General question, matched power tubes.

[Laudanum]

Posted this on another forum.  Not Bottlehead related but I trust the knowledge of the guys on this forum.  Here goes ...

Got a pair of "matched" EL34 tubes from a reputable online dealer for a Marshall JTM45 clone I built about a year ago.
Install the tubes in the amp. Go to adjust bias and the tubes are 9ma apart. Shut the amp down. Swap the tubes/sockets, fire amp back up and the bias problem follows the tubes. "Hot" tube is still "hot" in the other socket, and "cold" tube is still "cold". Same 9ma difference. My conclusion is that the tubes are not matched. End of story.

Am I missing anything?

Thanks

[Paul Birkeland]

9mA is pretty hefty on something like an EL34 (especially in a guitar amp) where it may only run at 30mA at idle. 

A good step to take would be to try that pair of tubes in a second amplifier, just so that when you contact the dealer, you can let them know that you were concerned about potential issues with your amp and took the time to rule that out.

-PB

[Laudanum]

9mA is pretty hefty on something like an EL34 (especially in a guitar amp) where it may only run at 30mA at idle. 

A good step to take would be to try that pair of tubes in a second amplifier, just so that when you contact the dealer, you can let them know that you were concerned about potential issues with your amp and took the time to rule that out.

-PB

Hi Paul.

I would but I don't have anything else that runs EL34's.   I do have two other tube pairs however.   A pair of Chinese EL34's (Valve Arts) were dead on when I got them.  They are about 2 ma difference now after 30+ hours.   And a pair if JJ's that were maybe 1.5ma difference when I got them and after close to 100 hours were about 3.5ma from each other. 

At any rate, dealer wanted me to swap tubes between the sockets and report back.  I had already done so  ... bias follows the tube.   So, he's asked that I send them back, so they are on their way back.  We'll see what he finds.