JTM 30 blowing heater fuse and then.....

[glp]

So a buddy brings me a Marshall JTM 30. Im no fan of PCBs but I tell him I'll take a look.

Well the heater fuse is MIA? Assume it blew and he (or whoever?) couldn't find a replacement.
So I understand these had some history of this fuse blowing?
Pop in the correct slo-blo, 500ma fuse.
Test the 2 6L6 power tubes in an Orange DIVO valve tester.
Both test good.
Bring it up on a current limiter and all looks good.
Plug into wall power, volume pops in and out for a few seconds before I see R126 100k resister SMOKING!
Shut it down, test the tube this resistor is linked to.... tube now FAILS the tester?

I can replace the tube..... lest someone thinks something else might've taken that tube out on power up?

Thoughts? Experience?

[g1]

I think you meant 100 ohm, not 100K for R126.
The tester will only test at low voltage, so the tube can still be bad when run in the amp.
New tube, resistor, and fuse and you should be ok. But before installing the tube, make sure the bias voltage is proper at pin5 of the socket.

[The Dude]

Most tube testers don't stress the tube nearly as much as the amp does. It's not uncommon for a tube tester to say a tube is good when it's actually not.

Edit: Dammit, g1, you beat me.

[Enzo]

A 500ma fuse is not likely to be a heater fuse. I would believe it was a high voltage fuse, and so consistent with tube failure.

[glp]

yup.... typo... its a 100 ohm 1 watt (R126). Its darkened a bit, but still measures 98 ohms!

[Enzo]

It may measure OK, but it clearly has been stressed. It may be mechanically weakened, and so less reliable. Considering they cost very little, why risk more tubes? Just replace it anyway.

[trobbins]

R126 is the screen stopper to one of the 6L6.

The 500mA fuse is a slow blow in series with standby switch supplying the full-bridge rectifier and 47uF filter cap for B+. There is no discharge resistor for that cap, so be aware.

There is a heater fuse, cos the preamp tubes are run from a DC supply.

There are no bias sense resistors, or bias adjust - that can be upgraded. Links LK118, LK119 can be replaced with sense resistors. ZD101 can be modified by adding a 1N4734A, 5V6 zener in series, with a parallel 50k trimpot, to act as bias adjust.