How do I get rid of the hum in my '63 Fender twin amp?

[chasingpeacde]

I tried once again the search option here with no luck so I thought I'd ask all of you gurus out there. I'm in the process of trying to get my old '63 twin amp going after about 20 years of sitting around. I'm told that this year has a fixed bias setup. I finally broke down and bought all new tubes for it. Then I found that I had a bad tube socket in one of my pre-amp tubes. I replaced that one with a new one. Now the amp plays and sounds great except for the constant hum that I'm getting. It's the same regardless of the ground switch position. What do you all suggest? Thanks.

[67polara]

You didn't by any chance unwrap the filament wires on the tube socket you replaced?  Did you rout the wires the same way they were a 1/4" off and you have a chance of hum happening.

Tony

[Danskman]

First, disconnect the ground switch and install a 3-prongs mains cable.
+1 on 67polara suggestion.
Maybe your '63 amp needs some other repair, like electrolytic filter caps replacement (power supply, bias supply filter caps, cathode bypass caps)...
HTH,
Danskman

[LooseChange]

You need to change out ALL the electrolytics. Add that grounded power cord. Be sure the Heaters are center tapped or use an artificial center tap through two 100 ohm resistpors to ground.

Power supply noise is a hum. Heater issues sound more like a buzz.
Hope that helps.

[Danskman]

heaters uses native 120V 60Hz voltage = hum
Power supply (rectified) gives ripples at double the mains frequency (120Hz) = buzz
Note that bad grounding in an amp can give 120Hz and upper harmonic buzzes
If it's a original power transformer, the heater secondary wires are center-tapped inside the PT (referring to the AB763 Twin Reverb amp schematic). You shouldn't need the two 100ohms resistors to ground to create an artificial center tap.
HTH,
Danskman

[LooseChange]

Clarify...
To my ears the sound of a heater circuit with a disconnected center tap (of any type) is a buzz.

[chasingpeacde]

Thanks for all of your responses! First, I guess that I should've told you guys that my amp had the 3 prong power cord added years ago. Also the caps were replaced as well at the same time. Bear in mind that this amp hasn't been operated for about the last 15 years or so since I was moving around alot for awhile and now it's been sitting in my basement for the last 9 years. I think what Tony (67polara) said made sense. I think that I might've unwrapped some of those wires to that pre-amp socket that I replaced. I'll start with correcting that and see where that takes me. How do I check to see if the heaters are center-tapped? What exactly am I looking for here? FWIW, it appears that the transformers are the original ones through date code markings. Thanks again guys.

[Danskman]

Buzz, hum, who knows?? What I was saying is that, following my own ears, a hum is lower in frequency than a buzz. remember I'm not a native-speaking english person... For me, buzz could be compared to hum like a honeybee vs. a bumblebee.
Didn't want to be The Definite Wise Man  
regarding the filaments wiring and the center-tap, your Twin could be equipped with
either a internally center-tapped heater wiring, with only two (generally green) wires coming out from the PT
or a non-CTapped PT, with two 100 ohms resistor two ground, generally located on the pilot lamp socket.

if the heater circuit is center-tapped, you should measure a very low resistance to ground when all the tubes are removed. otherwise, the resistance should be infinite.

Edited: I attached a pdf showing an extract of the original '63 Twin Reverb schemo and layout.

[chasingpeacde]

What I meant by a "hum" was a static-free sound that would sound similar to if you tried to say "Hmmmm" with your mouth closed. I went ahead and removed the head from the cabinet and de-soldered the wires from the tube socket, then twisted them back to the way that they're supposed to be, then re-soldered the wires back. After that, while I had the head on the table, I went ahead and started spraying my pots with electronic-grade contact cleaner because I had a couple that were crackling. Well, I got to the volume pot on the Vibrato side and while exercising the pot it got harder and harder to turn until it froze up. It got so bad that I couldn't turn it with a pair of Vice-Grip pliers! So I went ahead and ordered a new pot. Now I have to wait for that to get here to see if I still have a hum or not. Let's keep our fingers crossed! I'll keep you all posted. Thanks again.

[Dave]

Sounds like that contact cleaner is pretty good stuff. Does it double as Super Glue?
If you had unwrapped the heater wires when you replaced the socket, and now you have rewrapped them, there is a pretty good chance that you have fixed the problem.

remember I'm not a native-speaking english person...

If you hadn't said that, I probably would have never guessed it.

Dave

[Justa]

Thanks for all of your responses! First, I guess that I should've told you guys that my amp had the 3 prong power cord added years ago. Also the caps were replaced as well at the same time. Bear in mind that this amp hasn't been operated for about the last 15 years or so since I was moving around alot for awhile and now it's been sitting in my basement for the last 9 years

FYI,
Electrolytic capacitor life is reduced when they sit for years like this without voltage applied making them more likely to fail early.  Turning your amp on and allowing voltage to be applied to the caps on a regular basis helps build back up the dielectric layer between the plates of the capacitor.

Have you double and triple checked the wiring to the tube socket that you replaced?  Have you measured the voltage on all the pins to see if anything looks abnormal.

First thing I would look at like the other guys suggested is the heater wire location and wraps.  Make sure to check the entire heater wire length runs.  That is a long amp and it is easy to bump them out of place where you do not expect to.  I like to look at the position of all the wiring when I am done working on an amp before buttoning it up as I usually find a couple things out of position.

[LooseChange]

IMO, it's the electrolytics... And don't forget the Bias supply filters too.

[moody]

Thanks for all of your responses! First, I guess that I should've told you guys that my amp had the 3 prong power cord added years ago. Also the caps were replaced as well at the same time.

The caps may well need replacing again.  Without being used they won't last.