What would cause high frequency oscillation with neg feedback loop connected?

[jasonguitar]

BTW, been reading about coupling cap orientation so I did the scope tests, marked the outside foil and oriented the caps properly in the amp. Made an audible difference to my ears. Also read unclespot's blackfacing page where he swears by keeping the heater wires uniform which I believe also helped improved the hum level. Played it at rehearsal the other night and this amp sounds better than ever! Super reverbs kick but. Good times...

[Chuck H]

I have noticed in some of my builds of late that I'm getting more ripple than usual. I've always used the Sprague Atom caps without problems until now. I don't have any orders right now but I'm going to revamp my layout for axials and try some Nichicon PW series for my next amp. Some things that have helped with hum include:

I use a three star ground. One for the preamp close to the input. One for the PI, NFB loop, reverb and trem at mid board and one for the main and screen filters, PT ctr, rectifier, PA cathode, OT 0V, speaker sleeve and just basically anything that isn't a part of the preamp or PI is gounded with the chassis ground (green mains wire). This seems to keep hum off of the preamp where it is most likely to be amplified.

Elevate the filaments. A voltage divider off the main filter + node can double as a "bleed" resistor and a +V reference for the heaters. Aim for 30 to 40V. Use this as your filament wind CT. This is an especially good idea for amps that use cathode followers in the preamp because it reduces the heaer to cathode differential. This has been a problem with some recent production tubes.

Don't use a ground buss across the pot backs. This invites ground loops in some cases. Just be OK with the "mechanical ground" via the chassis mount for the pot cases and use the three point star ground for the actual circuit.

Try different preamp tubes. I've had a few preamp tubes that were quite hummy right out of the box. Particularly Sovtek's.

You can do a pramp tube pull and try to narrow down where the hum is comming from too.