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Tubes in JCM 800

Musicman

Well-known member
Joined
Feb 27, 2002
Messages
1,868
This may seem really stupid but I don't know much about tubes and have a question. I recently looked in the back of my JCM 800 while it was on which I never do. I noticed the EL34 tube closest to the smaller tubes 12ax7? was glowing red along with the smaller tubes. The other EL34 was not glowing at all. I took out the EL34 that was glowing out and the top was white and crusty while the other one looked like mirrored silver. I swapped them and the tube that wasn't glowing did and vise versa. Can someone tell me if this is normal? I've moved a couple times recently and I bought the tubes new not long ago and had the amp checked before the moves. I think it sounds the same as it did. Maybe it's nothing but I'm curious to know if it's normal. Thanks for any help you can provide.:2cool
 

audioman

New member
Joined
Nov 23, 2005
Messages
49
This may seem really stupid but I don't know much about tubes and have a question. I recently looked in the back of my JCM 800 while it was on which I never do. I noticed the EL34 tube closest to the smaller tubes 12ax7? was glowing red along with the smaller tubes. The other EL34 was not glowing at all. I took out the EL34 that was glowing out and the top was white and crusty while the other one looked like mirrored silver. I swapped them and the tube that wasn't glowing did and vise versa. Can someone tell me if this is normal? I've moved a couple times recently and I bought the tubes new not long ago and had the amp checked before the moves. I think it sounds the same as it did. Maybe it's nothing but I'm curious to know if it's normal. Thanks for any help you can provide.:2cool

The tops of the tubes should both be shiny/silver looking. Crusty and white means that there has been a vacuum leak and the tube is toast. Time for a new set of tubes and a rebias. The question is why did it go south? I would get your screen resistors checked, sound like the tube may have shorted.
 

Musicman

Well-known member
Joined
Feb 27, 2002
Messages
1,868
Thanks, I'll have to look for a decent shop in town. Should the other tube that was silver tipped and not glowing be glowing or not?
 

dartanion

New member
Joined
Nov 24, 2007
Messages
244
Yup, that white crusty tube is toast. Toss it and get yourself a fresh set of EL34s. No real need to replace the preamp tubes as they normally outlast power tubes by a long shot. Yes, setting the bias is important for any power tube swap.

The red glowing is normal as long as it is coming from the filaments (heater). The filaments heat the tube up to operating temperature, so this is normal.

A red glow that is not normal would be the plate(s) of the tube(s) glowing red. The plate is the largish metal structure you see when looking inside the tube. This should be readily apparent as the plates are normally gray or black and should remain that color under normal operating conditions. Operating a tube while red plating will drastically reduce the life of the tube.
 

RiverRatt

Active member
Joined
May 23, 2006
Messages
1,329
tubes.jpg


The silver color is the metal that burns off the getter when the tube is first fired up. This gets rid of any oxygen or whatever that's left inside. If every tube that you plug into that socket glows bright red, or if the plates start glowing, you need to turn the amp off NOW and get it serviced.
 
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