Selmer constellation 20watt with hum after 30 mins plus

[Arbutt]

Hello

Sorry its a long post , please read it all thanks

Ok this was given to me with a lot of hum and noise issues
1.Replaced all Filter caps, Not Original but changed again 2 x 47uf and 1 x 22uf
2.Replaced power tubes New JJ EL84
3. Replaced EZ81
4.Replaced Bias cap
5. Tested pre amp tubes all good,
6. Cleaned pots

At this point all seems to be good

Running on test after about 30 mins + it would go from quiet to excess amount of low freq hum
" Im writing this from home and in my notes and i didn't write down the freq ( Memory issues these days "
selmer_constellation_20w.pdf
Further checks
Monitored Bias- B+ voltage from cold and after 30 mins all good and no change
Remove V3 ( PI) and the hum goes,
Leave V3 in and Remove V1-V2 No change
Removed the board and cleaned it up, Checked and resoldered.
Decided to change C10 -C16-C20-C19 and tube base
Caps C1 -C11-C12 have been replaced by previous tech
When hum has returned, Using the scope i can't see it on the B+ or bias
Bias at 18- 20ma per tube
B+ @ 370v
Replaced R20-R19-R21-R22-R29-R33 ( Many Way out of spec)
Cleaned the board of wax
V3 pin 1 =160v - Pin 6 =229v- pin 8 = 56v - Pin 3= 1.3v Pin 7 = 28v
V3 pin 2 can't see any source of the hum

I begining to think i have a conductive pcb. Its made in mid 1960's
The fact that from stone cold its ok for a good 30 mins plus

Do you have any throughts ??
Many thanks in advance

[Jon Snell]

If you had a conductive PCB it would do it constatnly.
Fit a matched pair of EL84s. If one is getting weak or one is starting to go gassy, thay will produce 100HZ hum.
Unplugging V3 will tewll you if the hum comes in before the phase splitter or not.
C19/20 leaking could cause excess current draw.

[Arbutt]

Hi jon . If i remove v3 the hum goes not being caused by v1 or v2 circuit. Replaced virtually all the components for the v3 circuit and new matched el84

[Jon Snell]

Are we talking mains hum 50HZ or smoothing 100HZ?
I would remove C16 and test. If the hum has gone, look around the tone control section for a bad ground.If still there, what happens if you ground V2 pin7 and then again V2 pin 2.
Nice amplifiers by the way, I have repaired many and refurbished 2.

[Jon Snell]

Quick thought, remove R31.

Let me know the results of your tests.

[glebert]

Arbutt, does the hum change with tone controls? I found this post (https://oldamps.weebly.com/selmer-co...storation.html) which seems very similar to your amp in regards to the vicinity of the hum.

[Arbutt]

Thanks all your help
I removed the tone control board and Replaced all the tone caps and all resistors" as out of spec"
Been on for about 2 hrs quiet as a mouse
sounds good and all working
Till the next one
Mr A