Did my Crack just kill a tube? [Resolved]

[PancakeGiraffe]

Greets, all!

I built my Crack + Speedball around a year ago, no problems whatsoever. Just this morning, I received my WE 421A tube and chucked it in. Happy listening for around an hour.

All of a sudden, there's this faint high-pitched static present in both channels, so I turn the amp off and go have lunch.

When I get back and turn it on, the right channel's dead. Flipped the lid over and discovered that both LED's on the B side of the big PCB are off. All others are fine.

Funny, both input and output tubes glow perfectly. When I replace the 421A with a TS 5998, all the LED's come back on, as well as both channels.

The voltages on the Speedball for the 421A are as follows:
OA- 68
IA - 205
B-A/B - 0
IB - 204
OB - 76.5

OA - 90
OB - 2
G - 0
B+ - 202

As for the 5998:
OA- 78
IA - 192
B-A/B - 0
IB - 192
OB - 77

OA - 93
OB - 92
G - 0
B+ - 190

I've had this 'murder' happen before, with three other Thomson 6080WA tubes. I'd just assume it was the tubes at fault.

Though, PB did mention in another post that tubes don't just stop working one day - that they would sound garbled first? Or maybe I just wired my Crack up wrong and turned it into a serial tube-killer.

Any help would be greatly appreciated. Thanks bunches, friends!
- Kenneth

[Doc B.]

Your assumption is correct - if the amp works with one tube and doesn't work with another tube, it's not the amp that has the problem. Try cleaning the pins of the 421A, but it sounds like it probably has an internal short.

[Paul Joppa]

It occurs to me that the socket might have a loose grip on the right-side cathode pin - just enough that some tubes won't make contact reliably. It might be worth checking with a tube tester or another amplifier before throwing out the "bad" tubes.

[PancakeGiraffe]

Hi Doc and Paul!

Thanks for the valuable advice. I've cleaned the pins and just about swapped the 421A in and out ten times now. Did a bit of wiggling. Even retested the old Thomson tubes. No dice.

Unfortunately, I don't have a tube tester and this is my only amp, so I'm gonna have to assume yet again that it's a faulty tube.

Sigh. Any possibility that my amp could be the one shorting them out? (and the 5998 is just hard to kill) As glad as I am that it's supposedly wired right, I just don't want the same thing to happen to another new tube.

I mean, I can't be that unlucky... right? Four broken tubes?

[Paul Birkeland]

Usually a condition that will damage a tube will be very, very visible when doing voltage checks.  One exception to this is when you have a really bad oscillation, but the Crack circuit isn't the type of design to create those conditions.

The 6080 specifically is a super rugged tube.  Before we wrote the new Speedball manual, it was really frequent to see poorly mounted TIP50 transistors shorting at least one cathode of a 6080 to ground, which pushes the 6080 well beyond anything published on the datasheets.  I don't believe we ever saw a 6080 fail from this.

[PancakeGiraffe]

Thanks for the input, PB.

Guess it's just my rotten luck. Here's to hoping the next tube will be better!

Cheers for all the help, guys. I really appreciate it!
- Kenneth

[Paul Birkeland]

I still wouldn't toss the tubes, just put a ? on them with a sharpie and stash them away.  You may discover a broken wire or some other cause for this down the road that we haven't anticipated.

[PancakeGiraffe]

Good idea, Paul! Thanks, I'll do just that.