I'm trying to pin down a small hum problem in an old tube PA head I'm repairing.
http://www.theused.com/manuals/simpson/simpson_ma-8n.pdf
So far it looks to be narrowed down to the output tube - hum doesn't go away with both V1 and V2 out of circuit. Hum will nearly vanish by grounding the 6L6 grid resistor.
My best two guesses at this point are:
1) grounding loop problem around 6l6 socket - The original grounding scheme in this amp is typical - grounds placed on the tube socket grounding lugs(chassis) and not always in a stage by stage manner. When replacing the electrolytics I managed to keep the HV CT, 1st cap, and 6L6 cathode/cap grounds tied to a single point. The 2nd cap right after the choke is tied ~ 5in away on the 6l6 socket lug - possible loop there? I've tried the 6L6 grid resistor at both places - same result.
2) Slight imbalance in the heater grounding scheme. Though not shown on the schematic, the amp has a pair of 11ohm resistors tying the heaters to ground via the V2 socket lugs. DCR match is reasonable - AC to ground between each is ~ .25 volts difference. Stray AC measured on 6l6 grid = .5 V
Cramped space and common ties prevents the above 2 scenarios from being easily tested without some major deconstruction. Thought I would post here for other suggestions before proceeding.
http://www.theused.com/manuals/simpson/simpson_ma-8n.pdf
So far it looks to be narrowed down to the output tube - hum doesn't go away with both V1 and V2 out of circuit. Hum will nearly vanish by grounding the 6L6 grid resistor.
My best two guesses at this point are:
1) grounding loop problem around 6l6 socket - The original grounding scheme in this amp is typical - grounds placed on the tube socket grounding lugs(chassis) and not always in a stage by stage manner. When replacing the electrolytics I managed to keep the HV CT, 1st cap, and 6L6 cathode/cap grounds tied to a single point. The 2nd cap right after the choke is tied ~ 5in away on the 6l6 socket lug - possible loop there? I've tried the 6L6 grid resistor at both places - same result.
2) Slight imbalance in the heater grounding scheme. Though not shown on the schematic, the amp has a pair of 11ohm resistors tying the heaters to ground via the V2 socket lugs. DCR match is reasonable - AC to ground between each is ~ .25 volts difference. Stray AC measured on 6l6 grid = .5 V
Cramped space and common ties prevents the above 2 scenarios from being easily tested without some major deconstruction. Thought I would post here for other suggestions before proceeding.