'55 5E3 hums louder as it warms up.

[jhundt]

My old 1955 5E3 hums. It always has to a degree; it is not a silent amp. It has been on long-term loan to my nephew, who uses it in his home studio. He has always been very happy with it, but recently he mentioned that it was 'losing it's character' and that the hum was getting too loud for recording.

I replaced the three filter caps with FT 22/500's, and I banged in a new set of JJ tubes. The power tubes were old black-glass Sovteks that I bought from Mike Mathews 'New Sensor' company, back in the early 90's, marked 'made in the USSR'.

Now I think the amp sounds good, but it does still have a distinctive him. And the hum may indeed be getting louder after the amp is on for an hour or so. (or I may be thinking that it is louder, the power-of-suggestion coming from my nephew's comments).

Is there anything that anyone knows that might cause an amp to hum louder after it has been on for an hour?

 

[slider313]

Check every resistor to be sure they're within spec. A mismatched set of power tubes will cause hum. One of those power tubes may be drawing more current after it gets hot, causing the mismatch.

 

[dankilling]

V1 plate resistors may be going, and as they heat up they could change sonically

 

[Wally]

Plus one on knowing the voltages and biasing.
Turn the amp on and play until the hum increases. Unplug the guitar and pull tubes…starting with V1…to see if and when this hum goes away.

 

[dan40]

How old are the other electrolytics on the board. The smaller 25uf caps may be dried up if they are originals. Preamp tubes can also be a source of hum from either heater to cathode leakage or becoming gassy.

 

[NTC]

Are the grounds for the input jacks making good contact? Are the shorting connections in them still shorting?

 

[jhundt]

thanks for the suggestions everyone. I am giving the amp back to my nephew to play in his studio for a while, while I take a three-week trip. I will wait to see what he says before I try do anything more. But please keep the suggestions coming!

 

[King Fan]

Have a nice trip. When you get home, hum diagnostics:

Is it 50/60Hz, 100/120, or something else?
Is it worse with a bare plug in the input jack vs. nothing?
Is it raspy / buzzy or smooth / hummy?
Does it vary with the volume knob(s)?
Both channels?
Does the heater have a CT? real or artificial?
Where is that anchored?
Yes voltages, including all the B+ nodes…
All the other suggestions above (Wally's stepwise tube pulling is an especially telling early diagnostic…)

Let us know!

 

[Gunny]

The original 5E3 amps have no power supply choke, only a 5K resistor. Total reduction of hum is never possible. If you're willing to do a mod, install a choke. I did that in a tweed Champ kit and it made a world of difference.

 

[Lowerleftcoast]

Total reduction of hum is never possible.

???
Technically, I suppose you are right but a 5E3 can be quiet enough for studio use. Mine seem to be as quiet as my Deluxe Reverb. I doubt I could notice a reduction in hum if I installed a choke on my 5E3s. They are already extremely quiet.
The OP says there has been a change in the amount of hum. The hum may increase after running the amp for an hour. He has installed new 22uF filter caps and installed new tubes but still has a distinctive hum. Imo, a *distinctive* hum is not a trait of a 5E3.

 

[jhundt]

thanks, I will get back to this in three weeks.