6G2 Princeton Tremolo?

[Randall]

In studying this circuit, I don't quite grasp how it works. I see the DC bias voltage pass thru the intensity pot on it's way to the output grids, and I see trem triode plate voltage on the other side of the .1 blocking cap. If DC can't pass thru that cap, how does the trem oscillation effect the bias voltage?

[Enzo]

There is no DC on your speaker, and yet there is a substantial voltage there. We don't need the DC from the trem tube plate, what we need is the trem oscillation signal, a low frequency pulsing.

I will make up some numbers. Let us say the trem signal at the plate of the trem tube is 10v peak to peak. I have no idea what it really is. The 0.1uf cap lets that through just fine. In the absence of trem, the bias sits at -35v. MAx the trem, and that 10v p-p adds to the -35, so the -35 then sways from -30 to -40. Back and forth at the trem rate. THis of course changes the tube bias in real time.

As you dial down the trem level, your bias feed gets loser to the low impedance of the bias supply, and the trem action lessens until gone.

So that cap is a interstage couplig cap just like the ones between stages in the signal path.

[Randall]

Thank you Enzo. That was what I suspected in blotchy black and white, thank you for bringing into focus so it makes real sense. You are amazing and have taught me much.

[Jazz P Bass]

That type of a Tremolo circuit is commonly referred to as "bias wiggle".

[Randall]

This I did know.

[tubeswell]

The 6G2 trem is an AC-coupled signal as Enzo mentioned. Like any such signal in an amp's signal path, the strength of the trem 'signal' being outputted from the LFO stage, can be 'lost' through the impedance bridging of the bias supply circuit and the tube current in the output tubes. If the output tube bias is hotter, the trem signal current or trem signal amplitude (voltage swing) would have to be increased to compensate, because more trem signal gets 'eaten up' by the impedance bridging in the bias supply circuit (where the resistance in the bias supply -ve voltage divider has been decreased to make the output tube bias run hotter). This is why, if you increase the bias voltage to make the output tubes run colder, trem intensity gets stronger.