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working on the earlier version with the hum balance pot (sans DC filament supply on V1).
I cannot track down a schematic for this particular version, so I altered the later version to reflect the filament supply in this amp. I'm working under the assumption that the only changes are the fixed resistors, bridge rectifier, and reservoir cap in the later versions.
It came in with the complaint that it was noisey/humming badly. When I turned it on to confirm this, it wasn't overly noisey, but there was an audible hum that I wanted to take a look at.
(I owned a BV-60 1x12" combo that I remember having a quieter noise floor)
Plus, the Reverb pot was damaged, and needed to be looked at. So I pulled the chassis, removing all the transformer QCs, and leaving the back panel attached.
I was able to repair the reverb pot, and reflowed all the solder connections on the pots, jacks, filter caps, and a few other components which I felt could use some preventative care.
Re-assembled the amp, checked the supply voltages, and tested it on the scope. Everything looked great....
Until I connected the speakers for a final audio test. The amp, was incredibly noisey! way worse that it was, which is such a drag when that happens.
So I did some further testing, reading some voltages and didn't initially find anything that might suggest a broken ground, bad filter cap, or failed coupling cap. But, here is where it gets interesting/frustrating: (oh, panel controls, or whether input is connected has no affect)
• pulling V1 has little to no affect;
• pulling V3 has no effect;
• pulling V4 kills the noise completely, (but not the signal output - dry audio signal passes fine).
So, I reinstalled V4 (12AU7- one triode drives the reverb/other drives effects loop via cathode follower). I had a hunch that if I interrupted/opened the effects return, the noise would stop, which it did. Strange.
You might be asking "what about V2? What happeded when you pulled V2?". Good question. I can't remember
sorry, but here's what I do know.
If I AC shunt the plate of V2a (pin 1-TP 3) using a cap probe. The noise is gone completely; doing the same on the grid and cathode has no effect on the noise.
If I AC shunt the plate of V2b (pin 6-TP 4) using a cap probe. The noise is gone completely as well. Yet if I measure the DC voltages at those nodes, there is nothing that would indicate a problem so far. I've eliminated any of the existing tubes as the problem.
Here is a couple of other interesting things; I've found examples of people with this model, who've experiencing noise problems which I suspect might be cause by the same thing. But, specifically, I found a thread where a guy posts about a hum problem with the same exact symptoms (IE. pulling V4/opening the effects return, etc ) Many of them are looking for the 6212 schematic with the hum balance pot. But I couldn't find anything where the issue was identified and solved, at least searching this forum for a while and looking at Hoffman's site.
But, I know some of you long timers have been on some of these threads, so if anything comes to mind, I'm all ears (or eyes, I suppose)
edit: Oh yeah - I unified the schematic into one layout, and highlighted the points in the schematic I'm addressing above. There's a chance something might have been deleted, or I made an error combining the schematics. If you find anything, let me know so I can correct it
I cannot track down a schematic for this particular version, so I altered the later version to reflect the filament supply in this amp. I'm working under the assumption that the only changes are the fixed resistors, bridge rectifier, and reservoir cap in the later versions.
It came in with the complaint that it was noisey/humming badly. When I turned it on to confirm this, it wasn't overly noisey, but there was an audible hum that I wanted to take a look at.
(I owned a BV-60 1x12" combo that I remember having a quieter noise floor)
Plus, the Reverb pot was damaged, and needed to be looked at. So I pulled the chassis, removing all the transformer QCs, and leaving the back panel attached.
I was able to repair the reverb pot, and reflowed all the solder connections on the pots, jacks, filter caps, and a few other components which I felt could use some preventative care.
Re-assembled the amp, checked the supply voltages, and tested it on the scope. Everything looked great....
Until I connected the speakers for a final audio test. The amp, was incredibly noisey! way worse that it was, which is such a drag when that happens.
So I did some further testing, reading some voltages and didn't initially find anything that might suggest a broken ground, bad filter cap, or failed coupling cap. But, here is where it gets interesting/frustrating: (oh, panel controls, or whether input is connected has no affect)
• pulling V1 has little to no affect;
• pulling V3 has no effect;
• pulling V4 kills the noise completely, (but not the signal output - dry audio signal passes fine).
So, I reinstalled V4 (12AU7- one triode drives the reverb/other drives effects loop via cathode follower). I had a hunch that if I interrupted/opened the effects return, the noise would stop, which it did. Strange.
You might be asking "what about V2? What happeded when you pulled V2?". Good question. I can't remember
sorry, but here's what I do know.If I AC shunt the plate of V2a (pin 1-TP 3) using a cap probe. The noise is gone completely; doing the same on the grid and cathode has no effect on the noise.
If I AC shunt the plate of V2b (pin 6-TP 4) using a cap probe. The noise is gone completely as well. Yet if I measure the DC voltages at those nodes, there is nothing that would indicate a problem so far. I've eliminated any of the existing tubes as the problem.
Here is a couple of other interesting things; I've found examples of people with this model, who've experiencing noise problems which I suspect might be cause by the same thing. But, specifically, I found a thread where a guy posts about a hum problem with the same exact symptoms (IE. pulling V4/opening the effects return, etc ) Many of them are looking for the 6212 schematic with the hum balance pot. But I couldn't find anything where the issue was identified and solved, at least searching this forum for a while and looking at Hoffman's site.
But, I know some of you long timers have been on some of these threads, so if anything comes to mind, I'm all ears (or eyes, I suppose)
edit: Oh yeah - I unified the schematic into one layout, and highlighted the points in the schematic I'm addressing above. There's a chance something might have been deleted, or I made an error combining the schematics. If you find anything, let me know so I can correct it
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