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Post subject: Re: Silverface Champ
Posted: Tue May 17, 2011 6:54 am
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Rock Star
Rock Star
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Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
You sure it's not a VC in disguise? :D

Ok, ok... On single-ended amps Fender didn't use screen and grid stopper resistors the same as in push-pull amps (like Princetons, Deluxes, or bigger 35-40 watt 6L6GC Fenders). They did drop the voltage to the screen to help keep the 6V6GT from going ballistic. But, I don't think I've seen any single ended Fenders with grid stoppers.

It's the way most SE amps are setup for idle bias. The 220K-ohm resistor to ground between the gain stage/input tube and the cathode resistor/bypass cap (usually 470-ohm/25-50mfd)---sets the tube into Class "A" cathode-biased mode. If you added a 1k-ohm resistor to the grid (to stop oscillations) it would disturb this input/cathode balance of the bias.

This is something you don't need to worry about in push-pull, Class AB1 amps-- like Deluxes.

So, to help stabilize the amp, the ceramic disc cap is added to feed high frequency (esp oscillations) to ground, before they can be amplified by the 6V6GT and cause problems.

HTH!


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