What are these caps for?

[titser_marco]

Hi! Just finished doing the OD section of my D-style amp rebuild and I was wondering if you could help me out here. I followed the second schematic at the bottom (500pF caps) and I noticed that this cap caused whistling and other sorts of nasty high frequency things happening especially when playing notes at the high register.

I'd like to ask then: what are these caps for? What will happen if I increase or decrease them? I'm asking these because this might be the tweak I'm looking for. I still feel that the OD section is to fizzy for my tastes. I've tried disconnecting the 2.2uF Accent cap in the NFB section and while it did remove the fizz, it made the amp too dark for my taste. I was wondering then if changing these two 500pF caps may be the solution to my problem.

Thanks!

[xtian]

I belive those are snubbers, to help tame fizz and high freq oscillation.

[titser_marco]

I belive those are snubbers, to help tame fizz and high freq oscillation.

I see. What is the effect if I increase them to say .001uF?

[Leo_Gnardo]

I belive those are snubbers, to help tame fizz and high freq oscillation.

That, and they will only work if the cathode bypass caps have excellent high frequency response, iow act as a short circuit when high frequencies are applied. Not all electrolytic caps respond as you'd expect a theoretically perfect capacitor to act. You could apply a "band aid" cap in parallel to those cathode bypasses. IMO the best solution is simply run those snubbers straight to ground then they can't set up a feedback loop thru the tube resulting in whistling and chirp noises and RF junk beyond the audio frequencies.

Another place to employ this technique is to put the snubbers across the plate resistors BUT this also assumes the hi voltage filter cap serving hi voltage DC to those plate resistors has the expected hi frequency response which isn't always the case.

[JTU]

The cathode of the tube is like a ground for AC since it's cathode resistor is bypassed with a large cap. The output impedance of the stage and the capacitance of the cap (from plate to cathode) form a low-pass-filter for the signal coming from the tube.

You can use a RC-filter calculator to calculate the cut-off frequency.

There are also calculators to calculate the output impedance of a tube stage ( for exmpl. ampbooks.com).

[vibratoking]

You just have to try it for yourself.

IME, snubbers remove the fizz while simultaneously removing the 'life' out of the amp. Needless to say, I don't like them. YMMV

BTW, there are many threads about this in the Dumble section.

[rp]

IME, snubbers remove the fizz while simultaneously removing the 'life' out of the amp. Needless to say, I don't like them. YMMV

My experience too, removes life, looses treble air, sparkle, doesn't take much, I can hear the 47pf often seen across the PI. I usually loose that too. Shielded wire acts as a snubber. Guess it depends how much 'life' you have to loose before you start feeling sick.

All amps work out a little different, I always try to go w/o shielded wire or snubbers and hope for the best and then see what happens. On ptp that I usually go w/ I find I can forgo snubbers and shielded, but I never built a Dumble or high gain. On SF Fender you can often loose the snubbers w/o ill effects, I'll lift one side and check before 86ing them. Anyway, I learned that just because they are in a schematic doesn't mean you have to have them - again maybe Dumbles are different.