Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Choan on December 04, 2025, 04:02:14 pm
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Howdy everyone. I picked up a 76 SFDR (AB868) and noticing some 60hz hum and ring mod like oscillation on higher notes. It’s come with a bias balance modification that I don’t understand. It uses the Bournes style pot suggested on Rob's page but isn’t fully wired like that layout. I can’t get the hum to go away pulling/swapping with good pre, power or rectifier tubes, and it goes away entirely when the power tubes are pulled. It remains with a single power tube in either socket. Speaker swap doesn’t get it. I have it biased at 21ma with 406 on the plates. I noticed the the 220k resistors in the phase inverter aren’t tied as shown on the layout. I checked filter and bias cap grounds and they check out, and they appear to be newish FT and TAD with no visible leakage. I will check heater circuit with a lamp battery but there’s something funky about the way this bias circuit is wired, any thoughts? Also, Power tubes have original carbon resistors and ceramic caps which I intend to change. Howdy everyone. Picked up a 76 SFDR (AB868) and noticing some 60hz hum and ring mod like oscillation on higher notes. It’s come with a bias balance modification that I don’t understand. It uses the Bournes style pot suggested on Robs page but isn’t fully wired that way. I can’t get the hum to go away pulling/swapping with good pre, power or rectifier tubes, and it goes away entirely when the power tubes are pulled, it remains with a single power tube in either socket. Speaker swap doesn’t get it. I have it biased at 21ma with 406 on the plates. I noticed the the 220k resistors in the phase inverter aren’t tied as shown on the layout.
I checked filter and bias cap grounds and they check out, and they appear to be newish FT and TAD with no visible leakage. I will check heater circuit with a lamp battery but there’s something funky about the way this bias circuit is wired, any thoughts? Cheers and thank you!
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Looks like a bias balance arrangement with the small trim pot being an overall adjustment.
Remove the power tubes and measure the negative voltage at the grids as you turn each pot to get a feel for what they do. The large one will shift voltage from one one side to the other while the trim pot will adjust both simultaneously.