SF DeluxeReverb ugly distortion

[Zouto]

Almost fixed...

Good news:a 47 pF Mica across the PI's plate resistors plus swapping the 220K grid leak resistor on V4 (on Rev.pedal RCA connector) removed all that sick distortion and oscillations completely, the amp sounds finally fine.The remaining bug is a loud hum as soon as I turn on the Reverb pot.I swapped V3,V4, cables and Rev.transformer, tried some other reverb tanks with long cables at quite a distance from the PT for no bettering.Hums like some grounding problem.The reverb sounds ok, but the hum is much louder than on another SF DX Reverb and a SF Twin I tried last week.Any ideas for some final dehumming?Thanks,
Zouto

[jrfrond]

OK, so placing a snubber cap across the PI plates stopped the distortion. So, what you had was most likely blocking distortion caused by supersonic oscillation, which is leading me to believe that, together with the reverb hum, you might have lead dress issues. Not uncommon in SF amps.

[Bruce / Mission Amps]

OK, so placing a snubber cap across the PI plates stopped the distortion. So, what you had was most likely blocking distortion caused by supersonic oscillation, which is leading me to believe that, together with the reverb hum, you might have lead dress issues. Not uncommon in SF amps.

Supersonic means faster then the speed of sound, ultrasonic means you can't hear it.

[jrfrond]

Supersonic means faster then the speed of sound, ultrasonic means you can't hear it.

OK, OK..... I can't hear anything above 13kHz anyway!

Problems like this are the reason that everything I work on gets a look-see on the 'scope. On a 'scope, it would be readily obvious watching all that IM distortion do it's thing.