6L6 shorting Heater-to-Plate in Blackface Bassman

[prs_matt]

I have a Bassman head in for the second time this year with a scored out octal socket between pins 2 and 3. It currently has JJ 6L6GC tubes in it. Voltages all measured fine when it was turned back over to the client last time it was in the shop.

What causes these shorts?

How can I ensure that it doesn't happen again?

Thanks for all the advice.

[thermion]

I've seen alot of this type of failure, typically on original bakelite/ phenolic sockets. I think these shorts happen when b+ sees a path to ground via the heater line combined with the deterioration of the insulating action of the socket over time. Did the socket you replaced fail again or was it the other one? Are the resistors on the heater circuit burnt?
Try ceramic sockets as a preventative; also blow out the chassis with compressed dry air in case various conductive chaff, swarf, solder driblets, or other extraneous junk lie in wait for the chance to cause more angst.
Best of luck.

[HotBluePlates]

I've had a bunch of NOS (and old, used) tubes. The only failure I've had with an RCA 6L6GC was a plate to heater short.

It appeared to occur internally, as I removed the tube and checked continuity from pin 2 to pin 3 on the tube itself. Got a beep from my meter; there was no continuity or carbon trail on the socket within the amp itself. Unfortunately, that tube went in the trash, and I had no further issues from that amp in the intervening years.

I don't know specifically why that tube went bad, but it was the tube itself and no fault in the amp. So you'll have to look for issues both inside the amp and also in the tube.

[prs_matt]

thermion: I replaced both octal sockets with new Belton sockets. I will check the heater resistors. A year ago when I first looked at the amp I thought that a tube was the cause as the tubes in the amp were old RCA blackplate 6L6s--one of which had a broken and loose getter rattling around inside the glass.

I wonder if there was some damage to the heater circuit as I believe this is the amp that had a fried heater center tap... now that I think about it. I will have to check. I wonder if there is a heater ground issue and I should just put in an auxiliary 6V transformer and take the 6V winding out of service on the PT.

Thoughts?

[stingray_65]

Years ago I had a tektronics scope that would throw 10" arcs inside the chassis when turned to its highest sensitivity. I got it that way and was told by the previous owner it had been to several repair shops and every one said every component tested fine.

I had found a pencil line between two ceramic terminals and decided the graphite was conducting.
 funny the line had an asterisk in the middle.

I took a magnifying glass, tooth brush and some 1,1,1 trichloroethane to it. as I looked through the glass to make sure every bit of the pencil line was removed, I saw the asterisk had actually been a spider that had turned to carbon! I assume the pencil line was the beginnings of a web.

Ray

[LooseChange]

Once the tube socket is scorched it will conduct again. That carbon is a path.
I had a PC board with connectors for the B+ and Screen side by side. The amp got a bit of moisture and caused an ark.  After that it would ark all the time and blow fuses.  I had to cut notches between each connector on the PC board to prevent it happening again.

2 cents.

[FYL]

I saw the asterisk had actually been a spider that had turned to carbon!

A real bug...

"Meaning "defect in a machine" (1889) may have been coined c.1878 by Thomas Edison (perhaps with the notion of an insect getting into the works).", Online Etymology Dictionary, Douglas Harper

[LooseChange]

A real bug...
"Meaning "defect in a machine" (1889) may have been coined c.1878 by Thomas Edison (perhaps with the notion of an insect getting into the works).", Online Etymology Dictionary, Douglas Harper

I learn so much around here. 

[Frankenamp]

Well, it appears that the aforesaid bug has become a 'ghost in the machine'... a case can be easily made that the scope had literally become it's haunts.

[Willabe]

Well, it appears that the aforesaid bug has become a 'ghost in the machine'... a case can be easily made that the scope had literally become it's haunts.