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I'm trying to improve a hum (120Hz) problem in an old Simpson tube PA head:
http://www.theused.com/manuals/simps...pson_ma-8n.pdf
So far I think I've isolated it to the output stage - hum is still there with both preamp tubes removed. Grounding the 6L6 grid resistor(tone control removed - just a single 470K on the grid) pretty much eliminates it.
All electrolytics and coupling caps are brand new + cathode bypass. I've tried the virtual filament CT with 100r resistors and also tried tying one side of the heaters to ground with no difference in hum level. Also tried several 6L6s and a couple different 5Y3s - same result.
What appears to work is additional B+ filtering. I initially replaced the stock caps with 22uF's at the first three nodes. I then clipped in an extra 20uF on the first two nodes flanking the choke. This did improve things a little but the hum level is still what I would call "too much" even by guitar amp standards. I know that SE amps are notorious for hum but for comparison - my homemade Champ has zero audible hum in the output - only hum that's there is in the preamp stages and it's faint.
I've poked and prodded with the chopstick - doesn't appear to be related to wire proximities - it sounds fixed -as in a possible ground loop issue. Unfortunately I have no pics available at the moment.
Grounding is: 1st cap + 6L6 cathode, grid starred together.
2nd cap is tied to grounding lug on 6L6 socket some 5 inches away.
I would normally have tied the 2nd cap with the 1st cap. Possible loop there? The original 2nd cap was tied into the same place I placed it(6L6 socket lug).
Only other thing I can think of at the moment is possibly the choke. DCR looks normal(330r). Inductance is 3H. No idea what the original specs were.
I've run out of ideas so I thought I would try and get some other opinions before doing any more deconstruction.
Thanks
http://www.theused.com/manuals/simps...pson_ma-8n.pdf
So far I think I've isolated it to the output stage - hum is still there with both preamp tubes removed. Grounding the 6L6 grid resistor(tone control removed - just a single 470K on the grid) pretty much eliminates it.
All electrolytics and coupling caps are brand new + cathode bypass. I've tried the virtual filament CT with 100r resistors and also tried tying one side of the heaters to ground with no difference in hum level. Also tried several 6L6s and a couple different 5Y3s - same result.
What appears to work is additional B+ filtering. I initially replaced the stock caps with 22uF's at the first three nodes. I then clipped in an extra 20uF on the first two nodes flanking the choke. This did improve things a little but the hum level is still what I would call "too much" even by guitar amp standards. I know that SE amps are notorious for hum but for comparison - my homemade Champ has zero audible hum in the output - only hum that's there is in the preamp stages and it's faint.
I've poked and prodded with the chopstick - doesn't appear to be related to wire proximities - it sounds fixed -as in a possible ground loop issue. Unfortunately I have no pics available at the moment.
Grounding is: 1st cap + 6L6 cathode, grid starred together.
2nd cap is tied to grounding lug on 6L6 socket some 5 inches away.
I would normally have tied the 2nd cap with the 1st cap. Possible loop there? The original 2nd cap was tied into the same place I placed it(6L6 socket lug).
Only other thing I can think of at the moment is possibly the choke. DCR looks normal(330r). Inductance is 3H. No idea what the original specs were.
I've run out of ideas so I thought I would try and get some other opinions before doing any more deconstruction.
Thanks
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