Oscillating Parasites?

[lowell]

Hey all,
Got a Hot Rod Deluxe III here. I think we're all quite familiar with these amps at this point.

It has a bad hum, and crackling. They coexist and both come and go depending on a couple things. After much investigation, I've found that 2 things get rid of it. Decompressing the bright switch (bright off), and pushing the 1st shielded grid wire closer to the gray ribbon cable. Seems an obvious case of parasitic oscillation. I've tried a new tube, pushing on everything with my sharpie, checked continuity between things, voltages etc... actually even if I move my hand around the amp it comes and goes. This amp is like an antenna!!

[lowell]

Yep, disconnected the grid wire to the first stage, inserted a 68k... fixed.

Sure did seem like a bad tube or hum issue... sneaky.

[Pedro Vecino]

These strange symptoms around the first tube in HR Deluxe/Deville can be treated by connecting a wire between the shield of the coaxial cable and ground screw, next to the input jack.
I've only seen once and it was very disconcerting.

[lowell]

Agreed. And yes I thought about adding wire, or replacing the coax cable with foil shielded cable... I think the grid stopper has the same end effect - rolling off the RF frequencies thanks to the Miller capacitance.

[g1]

This is an issue that is present when it is all buttoned up with rear cover/shield connected, yes?
These things can give you weird problems just because you have them opened up, probes connected, etc.
Just want to make sure you are dealing with the original complaint, not some incidental fault while servicing.

[lowell]

Thanks for checking... well, I wasn't hearing any issues, except a slight occasional crackling. Customer said sounded like preamp tube was going. It DID kinda sound that way, surprisingly. I wasn't concerned but wanted to be sure. As I removed the back panel the problem increased ten fold. So my prognosis was indeed that there was PO, and adding that grid stopper fixed it. Buttoned it back up and no issues whatsoever. All in all, I think the issues are the same, just worse without the back panel.

[TimmyP1955]

A 68k in addition to the usual 68k on each jack? Or were there no 68ks? The schematic that I have shows the usual Fender input stage with 2x 68k and a 1M.

[lowell]

I moved the 68k to the tube pin directly, instead of it being on the main circuit board and having to travel through the ribbon cable. Made a huge improvement. Sorry i wasn't clear on that.