5e9a tremolo

[Leevi]

What is the idea of the 10M resistor connected to the oscillation circuit?
At least the connection is causing 24v voltage on the foot pedal tip.


https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_5e9a.pdf


/Leevi

[d95err]

I’m guessing it is to create a bit of a thump when activating the tremolo.

Oscillators sometimes need some kind of noise to get the oscillation started.

[Leevi]

Yes, you are right, I tested it and it works like you mentioned.
Thanks
Leevi

[PRR]

> What is the idea of the 10M resistor connected to the oscillation circuit?
> At least the connection is causing 24v voltage on the foot pedal tip.

Put your finger on it. Will come down to under a Volt. Quite safe, and normal for this plan.

As d95err says: An ideal oscillator may never start. (Simulations usually have to be tricked.)  Real oscillators do because there is always random noise, and because we set the idle gain to about 120% of the final gain needed to sustain oscillation. If we pencil 1V final and 1uV noise, we need about 76 passes through the amplifier to get that 1,000,000:1 rise of level. (1.2^76 is about a million.) Here we expect 10V or more final level, but in the sub-sonic band a tube's noise is high, maybe 10uV, maybe more.

In a 1MHz radio oscillator, 76 passes is less than an instant. At audio it is a part-second. But for say 10Hz tremolo, 76 passes is 7.6 seconds or several bars late.

With a perfect "kick-start" it is "possible" to get full level on the first half cycle with a minor glitch. But that needs critical adjustment, and isn't really needed. If you kick it pretty near full level, and it takes a few cycles to settle to final level, that's usually fine.