Just Killed Two Thomson 6080WAs?!

[tvr2500m]

I was looking forward to a little tube rolling, rolling in one of the French Thomson 6080WAs from a number of them I just tested. They tested great on the RoeTest V11 I built.

Plugged the first one in, let things warm up for a while while I did some other things. Went to listen and only one channel was working. Looked for glowing heaters, all was glowing, wiggled the tubes a bit in the sockets, no change.

Powered off, removed the Thomson, installed a Tung Sol 6080WA, and every sounded great again. Tested the Thomson - nothing. Heaters heated, no output from both sections. Grabbed another Thomson, tested it again. It tested great. Plugged it into the Crack. Nothing. Well, no gain, distorted. Test it. Effectively dead. ??!!

I've rolled a lot - a lot - of tubes, both 12AU7s and 6080s/6AS7Gs, through the Crack without any issue. I recall the Thomsons sounding very good and was looking forward to listening with it.

I did resistance and voltage tests on the crack with the working tubes. Speedball installed. The measurements in italics are the numbers I measured when I first built the Crack, and as I have been using it since (around 5-7 years without an issue). The measurements in bold are what I just measured. The 6080 Speedball board OA OB values are higher than the manual range specifies, but the values are similar to what they had been and I've been running since I built the Crack. I recall confirming that all the voltages were okay on this forum then, and got the okay, though I'd have to check that. Here's what I got for voltages:

Line voltage: 120V AC, 60Hz

PT-10
- Terminals 7 and 9 = 5.99V AC. Manual, 6.5V AC ±20%. 6.05V AC
- Terminals 12 and 11 = 167V AC. Manual, 175V AC ±20%. 166V AC

Speedball
12AU7 board
- OA, 80V DC. Manual, 60-90V DC. 88V DC
- IA, 180V DC. Manual, 170-270V DC. 177V DC
- B-A/B, 0V DC. Manual, 0V DC. 0V DC
- IB, 180V DC. Manual, 170-270V DC. 177V DC
- OB, 86V DC. Manual, 60-90V DC. 84V DC

6080 board
- OA, 112V DC. Manual, 75-100V DC. 114V DC
- OB, 109V DC. Manual, 75-100V DC. 115V DC
- G, 0V DC. Manual, 0V DC. 0V DC
- B+, 180V DC. Manual, 170-195V DC. 178V DC

I would appreciate any thoughts on this.

Thanks.

 

[Paul Birkeland]

Are these the ones off eBay?

 

[tvr2500m]

I don't recall exactly what I sourced all of them, but some of them were from eBay. eBay sourced Thomsons...? Uh, oh. That's sounds ominous. What's up with them?

I test every tube I get, often with more than one tester - a Heathkit TT-1, TV-7D/U, and I built a RoeTest V11. With the RoeTest, I know the exact test parameters. And I try just about every tube I get in a real, live circuit.

Plugging back in several of the the 6080s/6AS7Gs I have at-hand/have used/been using, Cracky works as its usual great-sounding self.

I also checked whether the initial test voltages were green-lit; they were.

So, it appears that it's a tube issue, not a Crack issue. The RoeTest is showing grid issues with the Thomsons. Expensive fuses

 

[Paul Birkeland]

There is someone overseas selling 6080s labeled with that mark, and they are all duds.

 

[tvr2500m]

This is possibly the freakiest thing I've ever seen in decades of tubed-stuff. From long experience, I have always had strong confidence that if a tube tests well, it has a likelihood of working well. If a tube tests especially poorly, it will perform poorly. Serious defects that show up on a tester will show up in use. There's a decent positive confidence interval there. I often test everything I buy with two testers, then try to also try/listen to them in a circuit to corroborate between the measurements and in-circuit performance. It's rare, but sometimes happens, that tubes don't go through at least most of this regimen.

But, these are just confounding. As per protocol, I had tested almost all of them when I bought them - they had TV-7D/U and Heathkit TT-1 (both of these testers I keep calibrated) test results Sharpie'ed on them. And I just tested all of them again before using them on a RoeTest V11 (nifty device, a fair amount of work to build). They all tested very well. Nice results on all of them. Like, really nice.

Put them in Cracky... ...Nothing. Like, nothing. No pops, fizz, hissing, hum. Just the extremely faint sound of some music playing with the volume turned way up. Testing them after use, pretty much dead. Heaters heat. Heater current draw good. So, the heaters work. Yay. But no, or almost no, transconductance. Picking a few at random to retest after they've sat for a while. Same thing.

Pop back in just about any 6080 or 6AS7G I have, Cracky is back to being wonderful Cracky.

A handful have survived. Tested well before use, sounded good in use, tested the same after use. I have a lot of duds.

Anyone want some pretty display tubes? Salvage for base rings or pin bases? Or, anyone want some of the funky, large Thomson boxes?