Ampeg SVT Classic - How Much Background hum is "normal"?

[wizard333]

Just got done with a SVT classic that had bad solder joints in the pre-amp, dead screen grid resistors, and a bad switch. Also retubed it.

Now that its back together and working, it has a fair amount of 60/120 cycle hum. A lot of it I dialed out while biasing, but its still there and audible, just below conversation level. Thats with the pre-amp turned all the way down at gain and master, and as quiet as I could get with biasing.

Is this normal for these amps? I've never owned one so I cant tell. If it was a 100W Plexi I'd say it was normal and call it a day, but I'm not sure what is "normal" level on an SVT.

Thanks!

[Jazz P Bass]

Output Hum

Can you measure the hum with a DVM connected to the speaker.
Set it on AC volts.
I am usually happy with a 4-6 mv level, max.
If it gets to 10, it becomes really irritating.

[Mars Amp Repair]

hello,
you can isolate the problem by inserting a plug in the 'power amp IN jack'. If the hum goes away, then you problem is before the power/drive stage. If it doesn't go away, then you might have a bad driver tube.

Since the power tubes have to be matched within like 8ma for the green/red bias led's to even set on that amp, it's probably not a power stage issue.

The preamp tubes can be sensitive to cathode/filament relation in that amp. You might try swaping them around or trying a new one in different positions. I believe in that version of the SVT, there are only 2 preamp tubes in the preamp chassis.

glen

[stokes]

You say "60/120 cycle",you should first determine if it is 60 cycle or 120 cycle hum.They come from different places in the amp.