67 Bassman Hum

[Valvehead]

Are your grid voltages equal now?

much better , they are .3 apart at most now .

as in -48.3v / -48.6v and so on

[Valvehead]

Update to the madness...........

I replaced the 4 220k , the .022's , (also the .1uf caps after the PI ) . Same hum with the PI tube pulled out.

I tried something else---pulled a pair of GT 6L6 out of a personal amp and put them in , bias is at 23ma per tube , powered up = NO HUM , no noise !!! Do i have a pile of noisy 6L6 ?? maybe so .

So previously knowing the amp passes signal , I put the PI tube back in thinking we are done ---The hum is back !!

i then swapped out all 3 of the pre- tubes (again ) + the PI tube ....no change .

[Valvehead]

Another update.
I got another matched pair of 6l6 and with a bit of adjustment on the hum pot the hum is very low now....90% quieter . Now Im trying to find the random static/ crackles in the pre amp...Thinking a noisy plate resistor...but I think i read that the old blue coupling caps are notorious for going south ??

[Tom Phillips]

...but I think i read that the old blue coupling caps are notorious for going south ??

I don't know where that idea comes from. In my experience, the blue tubular caps used by Fender in the 1960s are very reliable and, to this day, it is very rare to find one that has failed in any way. However, the yellow tubular caps used before the blue arrived are often found to have failed.
It's distressing to find old Fender amps that have had all the blue tubular caps replaced in shotgun fashion.

[The Dude]

You can possibly narrow down the gain stage that's crackling by simply removing preamp tubes until the crackling is gone. Of course, this eliminates only the tubes after the pulled stage and any before and including that tube could still be the cause.

[Greg_L]

I've got an original mid-67 AB165 here with me now. The blue caps are the only things that haven't gone bad.

[Valvehead]

Thanks for all the advice guys - I think its good to go.

The crackling was 1 of the 220k in the preamp , it actually crumbled when i took it out .

Amp is sounding great at this point

[pdf64]

Prodding every component while the amp’s running would probably have revealed that kinda thing.

[Valvehead]

Prodding every component while the amp’s running would probably have revealed that kinda thing.

i chop sticked , used freeze spray and also a heat gun