Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: RightLurker on August 09, 2014, 01:22:05 pm
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I'm working on a '67 Fender PR that's giving me fits. When the reverb is turned up, it is accompanied by a loud hum. The reverb works fine, it's just accompanied by hum. When the reverb knob is turned down to 1, the hum disappears. The reverb driver and recovery tubes are new and known good, as are the reverb cables. The reverb pan is good. Disconnecting the reverb cables from the amp doesn't change the problem. The hum appears and increases when the reverb pot is turned up (there is of course no reverb effect). The reverb transformer is new. All of the grounds in the amp check okay. I put a bias adjustment circuit in the amp about two years ago, and it worked fine and sounded great for two years - now this. Any ideas? Thanks in advance.
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Hi Right, If your grounding checks out OK, here is a couple of things :-
If the tanks RCAs outer sleeve is grounded on both remove one of the grounds
If the INPUT to the tank is near the PT it will pickup hum from the tranny, so either move the tank away or end for end it
I don't a RCA connector at the AMP end but rather solder the wire directly to the tube/transformer.
:icon_biggrin:
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Sounds like you've checked the obvious and probable culprits.
Minor possibilities remaining;
Maybe the .003 output cap from the rev recovery stage is bad. (low possibility)
Are you sure your power supply caps are good? Especially the one/ones at the highest B+. The Rev driver gets big volts (compared to any of the other 12A_7's) and while you would expect such an issue to produce hum elsewhere, the output stage (the other place which gets the big B+) is in a push-pull config which tends to cancel hum. But that B+, applied to the single-ended reverb driver, could produce hum there which would pass into later amp stages. This line of thinking gets my biggest suspicion. Maybe your first 1 or 2 electrolytics are not doing their job. The preamp tubes have later, lower voltage / lower current electrolytics of their own and can probably tolerate
"less than normal" filtering from the first couple of e-caps. You could just try bridging the first or first & second e-cap(s) with 20 uf /450. Be careful, please!
You could also check for AC volts on that first and second filter caps. If you do, check both before and after you bridge in your temporary replacement caps.
And yes, it is absolutely possible that one cap in a multi-section electrolytic can go bad while others still work.
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Disconnecting the reverb cables from the amp doesn't change the problem. The hum appears and increases when the reverb pot is turned up (there is of course no reverb effect).
I think that indicates that the problem is in the recovery stage?
Brad :think1:
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Thanks to all. Eleventeen's suspicion re: the filter caps has me thinking - it does sound like a bad filter cap. I'll report back when I get it sorted out.
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Reverb recovery circuit parts as Brad said above.
Push on the board with a wooden tool and look at the solder joints very closely with magnification.
Check plate load resistors and see if they on spec
Look at every part in the reverb recovery circuit
What's the voltages look like?