could it be a bad filter capcitor ? i dont see any external damage, but might be worth changing them out before changing out the transformer
Yes, it sure is pointing in that direction, A PSU filter cap or caps are going bad.
A change out, thats what I would do, it’s looking that way by the sounds of things.
Also to confirm, and assuming you don’t have a scope at hand, and most importantly of all, if you have experience at working on live circuits.
You could easily make up a home made DIY probe if you have some parts laying around to test out the HT and other power supply lines. This is to check audibly, if the hum is anywhere in those supply lines.
You will need an axial high voltage coupling capacitor, rated no less than about 300V and somewhere in the range between 0.022uF to about 0.1uF. This is to block the DC voltage but let any AC through which will be the noise if any of your PSU caps are on their way out the door.
A guitar lead that you can cut up and an amp to listen to.
1, Cut one end off the guitar lead. Strip it back about eight inches or so.
2, Solder one end of the capacitor to the guitar + signal wire and wrap some electrical tape around it and make it safe.
3, Straighten the other end of the capacitor lead and if need be, extend it with short piece of solid wire or a straightened out paper clip or something like that and solder it on. Then tape that up too and make it safe. Leave a little piece at the tip bare.
This is your probe tip !
3, Twist the shielding braid into a wire and wrap some electrical tape around it, but leave a little piece at the end so that you can securely connect it to ground/earth/chassis.
4, Plug the jack into your amp and keep the volume WAY down.
5, Turn everything on, go probe the + side of those electrolytes and the other + VDC supply lines and listen to the amp for 100Hz hum.
100Hz sounds like G# -ish on the sixth sting of the guitar.
Pull the tubes out one at a time. On the tube circuit board valve sockets, test pin 1 and 6, those are your annode pins, Remember, read the pin number counter clockwise when looking at them from the front of the socket.
If you have noise on those pins then I would certianly say to change out those filter caps on that board.
6, Take your time and be careful that you don’t make a slip, or else blue lightning and grey smoke will appear in milli seconds if your not careful.
WarningDON’T try this method if you don’t have any experience at working on live circuits !
OK, that’s a cheap, easy and effective way,
You should do the noise mod anyway, and change all of those electrolytes in there. They are probably about thirty years old and have drifted well out and have a high ESR.
MJMP can supply you with the full kit.
Rusty.