Old Amp Hum

[chunkitup]

Yea another hum thread. I just replaced all caps and resistors in this amp (1940 Gibson EH-150) and the amp is alive and plays well but there is a hum. I dont know if hum was acceptable in the 30's and 40's. Volume does not change hum. I replaced the 2 prong plug with a grounded plug. I pull the first tube-no change, second tube-no change, third tube- no change, fourth tube-hum goes almost completely away, barely audible. I try all the tubes in the 4th position and each one brings back the hum which takes the 6c5's off the list of possibilities. I pull the wire from pin 3 of v2and3 and the hum goes away. So why didnt it go away when I pulled those tubes?
Next on geofex hum possibility list: mismatched output tubes-I used 2 seperate pairs of matched tubes and no change in hum.
Next-Bias Supply Caps-No caps cathode biased
Next-Unbalanced Filament winding-There is a CT which is grounded, perhaps I should put in the faux CT
Next-Input Jack-they are grounded to the chassis but if removing V1 or V2 doesnt change it then it cant be the jacks
Next-Poor internal wire routing- Perhaps but it is the original wiring
I am ready to update this sucker with all new wiring but I dont want to. Could it be a tranny problem? or do you think it is probably a grounding problem?

I have included a pic of the innards. The tag board has all new parts but everything else is the same.

[Twist]

Did you replace the coupling caps also. If not then do it. Also I would increase the capacitance value of the first filter cap. Try 22uf or even 50uf.

[JHow]

I couldn't tell from your picture, but when you replaced filter caps, was it a can-type setup or were the filters axial-type in the chassis? In either case, are filter grounds a good connection?

[chunkitup]

And JHow is the winner..... sort of!!! When I got the amp the big blue Mallory (not original)in the pic was already there so that is the only cap I did not replace since it measured fine. But if you notice in the pic where that big blue guy is grounded. Sometimes it is right in front of your face and you cant see it. Whoever added big blue grounded it to the tag board. I grounded it to the main ground on the tranny bolt and the amp is dead quiet now. The customer was very happy to hear the amp that his dad had in storage for decades but was non operational. Cool little amp. I noticed some boutique amps these days that use 6c5's the same as in this amp. The tubes definitely sound good.