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I have a twin reverb that has a hum when the master is turned up. The hum stops increasing at 5, but remains constant. From the looks of the circuit board the amp has been reworked. The balance control is wired for bias adjust. Caps are orange drops on all coupling. Ive gone thru the amp with a lead checking grounds. I did notice a blue cap wired to pin 7 of v5 or the one that handles the clean channel. I dont see this on the schematic. Any ideas of what could have been messed up when an amp is blackfaced. The amp is real loud. A lot louder than my 73. Also the reverb doesnt work, this might lend a clue as to a common issue that could also cause the hum. If I can kill the hum the amp will sound great. Is there a home made jumper or something I can do to test it. I moved all the wires with a chop stick, no difference. The higher you idle the tubes the louder the hum ( and the amp). Thanks for any suggestions.
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update
The tube thats different from my 73 is v1 and the cap and resistor are on pin 8. I tryed substituting some other tubes from my 73(there brand new). The amp squealed violently when the channel volume was crack to one. I tryed 3 differrent ax7 and two at7 same result. When I leave out v1 and run the reverb channel the hum is lower, but still there. The cap that is on v1 pin 8 looks like some blue ones from radio shack. It's a .68 k 250 volt. What was the amp tech that wired this trying to do? If I rewire to stock will it correct the hum? -
Pin 7 on a noval socket is the grid of the second triode in the tube. As far as I remember, Fender used these grid caps in SF amps to prevent the amps from oscillating caused by a rather sloppy lead dress. This was partly the reason, why SF Fenders don't sound as good as the BF ones.
For the reverb problem you might wanna search this forum for hints. There are several threads here that cover this issue. Maybe that's what's causing the hum.
Pin 8 on a noval socket is the cathode of the second triode in the tube. I assume the cap you're talking about is a .68uF at 250v. That's a rather small value with a rather high voltage rating for a bypass cap in a Fender amp. Fender mostly used 25uF at 25v as bypass caps.
Smaller values will generally make the sound less fat.
Anyway, I don't think the HUM is caused by the cap itself but maybe this "somebody" who had modified the amp put the lead dress wrong and caused hum this way.
Do you exactly know what circuit design your amp has and do you have a layout for it?
I would one after one pull the preamp tubes and find out when (if) the hum stops. Just to determine if it's in the preamp or poweramp.
Also check the bias properly.
And be aware of LETHAL VOLTAGES!!!
Hope this helps
MattComment
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