I got a bad JCM800....help

[drummercat]

My guitarist picked up a used JCM800 locally,
The amp powered up fine, but when we play it around 10 mins, 2 of the el34 start to grow really hot, and then a burning smell came from those tubes.

I bought a new quad of el34 to it and the problem still exist, and I exchange the tubes from left to right, the pair of tubes at the same socket still growing red hot.

can you guys help me out???
thanks!

[ocnor]

Take the amp to a tech and have the tube bias voltage adjusted properly.

[Silverwulf]

Originally Posted by ocnor

Take the amp to a tech and have the tube bias voltage adjusted properly.

+100...don't start swapping out tubes without properly rebiasing the amp in the process.

[-=¤willhaven¤=-]

Maybe a properly biased amp with a blown bias resistor? It's been a LONG time since I've serviced an amp... I might be talking out my ass now.

But either way, amp tech time.

[Light]

Take it too an amp tech. There is nothing in there which is user serviceable. Even after it has been turned off for months, there are still voltages in there which can kill you instantly.

And yes, it sounds as though it is biased incorectly.


Light

"Cowards can never be moral."
M.K. Gandhi

[guido #2]

Is that just a head? Is the output impedance set right for the cab that you are using?

Also: Any time you replace power tubes, you must rebias.

[TheRockDoc]

I didn't know there was such a thing as a good JCM 800

sorry... *go ahead and call me a bad name- I can take it *

[JosephCampbell]

Originally Posted by TheRockDoc

I didn't know there was such a thing as a good JCM 800

sorry... *go ahead and call me a bad name- I can take it *

Poopy head.

[TheRockDoc]

Originally Posted by JosephCampbell

Poopy head.

Stop! I can't take it!!!

[magic man]

re-bias that baby. it should be ok.

a side note on the 800:

make sure you use proper 3 AMP fuses. fast-blow ones. if you use the slow-blow ones at Radio Shack you could end up doing too much damage before the fuse will blow...

[amra]

Originally Posted by TheRockDoc

I didn't know there was such a thing as a good JCM 800

sorry... *go ahead and call me a bad name- I can take it *

Dill Weed!

[gbdweller]

Ouch, all of this name calling is too much for me to take. Take your JCM800 to the gosh darn tech and have it looked at. Light is 100% correct in his advise that the voltages inside the head can kill and you don't want to die. That would be very stinky. Oh man, you guys have me talking like a poopy mouth now!

[drummercat]

Thanks all you guys for the infos and advice!
I will take it to a tech for repair this week!

actually one of my friend is a hi-fi super freak, he can build anything tube amp and speaker cabinet, just for home stereo.
I took the amp to him for repair some months ago, he said the voltage of 2 pairs of el34 are not balance, and he solder a pack of resistor between the tube socket and the circuit board......

I took the amp back and play for a while, the tubes never get "red hot", but it altered the sound seriously!! even I crank up the "pre amp" knob at max, there is too little distortion, (may be barely enough for blues) and the volume is too low also, compare to anything tube 100w (mine is a 2203 mk2)and....sounded very thin.

I asked him again and he say the amp push the tubes too much (at 470V on the power tubes),so he put a resistor inside to let the tubes have longer life, but it seems not a good idea for guitar amps.

I solder out the resistors he put, the amp sound great again but the tubes get real hot....

I know nothing about electronics, but do you think re-biasing will alter the sound that much? or just he`s doing the wrong way?

Thanks all again!

[gbdweller]

Put simply, tube bias is like the mixture adjustment on a carburetor. Lean being cold and rich being hot. Some amps offer adjustment and some have a fixed bias such as the 5150 (which is biased way too cold by the way). For most situations you want the cathode current as high as possible for the best sound without burning your tubes up. The resistor he wired in would not be the proper way to bias your particular amp or balance the power stage and it sounds like it set the bias way too cold resulting in a sterile, low gain sound. Your friend told you the tubes were running at 470volts. That's the plate voltage which is only part of the equation. That needs to be adjusted to end up with the proper cathode current (approx. 34ma for the JCM800 w/EL34's). Your amp is definitely in need of a good service tech, especially if it has been "messed with" as you have described. As you have advised that you are not knowledgable in electronics, I can't over emphasize the importance of not messing with you amp. If you touch the wrong thing, even with the power disconnected, you can end up dead, real quick.

[TheRockDoc]

Originally Posted by amra

Dill Weed!

I prefer fresh basil with my fried Marshall, thank you very much...