Valve testing

[Bluebark]

I bought a couple of 6l6s off the net and one passed a quality test and one didn't. I put them in an amp I was working on and turned the amp on. I got distracted for about five minutes and when I came back the bad tube was red plated. I shut off the amp and after the tube cooled, I tested it again and it tested better than the known good one. The tester is just a basic unit testing for shorts and emitter quality.

Why would this tube test better after getting red plated?

[shooter]

 


if someone beat the snot outta ya, you'd behave better also.


note to future tubes you test;
use a cathode biased amp, they are slightly more forgiving before they flame-out 
ALWAYs have a meter across the cathode R set to measure VDC, NEVER get distracted, make your measurement, power down, do math, tweak bias, repeat test.  now that it's properly setup, you can do a longer test, NEVER get distracted!

[Bluebark]

Shooter, I understand. My mistake was not powering down before walking away. The red plating on that tube turned out to be the pad for the trace that goes to screen pin lifted off and broke. I hard wired it and all is well. Just curious as to what effect the red plating had on the tube to make it "test" better after the RP. It's a GE 6L6GC.

Thanks for your quick reply.

[shooter]

got nothing except a paraphrased quote from a gigging guitarist;


"tubes that are near death sound the best, I have my amp-guy save all his pulls and I use 'em til I kill 'em"

[scstill]

"tubes that are near death sound the best, I have my amp-guy save all his pulls and I use 'em til I kill 'em"

I have a killing it story too.
When the family was out many years ago now, I decided to crank my Pro Junior
But it was too loud so I put in some ear plugs.
Running the gain pedal high, running the amp volume high, sounded sooo good.
After 30min or so it started to sound awful.  :-(
Turned out I blew out the speaker, And it was a vintage blue alnico eminence.

[Bluebark]

In other words, if I read this correctly, use it till it dies! I'll look for a tube book that goes into the physics of the electrons. Thanks all...

[shooter]

https://el34world.com/charts/Schematics/files/Books/Radiotron_4th_edition.pdf

[scstill]

If you know you have bad tube replace it.
It might fail in such a way that it damages other stuff like the OT.

[Bluebark]

Shooter, thanks for the recommendation on reading material. Supposed to rain for a few days (FL...go figure) so I will give it a look. 1500+ pages...haha. This is going to be a slog...

I think my problem is I ever had any formal training in electronics. I should have taken electronics in H.S. instead of home ec. But being one of three guys in a class full of girls did have it's privileges.

scstill, I don't know if I have a bad tube or not. The tubes are supposedly NOS GE L6LGCs from '62. They all tested good but one was weak. And as it so happened the weak tube was in the amp when it's screen resistor broke free of the board and the tube red plated. What amazed me was the fact that it tested better than the other two after retesting. I was like what the hell?

[scstill]

Just a thought but Maybe the initial test was setup incorrectly or misread.
I have done things like that before.
If it tests good now (couple of repeats) then it should be good
Does it work well in circuit now?

[Bluebark]

It did. After hard wiring the open trace, it seem to work fine and still tests good. But like I said, the tester only tests for shorts and emissions per documentation. Just tested it  and still getting a good reading of 62 on the meter and it climbs the longer it warms up.

[shooter]

good reading of 62 on the meter and it climbs the longer it warms up.

62 what?


in the tube tester, or in the amp?

[Bluebark]

Sorry Shooter...the meter in the tester. Testing pre red plating.