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With both volume controls set to zero, amp is quiet. Turning up NORM channel volume control (with nothing plugged into input), all i hear is hiss. Turning up VIB channel volume, i get hiss with a significant hum as well (REVERB control is at zero). I replaced V2...hum decreased slightly, but it's still there. I replaced input jacks, hum still there. I have moved a few of the wires associated with the VIB channel around, no effect on hum. Could there be a leaking cap causing this, or some other component failing? Any way to test for this without shotgun replacement? Thanks for your help!
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The same filter cap feeds both channels. I imagine if it was creating hum then it would be present on both channels. The cathode bypass cap could cause hum, but what I really would check closely are all the grounds associated with the preamp. This includes where pots are grounded to the chassis via lockwashers on the shaft, ground connections to the brass plate, and locations where the brass plate is soldered to the chassis, which usually also includes some component grounds. All these places can get corroded and/or the solder joints fail. Leaking coupling caps can cause hum. the leaking coupling cap causes increased which reveals itself as be higher than normal voltage at the cathode, lower than normal voltage on the plate (of a preamp tube). -
i'm getting some funny voltage readings around V5....the schematic says 415V from the filter cap, 270V at pin 1 (145v drop across the 220K), and 345V at pin 6. in this amp, i measure 391V from the filter cap, 382V at pin 1 (only 9V drop acros the 220k), and 292V at pin 6....i'm thinking something's up with the 220k resistor, but it meters at 215k. Maybe i should replace it anyways....Comment
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V5 is the trem oscillator and you will get some "funny" voltage readings when the trem is off. The negative voltage from the bias supply cuts off the tube current. No current means no voltage drop across the 220K. If you are troubleshooting hum just pull V5 out and see if there is any difference in the hum problem. If not, dont worry about that circuit.Comment
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could the extra noise on the VIBRATO channel just be from the extra gain stage compared to the NORMAL channel? it's there, but not as bad when the tone controls are turned down.Comment
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yes, the vib channel has more noise than the normal channel, and if you turn them both up to 10 (not at the same time) you will notice a difference. But your questions relate to hum, which is different than noise, unless what you are calling hum is what most would refer to as "noise"...Comment
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