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previous: Eth That tiny capacitor always confused... -- 9/21/1999 6:19 PM

Re: Cap across driver tube

9/22/1999 2:24 AM
Steve A.	Re: Cap across driver tube Eth: I had avoided those caps thinking that they might make my amps sound muddy (reasoning: look at all of the muddy sounding amps that use those caps) but you guys kept mentioning them so I finally tried them out on a few amps and liked the results (it smoothed out the sound a little bit and reportedly makes the design more stable- so I might as well include them in high gain amps- right?) While I don't dispute your analysis, what I see is the 47pF cap passing some of the high freqs from the inverted output of the PI to the non-inverted output, which should reduce some of the high freqs which could possibly cause an inaudible power-robbing oscillation over maybe 32khz. Steve Ahola P.S. I keep reading explanations of how the PI and NFB work in a Fender or Marshall type amp, but it seems to go in one ear and out the other! Maybe someone can write up a detailed analysis of the PI and output section of a tube amp and post it to a site... I'll help out with the drawings if needed. (Instead of looking through various notes I'd like to see a nice 12 page article that I could study every night before going to sleep... maybe after 6 months of that I'll have some idea what is going on in a PI!) Until then I'll just keep copying one PI design or another without really understanding what is going inside that handful of caps and resistors...

9/22/1999 2:24 AM

Steve A.

Re: Cap across driver tube

Eth:

I had avoided those caps thinking that they might make my amps sound muddy (reasoning: look at all of the muddy sounding amps that use those caps) but you guys kept mentioning them so I finally tried them out on a few amps and liked the results (it smoothed out the sound a little bit and reportedly makes the design more stable- so I might as well include them in high gain amps- right?)

While I don't dispute your analysis, what I see is the 47pF cap passing some of the high freqs from the inverted output of the PI to the non-inverted output, which should reduce some of the high freqs which could possibly cause an inaudible power-robbing oscillation over maybe 32khz.

Steve Ahola

P.S. I keep reading explanations of how the PI and NFB work in a Fender or Marshall type amp, but it seems to go in one ear and out the other! Maybe someone can write up a detailed analysis of the PI and output section of a tube amp and post it to a site... I'll help out with the drawings if needed. (Instead of looking through various notes I'd like to see a nice 12 page article that I could study every night before going to sleep... maybe after 6 months of that I'll have some idea what is going on in a PI!) Until then I'll just keep copying one PI design or another without really understanding what is going inside that handful of caps and resistors...

Replies:

Andy Fuchs You guys are off the mark on this I... -- 9/22/1999 11:57 AM