What does this capacitor do? (Bias Shifting Tremolo Question)

[Stovemananon]

Hello all!

I posted this on TDPRI but wanted to get your take as well.

I recently built this amplifier:



The tremolo I used is from a Silvertone 1482. (6AU6 Oscillator)

When I first installed the circuit, the tremolo had no effect. I read online that I could change the value of this resistor to increase the trem depth:

1meg resistor bias trem.png

This worked to a certain extent, but the trem depth was still very weak.

So I removed this .02 capacitor and it made a WORLD OF difference.

02Capacitor.png

It seems to have been bleeding the majority of the signal to ground which diminished the trem strength.

What it sounds like now:

https://photos.app.goo.gl/1o1fGE1LLEPjwGEA7


I see this capacitor in a lot of tremolo circuits.

Example: Princeton Reverb

princeton reverb.png

My question is: What is this cap for? I realize it's part of a high pass filter, but I don't get it. Why would you want to bleed signal to ground to diminish strength?

Thanks in advance!

[sluckey]

That cap smooths the LFO sine wave signal, making it look more sinusoidal and cleaner. The changes you made to increase the LFO signal are not usually necessary for EL84s or 6V6s, but are often needed for 6L6s. Which output tubes are you using?

[martin manning]

It's a low pass filter, fo around 7 Hz.

[Stovemananon]

Thanks for the replies!

I am using 6p3s Soviet tubes which I hear are the equivalent of 5881's. They are biased pretty cold.

In fact the bias was cold enough so that I could also try 6v6 tubes to see if I got a better trem strength. When I put 6v6 in, I did hear a slight difference, but not much.

So LFO is low frequency Oscillation?

Thanks again.