Peavey Classic 30 Bassman Mod RF noise

[tubeswell]

OK

So I done the 'Bassman' Mod on the Blue Guitar Website - sounds mighty nice too except for one tiny thing.

When I plug the guitar in I get this teency high-pitched RF hum that goes away when I touch the guitar stings or the chassis. Admittedly I was having trouble with this before I did the Bassman mod. I've done other Blue Guitar mods namely master vol, presence and resonance control mods, and I took the shield away from the resonance pot cable as Enzo suggested, but no diff, so I put it back. What am I doing wrong?

Just in case it was an input problem, I put a shielded cable from the input jack to the Grid of V1A, with a 68k Grid Stopper soldered strait to the #2 pin socket on V1, and a 1M to the shield-ground of the cable, on the other side of the grid stopper - tested fine for shorts etc before I stuck it in.

I also put shielded cable in-between the .01uF cap from the cathode side of the cathode resistor for V2B and the VR3/R13 junction (with the shield wired to the chassis). Apart from that I followed the mod. Could it just be that the filter caps are getting past it and are causing undesirable interference? They've been in there since the amp was new in 1995 (although apart from the RF that goes away when I touch the 'ground', I can't notice much else in the way of hum). I checked the current between R61 and R62 with the amp all wired up, and that reads 0Ω.

Its got to be something for crying out loud - GAAAAAAARGH!

[Enzo]

FIlter caps on that are not old enough to be dried out, and if they were, then touching the axe or not would make no difference.

How good is the ground connection between input jack and chassis wall?

When I expressed concerns about the shielding, it was not the shields I was worried about, it was the grounding paths the shields might be upsetting. I want any shields to be grounded at one end only, and do not substitute a shield in place of an existing ground path.

Does the mains outlet you are using have a good solid ground back to the electrical service panel of the house?

[tubeswell]

Well I think I figured it out - I think the traces remaining on the PC board between the input and the Grid of V1A (after I had disconnected R6, C10, C8 and R10) were acting as a sort of RF antenna, so I removed the relevant jumper to the tube board and it seems to have taken the RF noise down a bit.

[Enzo]

Of course, geez I'm stupid: you don't have C10 any more. That was its job. I get so used to looking at the schematic I forgot the changes you made, even though those changes are the point of the discussion. Man, I am getting old...

Your piece of shielded has a few pfs, wouldn't hurt to tack a few more onto the grid to see.

[tubeswell]

What size cap you reckon - a 37pF between pins 1 and 2 of V1 ( or is that overkill) I've got a 37 polyester and a 22 50V disc ceramic. Which will stand up better?

[Enzo]

Wait, which thing are we discussing? The cap I mean is from grid to ground, not grid to plate, try the 22 and see if it helps any. Hopefully there won't be 50 volts on your grid.

Between pins 1 and 2 we had that gimmick cap. Did you slice it off in the process?

[tubeswell]

Between pins 1 and 2 we had that gimmick cap. Did you slice it off in the process?

No I left it there

[tubeswell]

The cap I mean is from grid to ground.

I take it you mean the 22pF from grid to ground. If so, would I be better putting it as close as possible to the grid, (i.e. between the grid stopper and the grid),


Or should I stick it on the input-side of the grid stopper (and if so, does it matter whether its closer to the input jack or closer to the grid stopper) ?

[Enzo]

I'd try where is was most cconvenient and see if it helps any. SImple enough to move it around and see what happens if it does.