Distortion from 1 half of long tail phase inverter

[priesty]

I've spent the last few months turning a friends crappy Peavey 2x10 combo into a 50W Plexi a-la Kevin O'Conner's TUT3 book. There has always been a less than pleasant parasitic distortion present on the amp. I tried putting lots of plate resistor bypass capacitors on the pre-amp, but things didn't get much better.

Eventually, I built a signal tracer with a small battery amp, and have tracked down the problem to the non-inverting side of the phase inverter tube. Thinking it might be either wire routing or a dry solder joint, i re-soldered that side of the pi tube and routed the anode feed (and hence output) wire well away from other wires. Unfortunately the shrill 'screeching' distortion is still there on the 'front end' of any hard hit note. I have still got a 500pF cap between the anodes of the PI tube from my HF suppression experiments!

Are any of you veterans aware of what may be causing this??

[Jazz P Bass]

Are you using a feedback loop?
If so, you may want to try reversing the output transformer primary leads.

[priesty]

I forgot to say that I tried different 12AX7 valves in the V3 position and even a 12AT7, but the noise is still there.

[priesty]

Are you using a feedback loop?
If so, you may want to try reversing the output transformer primary leads.

Thanks for the quick reply Jazz, I can try that, but wouldn't the amp howl uncontrollably from switch-on with OT leads the wrong way round? The amp can play, but just sounds nasty!

[pdf64]

500pF? That's quite large, 10x higher than 5F6A.
Does the amp have global NFB around the power amp (schematic?)?
Try removing that cap, as my experience is that even the 47pF can cause 'ringing' on the leading edge of a squarewave; sometimes even with the same design, one implementation can better off without it, compared to another.
There's already several sources of HF phase shift going on there, the big unknown / variable being the OT, next being lead dress.

[priesty]

500pF? That's quite large, 10x higher than 5F6A.
Does the amp have global NFB around the power amp (schematic?).

The amp implementation of NFB is as per Marshall/bassman style amp, into the bottom of the phase inverter.

I'll remove the cap and see what happens. I might try disconnecting NFB after too to see if the problem goes.

Thanks!

[priesty]

Right, I disconnected 500pF cap and all that happened was the signal tracer parasitic frequency of oscillation went up.

I then disconnected the NFB, made no difference to the parasitic on the non-inverting PI anode. This pretty much rules out NFB as the problem.

The lead dress on the PI is as airy as I can make it, it is a bit near to the OT leads - 1" / 25mm.

So I'm kind of back to square 1! I'd post the schematic, but I don't want to breach London Powers Copyright.

[priesty]

I've just had a thought, my layout is such that the output transformer is quite close to the phase inverter valve, and the 'half' of the phase inverter which is oscillating is also the side nearest the O/T. Could this be significant?

[pdf64]

Have you got a screening can on the PI tube? From recent experience with a Scound City refurb, if missing this sort of thing can occur.
The layout and lead dress is the sort of thing I meant by 'implementation', as the way that the design is physically realised inevitably creates an overlay of unintended, unwanted capacitive etc couplings.

[priesty]

PDF, I don't have a screening tube installed at the moment, but do have a spare available, so will drop it in and let you know how I get on!

[priesty]

I fitted a shield can to the Phase Inverter valve. I ran the amp through a dummy load checking with a signal tracer and the screeching has gone!! So I guess the co conclusion is that flux leakage from the output transformer at high signal levels was interacting with one half of the PI valve to create a parasitic oscillation. This would make sense as the problem was more pronounced when the 16 ohm tap was used on OT (highest flux).
Thanks PDF!

[pdf64]

Brill!
Before we crack open the beer, check it with a real speaker, as there will be phase shift and increasing signal level with frequency, compared to a resistive load.
Pete.