From PP to SE by a Knob from Soma 84 to JCM - HOW ??

[kagliostro]

In an italian forum was posted a question about how to have in one PP amp an SE effect

I know the Soma 84 schematic where a pot is used in one branch of the Concertina PI to achieve that result

but what about PP to SE if the PI is a Long Tail ??

there is a way to obtain the same results ?

is a solution to put two pots as PPIMV instead of one PPIMV ganged pot and use one of it as to unbalance the signal to the final tubes ?

Thanks

Kagliostro

[HotBluePlates]

Look hard at your posted schematic. The 22nF cap and 1M pot are not part of the concertina, they are a coupling cap and a pot.

The concertina was probably used because the Soma 84 amp uses EL84's, and requires little driving signal to the output tubes. You could just as easily incorporate this modification to a long-tail circuit by placing a coupling cap and pot between one output of almost any phase inverter and the input grid of an output tube.

The Soma 84 assumed cathode biased output tubes (fixed biasing EL84's is a little bit silly); you could adapt to fixed-bias output stages by having a coupling cap between the output tube grid (and bias) and the pot wiper. That means you have a coupling cap before the pot and another after. You should make one of those caps 10 times larger than the other to avoid increased bass rolloff.

[kagliostro]

Hi HBP

Look hard at your posted schematic. The 22nF cap and 1M pot are not part of the concertina, they are a coupling cap and a pot.

if i don't understand bad also you agree that this pot is like one of the two pot of a PPIMV and lowering only one branch of the PI will result in an unbalance that will give a similar SE effect

about the use of caps to prevent the income of the bias DC to the pot OK, I agree with you

Thanks

Kagliostro