Trouble With a Paraphase PI

[Turbo7MN]

The cap on the cathode is not an electrolytic, 10,000pF is .01uF, so it's just a regular disc cap. Should I clip it anyway and see what happens? The tubes are fairly new Tung-Sol 5881s, not more than a few months old and not too many hours on them in that time. The only compnents in there that seem questionable are the coupling caps, which seem to be leaking a few mV.

[Daver]

D'oh! You're right about that cap. Not an electrolytic. I'd try removing it just for the heck of it and replace any leaking caps. Are the electro's in the power supply good? You could also try removing the negative feedback temporarily to see what that does. Try a lower gain tube in the phase inverter, like a 12AT7 , 12AY7 or 12AU7 and see if that changes anything. It might confirm that the PI is the culprit or send you on a different path. Keep us posted. Good luck.
Dave

[Turbo7MN]

Way ahead of you .

I gave the amp a full cap job as soon as I got it. Every electrolytic has been replaced.

I wired a SPST switch in to cut the NFB when I wanted a "dirtier" sound. It does the same thing whether the NFB is there or not.

I tried a 12AU7 in there yesterday. Aside from making the amp sound a lot brighter it did nothing to fix the distortion problem.

Any other ideas?

-Darren

[Satamax]

I would up B+ on teh PI to something like 260V or so, and raise the cathode resistors big time. Raise them untill you have no distortion at full blast, then back some, i'd set start of distortion at may be half rotation of the volume pot. How much bias have you got on the PI tubes for teh moment? How big is your AC voltage on the preamp output?

[Turbo7MN]

So I'll obviously have to drop the plate resistors then, they're being fed by about 300V - should I try 100K?

Not sure what you mean by "how much bias" - all my voltages match up pretty much within 5% of the ones shown on the schematic if that helps . I still haven't checked the preamp output but seeing as it happens even with a guitar input I think the preamp output is irrelevant now. I'll try 5.6K cathode resistors and see what happens.

-Darren

[Satamax]

Hi Darren.

Well, by how much bias, i mean how much positive voltage do you have on the cathode in relation with the ground. The schemmmo states 1.5V, but i find this a bit low, imho on a phase inverter. Since you're generating your gain before that stage, you musn't need much more. And yes, 100K will get you closer to cleaner headroom. I think you could even go down to 68K. I have even seen some differential PI with 47K on the plates. By raising your plate voltages, you increase your voltage swing capabilities and hence your headroom. We've taken the aproach of lowering the gain, didn't work, then, we do it the other way around. Lowering the sensitivity of the PI Up the voltage on the plates first, then raise your cathodes resistors. I'd start with something like 10 kils, and see what happens. Anyway you don't need big ones there.

[Turbo7MN]

I finally had some time to work on it tonight.

I replaced the coupling caps with .022 Orange Drops. I know ODs aren't the greatest choice for power stage coupling caps but I had them laying around so I tried them. No deal.

I also swapped the plate resistors to 68k. A little change, but there's still the nasty distortion on top of the sound. Next step is 10K resistors on the cathodes I guess.

[Two Tone]

I´ve had similar problems with an Ampeg Jet,solution was to lower the FB resistor(R16),150k is way too much,try 5-10k !

[Turbo7MN]

Seriously? That's kind of the opposite, I'd think. I'll try it anyway, but most of the time I prefer to leave out NFB altogether so I'm not sure how much benefit this would have.

On a more positive note, the combination of the lowered plate resistors and a 12AU7 has given a noticeable improvement in tone. I'll try upping the cathode resistors for sure if the NFB thing doesn't work out (it's the easier resistor to get to ).