Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jerrydyer on October 15, 2013, 11:19:42 pm
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Hey guys long time no talk... :sad2:
I am doing some tech work and I have a re issue twin reverb with a humming reverb tank. remove tank no hum. tried replacement tanks, all hum. I saw a lot on google but no one said that the hum disappeared when they disconnected the tank so????? thanks in advance.
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Are the correct jacks being used for input/output on the tank?
Generally, one side is left floating and the other is grounded (the standard tank has the output back to the amp grounded). I wonder if the cables were swapped if that would result in hum. After all, the output from a reverb tank is pretty weak.
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I did try every combination known to man to make sure. tried two other tanks as well. it seems others have had similar probs with the reissue some guys have had resolution some not. leads to me beleive there is a real issue here and I need to find a detour cause it aint gonna get resolved thru fender.
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Hum or kind of feedback ?
Reverb cables are good shielded ?
Reverb in circuit ;
1- If you remove 12AT7 tubes feede reverb transformer ? hum or not ?
If hum ; Reverb transformer ?
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defintately hum
swapped that 12at7 but did not remove it to see ... I will do that next prob a cheap little trannie right?
reverb cables are stock fender cable rca with red and white plugs.
thank you
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defintately hum
swapped that 12at7 but did not remove it to see ... I will do that next prob a cheap little trannie right?
reverb cables are stock fender cable rca with red and white plugs.
thank you
Yes try remove 12AT7 .
Trany , could be if hum with 12AT7 remove OR defective Fender RCA cable .
Do you have a 'scope ?
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Once, LooseChange fixed a reverb hum by relocating the 220kΩ resistor from the return jack to the rest of the circuit for that tube, so the resistor was grounded alongside the cathode resistor of the stage.
In that case, it seemed to solve a ground loop issue; one which doesn't appear in every Fender amp with reverb.
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Did you check the voltages of the reverb return and recovery tubes? Move the wires on the RCA plugs that are soldered to the board and see if the hum is lessened. I just remove the RCA jacks where they are located and run new wires from the resistors onto a new location on the rear of the chassis like they were on the old fenders. Those wires are so thin and fragile. Almost tinsel wire and they may be only hanging on by a single strand of wire. If it a customer fix then really to be sure the amp wont come back you have to relocate the RCA jacks and use a heavier gauge of wire. Maybe a cathode cap as well. Some of these amps act up with issues a little differently here and there. JMO
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thank you I will be back at it tomorrow
Im working at Luxxtone Guitars doing some warranty work for Guitar Center.
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One more thing of importance:
Get rid of that hum pot circuit on the back of the chassis. Just solder (2) 100 ohm resistors to each leg of the pilot light to ground. Clip the wires (pins 4-5 & 9) off the V1 preamp tube to the hum pot. I see and get a lot of these amps with hum issues. People turn the hum pot adjustment not knowing that there is no way you will ever know if the filament voltage is balanced and equal on each half of the tubes unless you are (A) an amp tech or (B) remove the chassis and measure the voltages of each tubes filament with a DVM.
Maybe someone can answer this question:
Why did they ever use a hum balance pot in a guitar amp? Common in Hi-Fi though
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Maybe someone can answer this question:
Why did they ever use a hum balance pot in a guitar amp?
Because they work! Glory days of tubes and NO high gain amps who cared?
Just like putting in a bias pot or (maybe even) a bias pot and a balance pot. They didn't need it.
I worked for a while at 2 music stores in my area fixing amps. I did have a couple of Fender PCB amps that came in that had the hum balance pot. They were not brought in for hum issues but while I had them in my hands I played with the hum balance pot and it worked great. Just by ear no meter. It was very easy to hear on the amp/s that had it.
Brad :icon_biggrin:
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One more thing of importance:
Get rid of that hum pot circuit on the back of the chassis. Just solder (2) 100 ohm resistors to each leg of the pilot light to ground. Clip the wires (pins 4-5 & 9) off the V1 preamp tube to the hum pot. I see and get a lot of these amps with hum issues. People turn the hum pot adjustment not knowing that there is no way you will ever know if the filament voltage is balanced and equal on each half of the tubes unless you are (A) an amp tech or (B) remove the chassis and measure the voltages of each tubes filament with a DVM.
Maybe someone can answer this question:
Why did they ever use a hum balance pot in a guitar amp? Common in Hi-Fi though
its a re issue