Peavey ValveKing 100 Overhaul

[physics]

Hi Guys

There is no interaction between the plate winding of the PT and the circuit in the manner described. The cap across the winding is too-low a value to have any such interplay; rather, it is there to help suppress noise, as described in TUT3 and elsewhere in the TUT-series.

I see, thanks.

The noise suppression function is bilateral in that noise coming through the PT to this winding MAY be suppressed slightly by this cap, while at the same time rectification noise can also be very slightly suppressed. Limiting factiors to effectiveness are the lack of known resistances or impedances for the cap to work against.

Yeah, the lack of any explicit AC voltage dividers seemed weird.

In any case, the cap you pulled out is a multi-layer ceramic cap (MLCC) which has quite a range of characteristics good and bad. I would not bother installing anything similar, and broadly would not use a ceramic in that position. "PP or go home".

I see. Given the apparent tendency for ceramics to fail short, I think I'm starting to develop a bias against them too. In any case, based on your input and other research I don't plan to put one back in right now, especially not one of the ancient bulk ceramics I was initially considering using as a temporary replacement.

Considering that the filter caps in the plate supply are rated for 550Vdc, the AC from the winding could be  as high as 385Vac, and may be higher with no load. A safety cap rated at 440Vac will work here. Choose a polypropylene types, which in such applications is a box-style format. Yes, there are ceramic safety caps but why go there? As mentioned, you do not really need this cap - the resistors across the first filter cap are far more important for safety.

Noted, thanks.

There is a Technical Article on my site that discusses stand-by switches. Such switches are illegal in some countries.

I forgot to check the website, thanks. It was an informative article, but I'm still left wondering about the physics behind cathode stripping/bombardment, and why it's not a problem at our plate voltages. Can you or anyone else recommend good books/papers on the physics of vacuum tubes? It's okay if they're math and physics heavy. Right now I'm reading through Theory and Applications of Electron Tubes by Herbert Reich, which came highly recommended from tubebooks.org. Anyone have any opinions on the book, or additional sources I should look into?

Anyhow, I added a fuse holder to the board for the HV fuse, left out the cap, and soldered a jumper wire in place of the standby switch. While I'm still uncertain about cathode stripping, between what KOC said and everything else I've found so far I haven't found a convincingly thorough argument for it being a problem at our plate voltages, plus my shoddy napkin physics seems at first glance to suggest that it's not a problem until around 3kV, give or take a bit due to electrode spacing. That, and the whole safety thing.

Next mods in the pipeline would be bleeder resistors and replacing the screen resistors with 1k 5W flameproofs. For the bleeder resistors, I'm figuring on installing roll-swaged eyelets in the relevant traces and then installing the resistor(s) in the eyelets. Anyone got any better ideas? Eyelets seem the cleanest, most repair friendly solution to me so far, though ideally they'd have flange swages if I read the NASA soldering standard correctly. I'll probably install eyelets in the old screen-resistor holes too, so that it's easier to service should they fail in the future.

Thanks KOC for all the info and input you've provided!