Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on July 11, 2019, 05:18:32 am
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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 (https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_5e9a.pdf)
/Leevi
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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.
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Yes, you are right, I tested it and it works like you mentioned.
Thanks
Leevi
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> 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.