Gridstoppers against RF, a necessity ?

[SY]

lndm: I haven't had that happen yet. Tell me more.

rdf: I haven't seen a correlation, but that's probably because I don't close feedback loops until the circuit within is stabilized.

[lndm]

I'll have my amp on the bench, scope at the output measuring noise or distortion, and once the pot goes past a certain point I get oscillation. Turning down often kills it, but i've had it bad before.

When this happens, I usually clean up my wiring and change devices and eventually make it disappear. Can't say I've nailed it though. e.g.

I used to run 6550's and I found I needed 22k grid stoppers to stop an oscillation at (20kHz i think or something high).

I ran a 33 as a driver once and it was microphonic. It oscillated at 5kHz.

The 12at7 I used in my phono amp kicked off an oscillation at 4kHz using the volume control, replacing it with 12au7 fixed the problem, but trying to duplicate it today, it wasn't happening.

My sig gen has show up trouble at 10kHz, possibly by noise injection.

I am reluctant to increase the grid stoppers too far for f response reasons. BTW, heavy buss and star gnd, decoupled stage rails. Phono, stage1, pot, driver,o/p.

[SY]

Interesting- I've never seen that. Normally I'd think "decoupling," but you've already gone down that path. Dumb question- have you replaced the pot?

[lndm]

Originally posted by SY
Dumb question- have you replaced the pot?

I'd like to LOL but I once had a computer monitor to fix. It wouldn't turn on and my wife said it must be the power switch. I was floored to find out it was the switch.

The pot is an Alps. Despite using it often, I seem to have forgotten it existed. Maybe I'll change it and see what happens. (Maybe it has a tiny flat spot)

[SY]

Wipers can do funny things. That's why I lust after good stepped attenuators (but live with Alps Black Beauties).