Ampeg SVT-3 Pro with bad hum

[xtian]

Client said his SVT-3 Pro is making crackling noises after warming up. When I plugged it in and played some music thru it, I found the output low and distorted, and a little hum present, even with master volume at zero. I measured less than 10mV across each the ceramic source(?) resistors, so I turned the bias up until I get ~20mV across each. Now hum is much louder. I pulled ALL five of the preamp tubes out, and hum is the same.

Next troubleshooting steps?

Anyone have schematic? Having trouble sourcing one.

[glebert]

Did you check this one at ET? https://elektrotanya.com/scm_electro.../download.html

[xtian]

Thank you. Did you know the amp won't run with the fan disconnected? Now I know. With no fan, the heaters for V4 and V5 don't work, and there is a bad hum.

Amp is running fine now, but I'm playing some music, waiting to see if owner's original complaint, crackilng noises, happens.

[xtian]

No issues, back to owner. In looking at my records I see I had another SVT-3 Pro in 2017 that needed a new 12AU7. I guess the cathode follower config is hard on those tubes!