Early 70's Ampeg SVT 60 Cycle Hum

[Aerostoon]

Hi all,
I am working on an early 70’s Ampeg SVT. This is the 6550 version. The amp has a very loud 60 cycle hum. I confirmed this with a tone generator and the 60 Hz tone was exactly the same as what the amp is producing. I have attached the service manual for the amp.
I have narrowed the source of the hum to the Power amp. I used a jumper across the molex connector pins 9 and 12 to serve as the power switch and have the preamp completely separated from the power amp. The hum is still there and quite loud. The owner said the amp hummed from the day he got it but when played loud he put up with the hum.
I installed a ¼” jack into the patch in (External amp) jack and grounded the tip to the shell and the hum does go down quite a bit but is still there. I read a thread where Enzo said doing this grounds the grid of the power amp V1 12DW7 tube and basically takes anything before it out of the system. Because of this I am suspecting that I have a ground loop issue possibly with the Patch in and out external jack connectors as I have read a few threads with this identified as the issue.
I am trying to figure out the grounding scheme for this amp. This amp does have a three prong grounded cord and the green ground wire is mounted to one of the power transformer bolts. When I look in the service manual page 4 for the power amp, it shows a resistor R53 10 ohm 0.5 watt for the patch in external amp jack. I cannot find this resistor in my amp. I loosened off the external amp jacks and let them hang loosely in the amp to see if there would be any change but this did not reduce the hum. Anyone have any thoughts or experience with this issue?
SVTHD Service Manual.pdf

[Enzo]

Ground loop? SO what happened? You plugged something into the EFX return and the hum stopped? If the hum continued, then the power amp has the hum.

So LOOK at the power supplies. Is the bias supply clean, free of ripple? Is B+ free of ripple. Try a couple different 6550s in the sockets.

[Aerostoon]

Thanks for the reply Enzo. When i plugged a grounded jack into the external amp patch in jack the hum was reduced by a little bit but still there. I will measure for ripple as you suggested and post my results.
I will find some different 6550's to sub in and see if that makes any difference.

[The Dude]

Also check the pot for the filament artificial center tap. I've seen them burn up in this amp in the past.

[nevetslab]

Thanks for the reply Enzo. When i plugged a grounded jack into the external amp patch in jack the hum was reduced by a little bit but still there. I will measure for ripple as you suggested and post my results.
I will find some different 6550's to sub in and see if that makes any difference.

In testing, you can reduce the tube count to one pair of 6550's. It could just be one power tube causing this, so go thru the six tubes, in one pair configuration (one upper, one lower on the output stage). Might get lucky. BTW, I believe you meant to say inserting a PLUG into the jack in the testing procedure.

[cerrem]

Also check the pot for the filament artificial center tap. I've seen them burn up in this amp in the past.

I would also say the same thing.. I have seen the little wirewound 100 Ohm pot to be broken on occasion... Also, the power supply RETURN is floating and connects to chassis at the EXT amp jacks... The speaker output jacks need to be floating with those 2-piece insulators...

[g1]

The speaker output jacks need to be floating with those 2-piece insulators...

Good point, I've seen/heard substantial hum from a ground loop due to this. Sometimes they get lost during disassembly or replacement. Also the real oldies had the xlr type cable to the cab, sometimes people replace that with a 1/4" chassis mount jack and neglect to insulate the ground.

[Aerostoon]

The 100 ohm hum contol pot seems good.

[Aerostoon]

I would also say the same thing.. I have seen the little wirewound 100 Ohm pot to be broken on occasion... Also, the power supply RETURN is floating and connects to chassis at the EXT amp jacks... The speaker output jacks need to be floating with those 2-piece insulators...

If you look at the schematic it shows the EXT amp jacks grounded to the chassis through a 10 ohm resistor. Are you saying just have the jacks insulated and not grounded to anything?

I installed the EXT amp jacks with phenolic washers and the jacks are completely insulated from chassis ground.

[Aerostoon]

In testing, you can reduce the tube count to one pair of 6550's. It could just be one power tube causing this, so go thru the six tubes, in one pair configuration (one upper, one lower on the output stage). Might get lucky. BTW, I believe you meant to say inserting a PLUG into the jack in the testing procedure.

I hope I do not sound ignorant but if you remove power tubes don't you have to change the speaker impedance load?

[The Dude]

It's not necessary for a simple quick test. You're only looking to see if the hum is still there.

[Aerostoon]

Ground loop? SO what happened? You plugged something into the EFX return and the hum stopped? If the hum continued, then the power amp has the hum.

So LOOK at the power supplies. Is the bias supply clean, free of ripple? Is B+ free of ripple. Try a couple different 6550s in the sockets.

I checked for AC on pin 5 of the power tubes. I am getting approx 7VAC on the bias supply. Maybe a failed diode in the circuit?

[g1]

Maybe a leaky cap.

[Enzo]

Weak/leaky filter cap in bias supply.

[Aerostoon]

Weak/leaky filter cap in bias supply.

If I remove the V1 12DW7 tube the ac to the bias disappears. I have tried a different tube including a 12AX7 in V1 to see if the problem follows the tube swap. With V1 in I can hear and feel V2 and V3 humming. I will check for a bad Cap in this area.