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orange glow on tubes

1/16/1997 12:21 AM
Dan Senne
orange glow on tubes
Can anyone please help with my question?  
I recently bought 2 NOS Tungsol 5881 's from Antique  
Electronics that I use in a guitar amp. One of the tubes  
has developed an orange glow on its side (on the plate?).  
The area of the glowing is maybe 1/8" x 1".  
he other tube doesn't glow. The  
bias is set to -40 VDC. If I lower it to around -35, the  
tube glows more, if I raise it to around -45, it glows  
less, but then the amp doesn't seem to sound as good.  
The 90 day warrenty on these tubes is about to run out.  
Should I assume that the glowing tube will not last as  
long as the other and return it, or am I just being  
paranoid? Pleas help!  
Thanks much in advance,  
1/16/1997 12:34 AM
The first thing to do is to reverse the tubes. If that tube is still glowing it sounds like it may either be defective or the two are not matched. If the other tube is now glowing then you have a problem with the circuit. You can connect a one ohm resistor from cathode to ground on each socket and measure the voltage at the socket end of the resistor on each tube to determine if the tubes are matched and if they are not you can individually bias them by running two seperate bias pots or send them back and request matched tubes.
1/17/1997 2:48 PM
john martin

Sounds like unmatched tubes to me also. They should both glow (or not glow) at the same time. I see this all the time in my amp repair shop. Always buy matched tubes. A.E.S. charges a few dollars extra for this, providing they have enough stock of a particular tube to get a match.  
1/23/1997 12:23 PM
Steve Tremblay

Hey man, you did not say what type amp. I recently worked on a Sovtek for a friend and he had the same problem, If the problem does not follow the tube, stays at the same socket you need to check the output transformer, this is the problem with his sovtek, A good Marshall xformer cured the problem. I could not find any other problem with the amp.
1/26/1997 8:42 PM
Ed Edwards

Just a while ago, while practicing, I smelled that 'heat' smell in the room just about the time the clean channel on my amp developed some terrible hiss, crackle and pop. I walked around and quickly identified two of the four EL84s recently purchased were glowing from their plates. I shut down (waited until cool) and began the swap process. Each of these tubes glowed no matter how it was paired. Then I got out the replacement tubes (the disgusting rattling Chineese unmarked things that were in it when I bought it used [a Peavey Classic 30, BTW]) and began pairing and swapping them. The bad tubes continued to glow wherever placed.  
Conclusion: Internal shorts in the filiments causing greater voltage drop across less windings, and so - superheating. How could this happen to two at once? Dunno---maybe preemie death. The inside of a tube amp is probably one of the highest vibration locations one could place such a delicate device, but it has been done since the '30s, so I suppose this may be on record as a mode of failure. OR, I could be wrong. (Weird how they overheat whever they're placed, though.)
1/28/1997 2:23 PM
Steve Tremblay

part 2, check for a cold solder joint on the tube socket. also could be a open grid resistor. Make sure you check the bias after amp has warmed up for a couple of minutes. also make sure all pins are tight on the socket.

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