Ampeg svt cl hum issues

[stirlo]

Hi all, my Ampeg svt cl has a nasty hum that seems to be coming from the poweramp stage - my repair dude has had it for about 8 months and has looked in all the expected places - it apparently isn't a valve or biasing issue. Does anyone know what else the problem might be?

[wizard333]

How loud is it?

Those amps seem to hum to some degree by nature, I've never been able to get one to be super quiet.

[Enzo]

Ask him to check the small tubes. First there are both 12AX7s and 12AU7s. MAKE SURE they are in the correct sockets. The 12AU7s go in the sockets behind the two bias controls

And if that is sorted out, replace the 12AU7s.

Make sure the -180 power supply is super clean.

Verify that ALL SIX power tube sockets have good screen voltage present.

[stirlo]

Ampeg svt cl hum issues

The hum is loud enough to know there's something wrong - you can hear it above the playing

[Enzo]

Please give attention to your 12AU7s as mentioned. That is by far the most common area of failure in my own experience with these.

[Amp Kat]

I agree and this amp also has three 12AX7's in the very front on the controls and preamp board. Try the send out and see if it still has the hum and also just put 2 6550's, 1 oin each side to eliminate power tube hum. Try the return in and see if the hum is pream or power amp related. These amps when working properly should be hum free and bias up correctly with the lights somewhat out of crossover.

[Mars Amp Repair]

Well,
My experience with the CL version is that the power tubes must be very closely matched (within 5-8ma) for the hum to be cancelled effectively as well as even for the bias LED system to work properly.

I've never been able to get the LED's to balance correctly with any less than the 6 6550's in place, however I just checked & you can for test purposes adjust the bias with 2 matched 6550's in there with bias tool or minimum hum if you ignore the bias LED's.

The actual bias can be anything you want on one tube & then just minimize the hum to match it to the other.

In this particular SVTCL, I could not get the hum to minimize as well as it does with all six in there, so I'm not certain what that test with 2 tubes really proves. glen

[Amp Kat]

I was trying to get him to isolate where the hum is coming from and at this time he or us really don't know where it's coming from. I've seen the preamp board have some freaky issues and shielding issues. Running two tubes will eliminate the possibility of power tube hum if that is the problem. If the problem is power tube related cutting back to two could show it barring of course a matched good known pair is inserted. Finding the source of the hum is the key and iosolating it to preamp,PI,power section can cut you're work in half. I've also had the leds very accurate but they aren't always written in stone by any means as the balancing comparator TLO-72's can get out of tolerance very easy. He could also have a blown 10 ohm resistor on the power supply which would cause hum. We are thinking tubes here but this amp has vibration issues that weren't addressed at factory and I've had chassis bolts unscrew themselves and fall on the board and blown resistors and heater traces destroyed on that pre board in the front. Wouldn't suprise me if this is another vibration issue causing something to come loose.