Power Tube Failure... Can they check AOK, but still be bad?

[67Mopar]

I have a set of SED 6L6GC tubes that I'm running in a Marshall 6100LM. With power off, I switched it to 1/2 power mode, turned the amp on... Pop! I lost one of the T500mA circuit protection fuses. I have some fuses arriving tomorrow, so I will check to see if the same thing happens, with no tubes installed.

I checked the SED tubes on my Hickok 6000A tester, and they test AOK. Question... Can tubes check good on the tester, yet fail when installed into the amp?

[xtian]

Don't know about your tube tester, but mine puts only a hundred or so volts on the plates. Power tubes could fail under 300+ volts, but not on tester.

[67Mopar]

Thank you. The tube doesn't get hot while testing, so I'll assume mine is the same as yours.

[Stevem]

I have that same tester, and yes you have a output tube that test good short wise on the tester, yet fail / arc when more voltage is applyed by the amp then what the tester has.

This is rare, but I have had it happen!

By the way Mopar67 I have a set of Edelbrock 77919 heads I am porting up for a buddy's stroked 440, nice heads I must say!

[67Mopar]

Thank you. I had a red plate issue @ tube V11 on a Marshall 6100LM. I discovered that pin 5 on V11 tube wasn't making contact. I pulled the tubes and tightened the contacts in the sockets. This is why I never use JJ power tubes... The large diameter pins stretch the socket contacts.

I biased them at 39 @ 100 watt (42 @ 50 watts). They seem to be holding steady, but I don't trust it... I'll run the amp out on the head box, so that I can keep an eye on them.

Lots of prep required, but those are nice heads. Good score!