment to see what error might make this happen. Not tonight though.

This is a pain in the @$$ for you but sort of a fun corker otherwise. The good news is that the amp is clearly detrimented and your guy will probably be delighted when it's fixed. No promise on when that happens though.

[The Dude]

If the clipping starts at the phase inverter, I don't believe it's a power tube issue.

[Helmholtz]

Please scope grid signals (which are essentially PI output).

[Chuck H]

I really don't want to derail Helmholtz line of diagnostics so please scope the power tube grids first.

Then, if you don't mind, take voltage readings on the power tube pins.

Then swap the power tubes in their sockets and scope the output again to see if the clamping stays with the socket or follows the tube.

The thing I notice about the waveform shown is that there is a crossover notch. As if the power tubes are being driven symmetrically but is then clamped or clipped. I'm actually suspecting a power tube or the OT.

[Helmholtz]

Please see post #2.