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Hello folks,
I repaired many issues with this Ampeg SVT-AR head and am left with one more issue to solve. With all volumes down there is a low but very audible 6Hz hum. It's about 3Vpp at the speaker output loaded. I've trace the origin to the 2nd 12AX7 (I'll call V2) in the preamp chassis and seems to be generated in the final triode of the preamp. I believe it's a cathode follower from the triode with pins 1-3. (BTW, It hums the same even if I pull V1).BTW, if I shunt the output signal of the preamp to the power amp section, the hum goes away. Also just putting a plug in the return jack kills the hum, too. Output tubes are balanced.
So, being a cath follower they have elevated the entire triode section so the grid has as I recall about 50V on it and the cathode has a fairly high DC voltage on it, too. All coupling caps in & out of the stage are blocking DC as they should.
Thinking it might be a filament hum issue, I fed the filament with a DC voltage which made no difference except unless I grounded one end of the DC voltage, the amp hummed even louder (probably a normal circumstance for a K-follower). I'm thinking tho, this may be a clue to the issue.
Anyway, I had to give the amp back to the cust as he's a gigging musician with the understanding I would do some more research.
I'm actually at the point that I think I might be getting a bit too anal about this hum and either it's just a design issue or possibly these amps are just that-a way.
Anyone have experience with this possible issue? Thanx, g
I repaired many issues with this Ampeg SVT-AR head and am left with one more issue to solve. With all volumes down there is a low but very audible 6Hz hum. It's about 3Vpp at the speaker output loaded. I've trace the origin to the 2nd 12AX7 (I'll call V2) in the preamp chassis and seems to be generated in the final triode of the preamp. I believe it's a cathode follower from the triode with pins 1-3. (BTW, It hums the same even if I pull V1).BTW, if I shunt the output signal of the preamp to the power amp section, the hum goes away. Also just putting a plug in the return jack kills the hum, too. Output tubes are balanced.
So, being a cath follower they have elevated the entire triode section so the grid has as I recall about 50V on it and the cathode has a fairly high DC voltage on it, too. All coupling caps in & out of the stage are blocking DC as they should.
Thinking it might be a filament hum issue, I fed the filament with a DC voltage which made no difference except unless I grounded one end of the DC voltage, the amp hummed even louder (probably a normal circumstance for a K-follower). I'm thinking tho, this may be a clue to the issue.
Anyway, I had to give the amp back to the cust as he's a gigging musician with the understanding I would do some more research.
I'm actually at the point that I think I might be getting a bit too anal about this hum and either it's just a design issue or possibly these amps are just that-a way.
Anyone have experience with this possible issue? Thanx, g