Cathode Bias Amp Bypass Cap Value

[Jag]

What effect does changing the value of the bypass cap, on the output tubes in a cathode biased amp, have?

Will increasing the value to, say, 100uf make it sound stiff? I've heard that the amp won't sag as much causing the amp to sound less compressed and stiff.

[hasserl]

Your description sounds like the effect of increasing the value of a filter cap, not a cathode bypass cap. As for the efects of a cathode bypass cap on the output stage, here is a paper on the subject you can download and read: http://aikenamps.com/cathcap.pdf If you don't want to read the entire thing you can skip to the last two paragraphs that summarize the findings. Keeping in mind this was written about sound reproduction where minimal distortion is desired, not sound production where we tend to want some distortion.

[Jag]

"Your description sounds like the effect of increasing the value of a filter cap..."

I know, that's why I asked. I got the info from a "repected guru" (his words not mine) and he was talking about the bypass cap. I even reminded him that we were talking about the bypass cap not the filters and he stood by the statement. I don't know if I consider him a guru but, I don't want to name him as he is well known.

This wouldn't be the first time I was right to second guess his words.

[tubeboy]

What effect does changing the value of the bypass cap, on the output tubes in a cathode biased amp, have?

Will increasing the value to, say, 100uf make it sound stiff? I've heard that the amp won't sag as much causing the amp to sound less compressed and stiff.

Increasing the value of a bypass capacitor on the output stage of an amplifier will result in a lower frequency response. Assuming your music program contains lower frequencies, this could actually increase current demands on the B+ supply and increase compression. It would depend on the music you are playing and how stiff the power supply is.

[Jag]

Thanks guys.

That's the way I've always understood it.

Still, when a "guru" gives me information that is contrary to what I understand, I'm unsure enough to question my knowledge. I know enough to mod, repair and build amps but I'm still working on understanding some of the theory and do's and don'ts.