Shared screen resistor question

[Reeltarded]

Have an amp that a peep of a peep owns. Peavey CL50 with a quad of 84s in output.

Each pair has a 5w 100r following a shared fatter 390r MO resistor that looks a little like glass bug guts oozed out of the middle. It reads open and the tubes are blackened around the plate vents. Amp ate mains fuse.

No tube tester but experience tells me they have had it.

So, I am going up to 1k on that shared resistor to go a little easier on the next quad but not sure what wattage rating that should be. The MO resistors always look beefy, but damn..

Anyone?

Thanks, Martin. lmao

[Firestorm]

Is the shared resistor after the screen supply node or is it the dropper to that node?

[Reeltarded]

That is a good question. I will have to get back inside and look. I can't read any of the schematics I found on my phone heh

i think the measurement from the out end of the burned 390r to the socket side of either 100r was unexpectedly high but I don't recall that number. At a glance I couldn't be sure what the circuit is doing there. The board orientation is very confusing.

(but I am me.. what isn't)

[cbass]

I belive the 390 Is the dropping resistor between the plate aand screen supply in that amp.I'd do a 1k 10 watt there

[VacuumVoodoo]

The 100 Ohm resistors are wired in the weirdest possible way: each is wired between 2 screen grid pins, then one end of it goes to the screen supply node after the 390 Ohm resistor. This way only one grid in each tube pair has the benefit of current limiting 100 Ohm resistor.
I usually leave the 390 resistor alone but remove the 100 Ohm ones and cut the track connecting pins 9 on V5 & V6. Then install 4 new 680 Ohm 3W resistors, one for each screen grid. This can be done by replacing one jumper with a 680 resistor for V5 and a little bit of trickery to fit in the last one for V6.

[Reeltarded]

Now I am more concerned as to why the 390r reads open.

Eeks

[cbass]

Tube Failed? drawing massive current through it maybe?

[cbass]

or bias circuit bad conection/ failed?Causing all four tubes to run away

[cbass]

Sockets arching could be a posobility

[randalp3000]

found this, hope it helps.......I also just picked one of these up for my shop for two bills


edit, I only see 2 100R and no 390r, not much help, sorry

[VacuumVoodoo]

R58 400 Ohm, connects to C37 after HV rectifier bridge. 400 Ohm is a non standard value, 390 Ohm is.

[JazzGuitarGimp]

Though I don't remember what it was, I recently saw a schematic using a quad of output tubes where only two of the tubes had a screen current limit resistor. I thought it was strange though I assume there's a good reason why this is done.

[cbass]

Peavey uses that 400r 10 watt in a lot circuits.I just assumed reel meant it measured 390 I guess not if its open though.Perhaps the 390 is a replacement

[Reeltarded]

Aha. I based what I said on lots of assumptions. You know what they say. If you assume you make an ass out out cbass.

[cbass]

Bad filter cap could cause that resistor to blow too.