Blues Junior distortion problem

[nickb]

I would expect the wave shape to change after the tone stack due to the altered ratio of harmonics. I'm not saying the problem isn't there but you could easily be misleading yourself. The only waveform that does not suffer from that issue is one with a single frequency i.e. a sine wave.

I think the earlier idea of signal tracing is the way to go. You use a second amp with a probe to listen to various points. The probe should have DC blocking, a high input impedance and adjustable attenuation.

[jpcar]

I would expect the wave shape to change after the tone stack due to the altered ratio of harmonics. I'm not saying the problem isn't there but you could easily be misleading yourself. The only waveform that does not suffer from that issue is one with a single frequency i.e. a sine wave.

I think the earlier idea of signal tracing is the way to go. You use a second amp with a probe to listen to various points. The probe should have DC blocking, a high input impedance and adjustable attenuation.

So, finally got to the bottom of this.
It was handy having the second Blues Junior for this.
I lifted the leg of the R20 - resistor just after the master volume wiper on both amps. Jumpered from one (master wiper) to the other (other side of that resistor).
No distortion happening before the master vol.
So I then jumpered two wires from R33, R34 (after phase inverter) - removed phase inverter tube from one amp and power tubes from the other.
Still no distortion. So the problem is something between power tubes and output transformer.
So I replaced R35, R36 (which were measuring ok) and replaced the two power tube sockets. They looked pretty worn. Problem gone. Sounds good now!

Thanks for all the help.

[Sowhat]

replaced the two power tube sockets. They looked pretty worn. Problem gone. Sounds good now!

Glad you decided to signal trace... mmmm, tube sockets, yeah, we've all seen that before, good one and hard to determine without a scope. Who say's you don't need a scope to do this stuff?

[nickb]

Glad you found it.

I've seen many of these with hairline breaks in the PCB traces around the power tubes. It's possible that the act of replacing the sockets also bridged a track. You can get upgraded replacement PCBs on the net.