scratchy guitar pots with 5E3

[k51502]

hi,i'm having a bit of a problem with a 5E3 that i built.it's causing the pots on my guitars to get scratchy.the guitars themselves are not at fault,they work fine in my other amps.
so i thought maybe i was getting some DC leakage from a cap in the amp.
so i changed the bypass caps and it still persists,next i changed all the coupling caps again it's still there,i even changed the filtering caps and it's still there.i'm at a loss now,even poking around the wiring with the amp on doesnt seem to lessen or worsen the problem.
so can anyone give me some pointers please.
i'm using Sprague electrolytics throughout the amp and mallory coupling caps.
one thing i'm curious about,can the input jacks pick up any DC or interference due to the close proximity to the board.
hoping someone can help

cheers

karl

[dai h.]

possibly the board is conducting (i.e. so called "tweed disease" which can even happen on new boards apparently) or maybe the tube itself is bad

[k51502]

well after much searching i found out the problem,it was a parasitic,possibly the tweed disease you mentioned.the solution was to remove the 4 68K resistors that the inputs are attached to from the board and mount them to the actual jacks,bingo problem solved.
it's amazing that for no obvious reason things like this can happen,ah well i've just ordered a turret board from ThunderTweaks and will install that instead of the fibre board.

thanks for your help

karl

[dai h.]

well it sounds like you fixed it, so maybe you don't need to replace the board? To be specific I'm talking about the insulating board material absorbing moisture or possibly carbonizing thus going conductive and causing voltage to show up where they shouldn't be (like grids). Some board material is more prone to absorbing moisture than others (like paper and cloth phenolic versus glass epoxy, say). So did you measure a voltage on the grids at the input and this went away when you lifted and moved the board connections (if I'm understanding you correctly)?

[k51502]

i didnt measure anything i was reading one of Gerald Weber's books last night and he mentioned that DC can be imposed on the grid wires so i decided to alter the layout move the wires aroung etc. and the result was no more scratchy pots.i'm still going to replace the fibre board,i prefer the heavy duty turret boards that ThunderTweaks make their excellent.i just should have done it to start with

karl

[dai h.]

re: Gerald Weber:

not everything is wrong but watch out what you read in his books and articles because some of it is completely wrong

[k51502]

ah dont i know it Dai,basically i steer clear of his opinions but i for the majority of the time i heed something in his books i try to double check it with another source.

thanks for the
help Dai

karl

[dai h.]

no problem, glad you got it worked out

[pezed]

I have had this problem with my 5e3 and it was a bad tube in the first preamp.. it only does it with 1 tube i have, a really old 12az7 which sounds excellent, but for some reason its scratchy on the controls and hums. weird.

[Bruce / Mission Amps]

I've also encountered a few older preamp tubes that seem to "leak" (for lack of a better term) B+ from either the plate or cathode bias.
Not sure why it happens but it does.