# Fender Vibro Champ grid stopper capacitor - yeh or nay?



## Lincoln (Jun 2, 2008)

On the power tube of some champs and vibro champs there is a 330pF cap across the grid and cathode. Not all of them have it, schematics don't show it.

I didn't understand why it was there so I started reading. They say it was put there to stop high frequency oscillations caused by sloppy wire (dress) placement. They also say it's a real tone killer and limits frequency response. 

I'd really like to hear some opinions on whether to put this cap in or leave it out, and if it's really as much of a negative as some people say it is.


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## WCGill (Mar 27, 2009)

Only one way to find out-try it and see if you like it in or out. Some Champs have the first pre-amp tube cathode bypassed, others don't, big difference in tone for this one.


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## Wild Bill (May 3, 2006)

Lincoln said:


> On the power tube of some champs and vibro champs there is a 330pF cap across the grid and cathode. Not all of them have it, schematics don't show it.
> 
> I didn't understand why it was there so I started reading. They say it was put there to stop high frequency oscillations caused by sloppy wire (dress) placement. They also say it's a real tone killer and limits frequency response.
> 
> I'd really like to hear some opinions on whether to put this cap in or leave it out, and if it's really as much of a negative as some people say it is.


Many of the SF amps had such capacitors. I have never known a case where it made any difference. For that matter, I have never known any case of high frequency oscillations due to poor lead dress. If I ever ran across such, I would just pretty up the wiring!

For that matter, I'm not sure if that 330 pf would be all that much of a "magic bullet" solution in the first place.

The reactance, or AC resistance of 330 pf is so small at the high frequencies of a guitar (tops out at 5 khz) that it doesn't have much effect on chopping highs so that you could notice. If there was an ultrasonic oscillation too high for you to hear that would be different - capacitors show a lower resistance to an AC signal as the frequency is increased. At bass frequencies that 330 pf cap would look like a resistor of millions and millions ohms.

I suspect that what originally happened was that CBS had maybe ONE case of an amp oscillating when it came off the assembly line, due to some newbie doing a horrible wiring job. Instead of rewiring the amp some suit asked an engineer for a cheap solution and he came up with the 330 pf caps. 

Or maybe it was just put in "just in case". The idea is very common with hifi amplifiers and it sure looks like the new CBS engineers were all hifi experts, since all the SilverFace changes were hifi type mods, rather than something important for a guitar amp. Experience with guitar amp circuits would have been relatively rare back in the mid 60's. The parameters involved are quite different from listening to some classical music on vinyl.

Short answer? Don't bother with the damn thing! If you have oscillation problems, spend the time and clean up your wiring. The factory might not want to spend the time but when your time is your own - do it right!

Wild Bill


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