JCM800 Burning Tubes - HELP!!!

[pdf64]

Assume you meant 750 ohm? It's going the wrong way, as it's there to limit screen grid current, so definitely replace it with the HT having blown previously. Peter.

[vintagekiki]

If R62 or R63 1K/ 5W resistor on that socket measures 750K ohms !!!, this resistor is burned (defective) because defective output tube. R62 or R63 value is from 470 ohm to 1K – 1K5. If is some of R62 or R63 defective amp power is low.
Fuses value F1= T2A; F2 = T500mA where is T = trage, slow
About biasing
R51 > 56k/ bias voltage is high/ idle current is low
R51 < 56k/ bias voltage is low/ idle current is high
Check RV1 22k ohm, may be defective or his value is larger and check C35, C36

If you distrust in your OT change with Fender OT for test.
The best hint is replase both octal sockets, both output tubes, R60, R61, R62, R63, C35
And biasing ...

[mikeboone]

Yes 750 ohms. I had "K ohms" on the brain... I would imagine that a 25% drop in R would be a reason to change.

The socket where the tubes burned has been changed. R60 & 61 are measuring 5.6K. I am going to replace R62.

Mike

[Amp Kat]

Thanks neighbor... Hows BR doing after the storm. Houma is recovering slowly but surely.

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Have to say Gustav rocked our world here and were still cleaning up and only about 50% thru ! and I am fully aware of what it did to you as one of my buds dad lives there.

Mike at this point I would rely on what Chuck said about checking other components around the tube socket. You also need to monitor your bias voltage and see if it changes after warming up. These amps are problems and I never had good success with them. I think those OT's in these are similar to the small JCM 900'S that are so proned to failing but I haven't seen but about 1 in this model go bad. I'm not crazy about that power board with all the fuses on it also as theres so many connections in this amp it's crazy. Let us know what happens.

[mikeboone]

Thanks guys for all the help...

I am going to dive back in this weekend and I'll give a run down of what I found.

...Or I'll be back with more questions!!!

[mikeboone]

OK guys... I replaced the 1K/5W resistor and presto chango - it works!!! So far.