Marshall Amp Forum - View Single Post

MartyStrat54 · 10-31-2009, 04:10 PM

I used to be caught up in the balanced PI tube bit, until I continued my research on it. The bottom line is your Marshall amp is not "balanced." It has multiple components that are + or - 20 per cent being used in the circuits. The gain stages vary from each amp.

In our experiments, we have found that you can use a regular gain tube or a high gainer in the PI slot and it can be lop-sided or even and we couldn't really tell much difference. I run a super high gain in my PI, just as a way to use an NOS tube that I cannot use in the V1 or V2 positions. If you ran a scope on the amp, the wave form would be asymmetrical, but I doubt if your ears could tell it. Even if you put a balance PI in, the waveform of your power tubes would still be asymmetrical due to imbalances in the power circuits.

Now the technical paper I read on this matter goes on to say that some guitarists and many harmonica players will actually use a 12DW7 in the PI. This creates a heavy asymmetrical waveform, but it is rich in even order harmonics. BTW, a 12DW7 has a gain of 100 on triode A and a gain of 17 on triode B. So one side is like a 12AX7 and the other is like a 12AU7.

Save money. You don't need a perfectly balanced tube. Just get one that is close if you feel the need, but don't spend extra cash on "perfectly" balanced tubes that probably aren't perfectly balanced.