Help with paraphrase PI & PPIMV - or should I go with concertina?

[Ottbucket]

Hi all,

I understand this power section uses a paraphrase but not experienced enough to have ever fooled with these. I understand its operation pretty well (ie: 40:1 voltage divider from top feeds bottom tube which should have a gain = 40).

I am not that interested in maximizing the output power (and may indeed end up with 6V6 in place of the 6L6GBs) .

My question is this: Can a PPIMV be used with a paraphrase? Seems to me that it can't be done because of the voltage divider needed to feed the paraphrase.

If it can be done, how would it be implemented? Or, is it just simpler to go with a split load?

Also, seems that a paraphrase would prohibit one from swapping 12A_7 types since they have different gains and would require different v divider values.


The amp is not working now and is nearly gutted except for the PI and output tube so I'm not looking for a quick fix.

Feel free to get as detailed as you want/can - I want to learn.

[nickb]

Essentially, you can just replace the 470K resistors with pots like this:



You should tweak the values of R6 and R5 (R5=R6 always) to get the same ac voltage at the power grids (taken from the pot wipers). It doesn't quite go to zero gain but near enough but I can't see that you'd need to do that anyway.

[R ski]

From my experience with tube power amps, if your after a clean sound the splitter will do fine,,,,,,I tried this with old hifi amps...they tend to sound horrible if driven..then you should consider stripping down and configure a long tail splitter found in Fenders or Marshal ....when driven they don't sound strained like some other splitters.....easy to find on the net.....

[J M Fahey]

Can a PPIMV be used with a paraphrase?

Yes.

Seems to me that it can't be done because of the voltage divider needed to feed the paraphrase.

No, PPIMV means POST PIMV, meaning it's AFTER the PI.
Doesn't care how you managed to split phase and get power tube driving signals.

It's just a dual master volume, between PI and power tube grids.

If it can be done, how would it be implemented? Or, is it just simpler to go with a split load?

Just check the 1000000 posts/schematics showing how it's done and do the same.
No need to reinvent the wheel.
Even less on tubes.

Also, seems that a paraphrase would prohibit one from swapping 12A_7 types since they have different gains and would require different v divider values.

That's a problem with ALL 12AX7, or tubes in general, mismatching between individual ones.
You won't solve it, just work with what's available.

FWIW there's noticeable mismatch even between triodes inside the same bulb, go figure.

Don't overthink it.

[trobbins]

If you are likely to overdrive the output stage, as per a guitar amp, then the PPIMV will assist in taming gross blocking distortion, although adding grid stoppers is still worthwhile to the output stage. Similarly, a grid-stopper on the input 12AX7 triode may tame PI overdrive.

[ArdeliaAhrens]

I agree!! Can't be wrong if using the Fender or Marshall splitters. That's what exactally I did before..

[loudthud]

You don't need a dual pot. Put a pot on the PI siganl feeding the top power tube with the voltage divider after the pot. It automatically attenuates the output of the bottom PI tube.

[trobbins]

That's neat and simple.

The bottom power tube grid leak could be lowered to be somewhat equivalent to what the top power tube grid leak has changed to. Grid stoppers would help to.