Differant brands of power tubes/bias Q's.

[Mike M.]

This past weekend I thought I'd try some differant power tubes in my DSL 50. Bought a multi-meter and checked the bias of the stock power tubes before I pulled them. Each tube read 38mV's. Put in the new JJEL 34's and let them warm up for about 10 minutes in standby. Clicked the standby off, checked the bias and each tube read about 15.4mV's.

Is this normal to have such a differant reading when going from one brand of power tube to another? I was able to set the bias no problem, with each tube reading about 39mV's. The amp sounds fine, no problem but again, I was really surprised at the differant readings from the stock tubes vs. the JJ's.

Normal?

 

[wixedmords]

Re: Differant brands of power tubes/bias Q's.

Yes, it can happen like you speak of Mike. This just shows the importance of biasing.
Imagine if the differential went the other way, to 61mA, someone who wasn't the wiser would be running very hot.

This differential isn't the norm though.

 

[Mike M.]

Re: Differant brands of power tubes/bias Q's.

Yes, it can happen like you speak of Mike. This just shows the importance of biasing.
Imagine if the differential went the other way, to 61mA, someone who wasn't the wiser would be running very hot.

This differential isn't the norm though.

Thanks for helping. This was the first time I ever tried this and while I knew there would be a differance I did'nt think it would be so extreame.

Seems like a very imprtant safety tip (for yourself and for the amp) would be to turn the bias down before installing new tubes just in case the newer tubes run much higher.