Can I slow down this trermolo? - GA-5T

[atmars]

I have been working on amps for a while now, but I don't know anything about tremolo circuits. Is it possible to slow this one down, and if so, how? Also, how does the mod work? Lend your knowledge trem experts.

http://www.gibson.com/Files/schematics/GA-5T.jpg

[Enzo]

Your trem is a LFO - low frequency oscillator - conected to the signal path to modulate it. You want to slow down the LFO. I usually want to do the same. Most trems I hear are way too fast at the lowest setting. And turn the speed over about 3 and they turn into a buzz instead of a tremolo.

The LFO is a phase shift oscillator. The signal at its plate is fed back to its grid through three RC sections to delay the signal a little bit. This feedback causes oscillation, and the RCs - resistor/capacitor - set the frequency.

Look at your trem tube. See the .01 and the two .02 caps in series from plate to grid? And between them, high value resistors to ground? One of those is variable to make speed adjustable.

The simplest thing to do is increase those caps. That makes each cap stage take longer to charge, thus lowering the frequency.

I turn the speed control all the way down, then add capacitance until it slows down to the minimum speed I want. You can increase the .01 to .02. Or increase one or both .02s to .047s. The numbers are not critical, I usually find changing the value by a factor of 2 gives a more noticable change. DOn;t go nuts, some circuits are not happy if you try to slow them down too far. Just experiment a little.

You could increase the resistors instead, but I find the caps a better way. One reason is that I cannot increase a resistance by tacking in a parallel part, but I can hold another cap against the original one and listen for the difference.

[NorCalTuna]

Hey enzo, fun post~

Is the .047 to the bias resistor a filter to keep the thump of the oscillator out?

thanks!

[Enzo]

The .047 is a coupling cap just like between any two stages. It keeps the 200v of DC at the plate of the trem tube from getting to the grids of the powr tubes. You really don;t want +200v on those grids.

It allows the LFO signal through, it is after all a signal. That LFO signal then modulates the music.

[atmars]

You can increase the .01 to .02.

i tried this out and worked. decent amount of slowdown, but I didn't go any farther, as it seems like the trem doesn't start up immediately. It seems to take about 2 or 3 secs to get rolling nice and strong.

[Enzo]

Have you replaced the original timing caps? Or just add new ones parallel? If the old caps were leaky, adding new capacitance in parallel won;t cure that. And leaky caps can cause balky oscillators.

Try nice fresh caps of the desired value.