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!
Topic: Help with modded SFDR bias circuit (Read 114 times)