5e3 hum

[Noval_novice]

I have an issue with a 5e3 kit build I've just completed. There is a tremendous amount of hum (and no guitar signal) when the bright channel volume is turned past 1 or 2. The normal channel works well and sounds good. I know the problem must be between the bright channel input jacks and the volume 1 pot. Anyone have suggestions as to the most likely culprit?

Thanks in advance!

[FYL]

Begin by swapping V1 for a known good tube, check the input jacks and wiring to V1, check wire routing.

A few suggestions:
http://www.geofex.com/ampdbug/hum.htm

[Phil_S]

Do you know if it's 60Hz or 120Hz? 60Hz is roughly B below your low E string and 120Hz is about 4th fret on your low E.

Got a picture to show us?

[passfan]

Check how the tone circuit is grounded or not. Seems I remember having a problem in that area before with a misplaced connection bleeding my signal to ground. It hums because of the loop.

Also I remember building off a cieratone layout where the ouput section ground was looped through the tagboard to the preamp grounds creating a loop. Check out the original fender 5e3 circuit schematic and ground as per. I had a horrible hum until I discovered that and it made me mad. You owe it to yourself to get a Jensen Electric lightning 12" to mate up to it, you'll be glad you did.

[Noval_novice]

Thanks for the suggestions. It was just due to my sloppy V1 wiring. My filament heater wires (and the artificial center tap) happened to be extremely close (read "touching") pin 6 of V1 --> presto! Hum city!

[Phil_S]

Yeah, 60Hz. That's why I asked. It tells you where to look. Glad you got it.

[Noval_novice]

Phil,

Just to be totally dense, what sections of the amp are likely to cause 60Hz noise and which are likely to cause 120Hz noise?

[FYL]

50 or 60 Hz = mains induced noise from heaters or any AC-carrying wires from the PT.

100 or 120 Hz = recto related buzz from the valve and filtering caps. Usually compounded by signal routing/wiring as well as grounding.

[Noval_novice]

Right. That makes sense. Thanks, guys.

[Noval_novice]

Ok. So now the amp actually works with both the normal and bright inputs, but at high volume settings I get some fuzzy decay to notes like bacon frying. It is more noticeable with humbuckers than single coils. It is still noticeable if I hook up to an extension cabinet with known good speakers. Cold solder joint? Oscillation? Any ideas? Now if only I could find a chopstick . . . .

[FYL]

but at high volume settings I get some fuzzy decay to notes like bacon frying. It is more noticeable with humbuckers than single coils.

Could very well be blocking/farting. The 5E3 was designed for nice and easy playing using single coils, not maxxed with big bad modern humbuckers. Too much drive and the cathodyne enters bloatware mode, then recovers slowly.

Correcting this while keeping the 5E3 tone is very easy:

- Add a 100K or more Rs grid stopper to the PI section of the cathodyne (cf. the schemo below). Some go up to 1M.
- Lower coupling caps from 100n to 22n (best with humbuckers), 47n or a combination (22n between V1 and the PI, 47n after the PI).
- You may also play with V1's cathode bypass cap, going from 25µ to 10µ or 4µ7 can give excellent results with humbuckers.

[Noval_novice]

If it is blocking distortion from the cathodyne, it is necessary to put higher grid resistors on the power tubes? Right now I have 1.5K in place on both.

[FYL]

If it is blocking distortion from the cathodyne, it is necessary to put higher grid resistors on the power tubes? Right now I have 1.5K in place on both.

1K5 is OK. The ugly disto comes between the gain and the PI stage of the cathodyne, not after it.

[Noval_novice]

Installed the 100K grid stopper on the cathodyne PI but it didn't help. Should I change cap values as suggested above or look for some other problem?

[FYL]

Installed the 100K grid stopper on the cathodyne PI but it didn't help.

Try higher Rs. Some find the right spot at 330K, 470K or even 1M.

Should I change cap values as suggested above

Smaller caps are a must if you use humbuckers, so yes, go ahead.

look for some other problem?

If the stopper + caps mods don't work...