Unstable EL34 pair

[Leevi]

I got a new Marshall Plexi clone wired and tested it little bit.
I noticed with a big volume that suddenly the volume dropped dramatically and one of the power tubes was
glowing orange. I switched the amp off tried again and everything looks to sound Ok but
the tube that was glowing orange had now a blue light inside.

Is that an indicator that the tube is faulty? Tube meter showed that both tubes are fine.

I changed a new power tube pair to the amp and didn't meet problems anymore.

/Leevi

[plexi50]

Need to test that tube in a tester and flick it with your finger in the short position (lightly shocking the bottle) to see if you can rattle the insides to create a short that may not show under normal tube tester conditons. And also you may have a bad bias r,cap, or coupling cap

I find that power tubes can hide there internal faults sometimes unless you lightly shock the bottle at which point a tube will show a momentary short. The key here though is to make certain that you tube socket pins in your tester are grabbing the tube tightly

Sounds like a possable rouge power tube to me. Test it or get rid of it

 

[birt]

if you do a very short test in your amp (as plexi 50 said, tap the tube to see if you can get the problem to show again) swap the tubes around to see if the problem stays with the tube or the socket. if it's not the tube you need to search IN your amp instead ;-)

[stratele52]

Tube defective or bad connexions / solder . The bias negative voltage do not go to the tube grid. Check pin at EL34 pin 5 if you have a negative voltage , like -30 volts DC. Look at the other tube wich is working good

[Leevi]

Thanks for the responses.
There is a new phenomenon with the new tube pair I changed.
You can hear some an extra tone like snap or crackling from the amp after it has turned on
and before the tubes have fully warmed up. Can that be a tube issue or is it telling about a bad contact somewhere?
/Leevi

[stratele52]

Tube defective or bad connexions / solder . The bias negative voltage do not go to the tube grid. Check pin at EL34 pin 5 if you have a negative voltage , like -30 volts DC. Look at the other tube wich is working good

check that . probably a bias problem

[plexi50]

if you do a very short test in your amp (as plexi 50 said, tap the tube to see if you can get the problem to show again) swap the tubes around to see if the problem stays with the tube or the socket. if it's not the tube you need to search IN your amp instead ;-)

Dont do the short test while the tube is in the amp powered up. If the tube does have an intermitant short and you can duplicate it while the B+ plate voltage is on the tube you can damage your OT not to mention high voltage arching that could occur if the tube does have or show a short when you tap on the tube

This is just something i discoverd while using my tester one day several years ago and noticed i had a tube that had 100% grid emmision and was biased correctly. But the tube itself was putting out little wattage. My tester didnt show a normal short. But when i hit the tube the insides rattled enough to show that there was a short in the tube at which time the short light came on

It hasnt failed me so far as to telling me if the inside of a power tubes structure is solid and not influenced by any sudden shock to the tube. It tells me if the tube is good or bad structuraly