RF wrote in message
...
Peter Wieck wrote:
On Feb 25, 7:33 am, "N Cook" wrote:
Amp seems to work perfectly well, being repaired for minor problem, no
succeptibility to microphony or tinging distortion.
Testing the 2x EL34, gains are good and matched but after a minute on
heated C/H insulation check, the resistance gradually decreases to 1M
for
one and 0.5M for the other.
What problems will this lead to if continued use, and how quickly ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list
onhttp://home.graffiti.net/diverse:graffiti.net/

It means that the tubes are not precisely matched. Sadly, "matching"
tubes these days is typically based only on "gain" and done for a
quick claim to inflate prices.

No problems if each tube has an individual bias adjustment, and you
watch over it. Even if they are biased together and that bias is
attended-to, there should be no problems. How many hours on these
tubes and are they driven to extremes or not?

What are you testing with?

http://hereford.ampr.org/Tube4.php?tube=6CA7

Peter Wieck
Wyncote, PA
Huh, he is talking about heater to cathode leakage. Nothing to do with
matching the tubes. The amount of leakage you have is no big deal for
power tubes, if it gets below 100K then have a hum problem.

That was the sort of info I was after. I was wondering if there could be
some sort of thermal runaway situation.
I'd not noticed any hum but will make a point of trying to measure it for
later reference purposes.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/