74 Super Lead radio interference

[Reeltarded]

That is when a cap on the input will most likely work.

[chadb]

Not sure how large a hole in the chassis has to be to allow radio waves in, but if any of the holes you've described as 'hacked a bunch of holes in the chassis' are larger than, perhaps 1/4", you might consider covering them up with metal plates that are well-grounded to the chassis. Certainly, if there are any holes large enough to host a tube socket, I'd start there.

I'd never actually thought of this. There are in fact 2 empty 9-pin socket holes between the 3 preamp tubes. Whoever modded this thing added a tube buffered effects loop that I removed. Off to the hardware store for some 3/4" steel hole plugs! Thanks!

[chadb]

Sorry Reel I mispoke. What I meant to say is "If I AM getting this interference without an input cord plugged in."

[Cameron]

That is when a cap on the input will most likely work.

Yes but put the cap from ground to ground. On the old Marshall there is a stand off right near the input. Put a ground lug there and put a .01 cap from the input ground to the standoff ground. It will send the radio signal that comes through the cables ground to chassis ground.Also ground the first cathode ground to the input ground.

[Analog Assassin]

What I did in my amp was use RG174 shielded cable from input jacks. Took input resistors off the board and soldered them directly to the tube socket. Capacitance just looks like you added a few inches of guitar cable, negligible effect on tone, but no more pesky RF problems, not even any cell phone interference when I leave my phone on top of the head.