Ampeg SVT II Pro hum

[jabbott]

First post. Part time tech for music store in Missouri. I've worked on a few Ampegs here but this one has me stumped. Output has low hum all the time, that the customer is wanting removed. The unit has all new tubes and the power supply caps have been replaced. I am beginning to think this is just the nature of this amp as everything seems to be functioning normally. Grounding the power amp input has no effect on the hum so not in the preamp. The scope shows a low level sine in output.

I've checked main board grounds and filament voltage and balance. I've checked main board and power tube board for solder issues. All tubes check good. I'm wondering if this might be magnetic coupling from nearby transformer? Any special cable routing that might have been changed during the capacitor replacement?

Thanks for any help.

[The Dude]

This could be many things. I sometimes see open screen resistors in those amps. I'd check each socket for screen voltage. If you had an open screen resistor, it would create an imbalance that might cause hum.

[Jon Snell]

It would be helpfull to state the hum frequency.
That will tell us whether it has Power Supply or ground loop issues.

[jabbott]

I think I checked the screen resistors when I had the tube board out and they tested good. I just saw TIB0003-1 and this amp has not had that applied. I believe the hum was 120 hz. Ripple on both screen and plate supply voltages seemed excessive but might be normal for this one. Around 5V and 15V p-p respectively IIRC (not sure).

Thanks guys.

[Jon Snell]

120HZ hum indicates your issue is with the power supply HT or an imbalance of output valves.

[g1]

Does this model have the lights for adjusting bias? I think that may allow enough imbalance to create some hum, have you tried adjusting it, and does it affect the hum level?

[jabbott]

The bias pots do affect the hum and the hum is at a minimum with both pots turned fully anti-clockwise and but no leds are on. I seem to remember a post that mentioned grouping power tubes for the best balance but don't recall if that was for this model.

I won't be back in the shop until later next week, hopefully. Dealing with a kidney stone currently.

Thanks again for the help on this.

[g1]

The bias pots do affect the hum and the hum is at a minimum with both pots turned fully anti-clockwise and but no leds are on. I seem to remember a post that mentioned grouping power tubes for the best balance but don't recall if that was for this model.

I won't be back in the shop until later next week, hopefully. Dealing with a kidney stone currently.

Yes, that was nevetslab that posted about grouping the tubes and balancing push to pull.
Hope that stone clears real quick.

[Jon Snell]

These amplifiers hace a DC coupled triode setting the bias and providing enough current to drive the output valves into Class AB2. (Squeezing everything out that it can).
I would check the 12BH7/12AU7/12AX7 (whichever you use) for misbalance if the output stage with good valves cannot do as the crib sheet attached suggests.

[jabbott]

Yes, that was nevetslab that posted about grouping the tubes and balancing push to pull.
Hope that stone clears real quick.

You and me both! Thanks.

[jabbott]

Just thought I'd let you know how this ended up. The music store released the amp to the customer and he reported
to them that it was much better now. One thing I did notice was when the metal cover was installed and the amp cold started,
the top cover would vibrate loudly on the inrush, then slightly during operation. I placed some pads to dampen the vibrations
which helped that. I'm wondering now if the customer's complaint wasn't about the vibration and not the hum in the output.

It doesn't help me when those taking in the repair don't ask the right questions or demo the problem with the customer.

I appreciate the input you all have provided. Thanks.

BTW. Kidney is doing better now.