Princeton Reverb, no distortion

[rutledj]

I built the AA1164 circuit. It sounds great but there is absolutely no distortion on this amp at any volume. Is there any recommended changes I can try that would give me more of an expected distortion around 5 on the volume knob?

[xtian]

I built the AA1164 circuit. It sounds great but there is absolutely no distortion on this amp at any volume. Is there any recommended changes I can try that would give me more of an expected distortion around 5 on the volume knob?

Your amp is not operating properly. Have you measured clean RMS output into a dummy load?

For troubleshooting, you can start by posting a voltage survey and photos of the circuit.

[ViperDoc]

Also recommend stating the speaker you're testing the amp with.

[pdf64]

Totally clean even with everything on 10 may indicate that something isn’t right.

… around 5 on the volume knob?

5 on a 1 - 10 scale isn’t even halfway.
So with a 10% audio taper volume control, not much signal will be passed.
Fender volume and treble pots have an unusual taper, about 30%. If you want a DIY amp build to respond similarly, similar spec control pots will be needed.
Alternatively, just turn the knob higher

[Stevem]

Of most importance in your voltage test you reply with is first , your AC wall voltage, the D.C. Voltage on the red wire feeding the OT and the cathode voltages on the preamp tubes as that can give a big clue as to how good the tubes are which inturn is a direct example of how much output drive signal there is to drive the other tubes down stream.

Low cathode voltage could mean that one side of the tube may only have some 70% or less current gain then what it should if you had a tube tester that can test for gm to prove it out with.

If both the A and B side of the first preamp tube are weak then your really going to have to open up the volume pot to get clipping out of the circuit!

[rutledj]

Marked my voltages on attached schematic. Nothing seems out of the ordinary.

Princeton.jpg

[didit]

Hello -

An output tube bias of -40V is downright cold. Try targeting -25-21V.

Best .. Ian

[rutledj]

Just increased it to -31v. This gives me 23mA through the power tubes (I have a 1 ohm resister in series with the cathode to measure this).

So should I be able to achieve pre-amp distortion in this amp? With a scope on pin 7 of V3B, I can turn the vol way up and there is not flattening of the peaks at all. Stays a perfect sine wave (with 1khz, 100uv tone fed in).

[pdf64]

Just increased it to -31v.

I think it would be better to view that as a decrease (in the magnitude) of the bias voltage.

So should I be able to achieve pre-amp distortion in this amp? With a scope on pin 7 of V3B, I can turn the vol way up and there is not flattening of the peaks at all. Stays a perfect sine wave (with 1khz, 100uv tone fed in).

That’s inside the power amp’s NFB loop. ie not within the preamp.
It’s not generally feasible to make sense of waveforms within a loop.

You understand that as signal level is increased from 0, the earliest clipping point will almost invariably be the output valve control grids?

The valve in V1 may be a bit weak, not drawing much current; how about if a stronger valve is swapped in

[didit]

Just increased it to -31v. This gives me 23mA through the power tubes (I have a 1 ohm resister in series with the cathode to measure this).

Perhaps -21V is a bit to hot, but -31V is still cool. Suggest 28mA will still be within 14W dissipation limit. Those are however your output tubes.

So should I be able to achieve pre-amp distortion in this amp?

In short, no.

Best .. Ian

[rutledj]

Finally found the issue. Forgot I had added a 470k grid stopper on the input of the PI (think I read that somewhere that it reduces some distortion in the PI). Replaced it with a Paul C suggested voltage divider to bias the PI. It made a huge difference and I can not get natural distortion from this amp (whether preamp or power amp).

Thanks for your suggestions.

Rut

[Stevem]

Just for future reference, I have found that .150 volts of input signal @ 1k is more then enough to drive any amp with even halfway good tubes to maximum output and clipping.

Lower test frequencies will send the amp into clipping even earlier!