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Vintage threads from the first ten years

Cap across driver tube

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9/20/1999 9:18 PM
Mike	Cap across driver tube What is the purpose of having a cap across pins 1 & 6 of the driver tube?

9/20/1999 9:53 PM
Graywater	Mike, What amp? What driver tube? Under what circuit conditions? What value cap? Tell us more........... GW

9/20/1999 11:12 PM
Mike	Aren't they all the same? Just kidding. Let's say like the Fender Bassman Model 5fg-A, there is a 47uF across pins 1 & 6I currently have a driver setup like the Bassman Model AA864, without the speaker feedback(yet), the 22k and .1uF are directly ground.

9/20/1999 11:24 PM
Benjamin Fargen	Mike, It helps to curb parasitic osciallations and also seems to tame shrill high end qualities from really bright preamp circuits. You can play with that value up to 180pf to suit your tastes. Benjamin.

9/20/1999 11:46 PM
Mike	I understand. Does adding a presense control, through speaker feedback add brightness?

9/21/1999 11:55 AM
Andy Fuchs	This cap is for what is commonly called "loop compensation". This is simply producing some losses within a feedback loop, to make up for say, transformer rolloffs. This makes compensating the circuit with feedback a little easier to do. It's value can vary depending on the transformer characteristics, and the sound your seeking.

9/21/1999 4:30 PM

Huh? :>)

Is Mike talking about the small 47pF to 100pF cap across the PI output?

If that's the one Mike ment, I think it's to cut extremely high frequencies that can cause oscillations and stability problems.

Which one are you describing Andy?

That sounds a little more like a description of a frequency selective resonant feedback loop or maybe something like a Z circuit Doc talks about across a Xover.

Bruce

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