Fender 68 custom vibro champ reverb issue need schematic

[vmazz]

I have a 68 custom vibro champ with what appears to be a dropping resistor for the B+ which keeps burning. But without the schematic I cant be sure what else it is connected to. It appears to be between the preamp nodes of filter caps. But only burns when the singe 6v6 is installed. It is R26. Anyone have acces to the schematic? or any experience with these reissue amps. I was wondering if it was connected to the screen but doewsnt look to be directly.

[Chuck H]

Do you have another 6V6 to try?

I can't find a schematic either and looking on line it seems all the board traces are on the under side. That'll slow you down but it shouldn't stop you. You could take some time checking the likely terminations for the circuit path just using your DMM.

[vmazz]

Yes Ive tried several other 6v6s. I have exposed the traces on the other side. only one going to the next node 22uf 450v capthe other end of the resitor going to the prior stage 22uf 450v cap. What am I missing? Why would this resistor have so much current drawn across(even when I pulled the cap on the ealrier stage) I dont see any burnt traces etihter.

[Helmholtz]

Try to make a drawing of the power supply wiring.

[x-pro]

this one?
https://www.youtube.com/watch?app=desktop&v=sXQ6mI6XcRw

[g1]

You said the 6V6 screen does not seem to go there, but where does it go?
The 68 Custom series are generally re-issues of the SF amps with some mods. I would think it should follow this schematic (attached) generally, they may have added an extra resistor right at the screen, but I think your burning resistor should be the screen node.
It's possible something else downstream is pulling too much current, and the 6V6 screen current just puts it over the edge of it's wattage spec.

[Chuck H]

Well what could there be about a plugging in a known good tube that would cause excessive current through that resistor? THAT is the question. And I have no answers without a schematic. There may be something unusual abut the design.

[vmazz]

Shorted filter cap. Replaced both Pi and Preamp caps and node resistor.

[The Dude]

Glad you got it fixed, but a shorted filter cap doesn't make sense with, "only burns when the singe 6v6 is installed". The shorted cap would be in the circuit whether a tube was installed or not.

[Chuck H]

I think g1 had it right. The shorted cap was a preamp filter so there would be some resistance between the source and the short. Add the power tube and the current draw went up enough to cook the resistor.?. Depending on what resistor values are used where and which node R26 is it seems plausable.

[g1]

I thought maybe if the cap wasn't a dead short it could have been right on the edge with the tube removed. So it burnt the resistor in normal operation, but removing the tube masked the fault.