"Shield" brand 6BQ5 Torched Screen Resistors

[boots]

I acquired a 6BQ5 that looks new in the box with the brand "Shield" on the tube and on the box. They also say "USA". I assume this was a re-branded tube. I don't know who manufactured it - there are no dots, stop signs, etc. Has anyone ever heard of Shield tubes?

I popped it in my modified HO (2x 12AX7 & SE 6BQ5) amp for an "ear test". It sounded OK for a couple mins, then went poof and smoke rolled out of the amp. It burned a pair of 1k 1W resistors in series with the screen grid of the output tube. I replaced the resistors and the amp seems fine (with a different tube!). I thought at first the screen must have shorted internally to the control grid, but checking with a meter shows no internal shorts. I'm wondering what happened? I'm a little scared to use that tube again. Could it have taken off into parasitic oscillation? No debris floating around inside the bottle. Only a little "soot" around the bottom end of the plate, which I understand is normal for 6BQ5's.

Any thoughts? Thanks...
-Boots

[Dan T.]

Does your 6BQ5 tube socket use any of its terminals as a tie point for circuitry? Doing this can cause a tube to burn up, among other problems.Most people know not to do this with 6L6s or El34s, but it's bad for other tubes too.I ruined some rare CBS 6BQ5s that way.

[boots]

No, there are 3 unused pins on the socket, just like there should be. I checked the tube again with an ohmmeter while jostling it around to see if I could get any intermittent shorts to show up, but I could find nothing wrong. I don't have a tube tester (other than an actual amp!), so I guess I'll just put that tube on the top shelf and forget about it. The amp is running fine after replacing the screen resistors and putting in a different tube.

Still wondering if anybody has ever seen any tubes with the "Shield" brand and logo? I'm now suspecting they might have been made by Sylvania - perhaps they are "seconds" that didn't pass QC? The bottle says "6BQ5 USA". No other markings.

[Pedro Vecino]

If pin 1 is not used in the socket, almost certainly defective tube. There are shorts that occur only in presence of high voltage.
Regards

[Diablo]

I have half a dozen NOS Shield tubes (7025) that work just fine. I got them as part of a large collection of tubes from a radio/tv repairman.