My JCM 900 - something's not quite kosher.

[zeegler]

Long story short: I'm playing one day, had the amp on about 20 minutes, and I smell something hot. I look at the back of the amp just in time to see one of the power tubes sparking inside (like welding type sparks), and the amp goes dead and shuts off. I didn't have time to turn it off or anything. So it turns out I blew an EL34, and it took the fuses with it. I replace the fuses, put in a set of new tubes, and bias the amp. Now here's the problem. Every time I turn on the amp, it stinks like burning hair once it warms up. I figure it's just because the tubes are new. Sure enough, the smell went away after a few weeks. However, for some reason the amp seems to have substantially less gain than it did before, and sounds filtered, as if I have a wah pedal engaged. I'm getting really pissed at this because the amp just doesn't sound right anymore, and I'm going to be rehearsing with a new band soon. Before I take this to my amp tech, does anyone have the slightest clue what might cause this? I love this Marshall, but if I can't get it to sound the way it used to, I'm gonna be a very sad man.

[jet66]

I wonder if a screen grid resistor might have been cooked. I had one start to go (a 6L6 amp) and even though it would bias in to spec, it didn't quite sound like I thought it should. Sure enough, one of the 470 Ohm resistors was reading about 280 Ohm. I replaced it (all four just to be safe, I bought the amp used) re-biased, and it was like a whole new amp.

I don't know if that will be your problem as well, just an experience I had with a flaky screen grid resistor. I don't know if there was ever a bad tube in it, but when I got the amp, it had KT-66's in it, and the bias was running around 80-85% dissipation, way too hot, and that was with the pot trimmed all the way down. I think that might be what cooked the resistor.

[Aso]

When I blew a tube in my JCM 900 SL-X a couple months ago it fried all the fuses just like yours did. I took it in to Savage audio and they had to fix a resistor or two on the PCB board inside the amp along with the tubes. That may be the issue with your JCM 900 also.

[zeegler]

Thanks for the info guys. I'll have to look into that.

[rocksoldier]

Different tubes?

[DonP]

Screen grids would be a good choice as been mentioned. They are 5 watt and are designed to burn up instead of taking out the trannies. Make sure you get ones that will burn open and not let your tranny fry.

Hopefully it's not the tranny....

[zeegler]

Different tubes?

Nah, the change is too drastic for it to be caused by different power tubes. I went from Mullard clones, to Mullard reissues anyway, so they should be close enough.

[Learn Guitars]

Nah, the change is too drastic for it to be caused by different power tubes. I went from Mullard clones, to Mullard reissues anyway, so they should be close enough.

I smoked a set of tubes on my 900 a couple months back, along with the tubes a few things on the board went too. Had the entire amp gone over along with a new set of EL 34s.

[zeegler]

Sounds like I should dig out the schematics, and get my meter out.

[Cygnus X1]

Sounds like I should dig out the schematics, and get my meter out.

I agree with DonP.
Take a close look at those screen grids.

[dmnall]

Zeegler,
Did you figure out the problem yet?? Was it the screen grids?? Also when was the last time you replaced the preamp tubes?? What model is your 900?? Mk3, DR or MK4 *sl-x*???

Cheers,
Charlie

[zeegler]

Zeegler,
Did you figure out the problem yet?? Was it the screen grids?? Also when was the last time you replaced the preamp tubes?? What model is your 900?? Mk3, DR or MK4 *sl-x*???

Cheers,
Charlie

I won't get a chance to check them until this weekend, so I'm not sure yet. I'll be sure to let you know how they check out.

I actually have this crazy idea of going through the amp and replacing all the components one by one with top quality stuff. I'm silly like that sometimes.

[dmnall]

I won't get a chance to check them until this weekend, so I'm not sure yet. I'll be sure to let you know how they check out.

I actually have this crazy idea of going through the amp and replacing all the components one by one with top quality stuff. I'm silly like that sometimes.

Nothing crazy about changing everything in there.. Just the issue I see is a lot of work to do that, lol.. I am still interested in knowing what went the problem...

cheers,
Charlie

[DonP]

I actually have this crazy idea of going through the amp and replacing all the components one by one with top quality stuff. I'm silly like that sometimes.

Yes, that is crazy / silly. Change it to a PTP amp with NOS mullard mustard caps and carbon comp resistors