Carvin X50B tube amp

[N Cook]

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 ?

 

[Peter Wieck]

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 ?

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

 

[N Cook]

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 ?

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

Unknown age, matched GT tubes, no date on tubes
used by a pub rock band
tested on Avo CT160

 

[Peter Wieck]

Unknown age, matched GT tubes, no date on tubes
used by a pub rock band
tested on Avo CT160

OK... Attend to the bias to the extent possible, and suggest that they
travel with a spare set of tubes. If you have a new 6CA7/EL34 to test
against, try that just to make sure you are not too far off.

At least you are using one of the very few testers that actually gives
reliable information.

Peter Wieck
Wyncote, PA

 

[N Cook]

Unknown age, matched GT tubes, no date on tubes
used by a pub rock band
tested on Avo CT160

OK... Attend to the bias to the extent possible, and suggest that they
travel with a spare set of tubes. If you have a new 6CA7/EL34 to test
against, try that just to make sure you are not too far off.

At least you are using one of the very few testers that actually gives
reliable information.

Peter Wieck
Wyncote, PA

This one was used by REME Wing Hebrides in 1984

 

[RF]

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.

 

[N Cook]

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.

 

[N Cook]

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.

Measured no more than 1mV rms hum over 4 ohm output for this one , level set
for 2.25 W of 400Hz output, but disconnected and monitoring for shorted
input