Bias setting question

[uncle looie]

I just picked a Bias Rite from Weber and I've got a couple of questions about it. I read the plate voltage on an Alessandro Working Dog 6L6 version that I'm retubing and got 427 on one tube and 420 on the other. I split the difference and went with 424 and came up with a bias setting of about 49ma. George put a sticker in the chassis recommending the bias be set at 35ma. Wouldn't that be a hot bias if the 70% dissipation was 49ma? Or...maybe I did the math wrong? Where does the "30" come from when figuring out the bias for 2 6L6's using the formula "30 divided by X, where X= plate voltage"? From the tube type or the wattage of the amp?

 

[Wakarusa]

427 on one tube and 420 on the other. I split the difference and went with 424
Probably better to tune to the highest voltage rather than the mean. Doesn't make much difference in this case tho'

and came up with a bias setting of about 49ma.
Fair enough.

George put a sticker in the chassis recommending the bias be set at 35ma. Wouldn't that be a hot bias if the 70% dissipation was 49ma?
Nope, that'd be cold. Just under 15W or about 50% of max.

Where does the "30" come from when figuring out the bias for 2 6L6's using the formula "30 divided by X, where X= plate voltage"? From the tube type or the wattage of the amp?
No idea. Never heard of this method before.


General comment: The 70% of max dissipation value is a "not to exceed" limit not a target. The proper bias point is whatever setting sounds best to you that is at or below the 70% limit.

 

[uncle looie]

Hey Todd, I appreciate your comments. I'm one of those guys that barely know my way around a meter, so I appreciate the help. I swapped the tubes around after my first post and got 424 on one and 422 on the other, so I left them this way and still went with the 424. I had a rehearsal to get to so I went ahead and biased it at 35ma (per George's rec's) for the time being. If I'm reading your post right, though, I really should just look for the sweet spot under 49ma then?

 

[Wakarusa]

When the voltage is close it doesn't make that much difference. For 21w dissipation it'd be:
424V -> 49.528302 mA
427V -> 49.180328 mA

Certainly not enough difference to worry about in this case.

I really should just look for the sweet spot under 49ma then?
Yup. If you can't tell the difference tonally between 35mA and 49mA (ferinstance) go with the colder setting (35mA) for longer tube life.

Also, whenever you rebias it is good practice to run the amp for a while and then re-check (in case things have drifted). Also check for hot components and orange/red plates during the test.

 

[uncle looie]

I usually do that because I had '65 RI Twin at one point and after I got the bias set at a shop, I noticed one of the power tube's plates was glowing cherry red. Now, I usually put it in a dark room and look it over to make sure nothing's amiss. That bias rite meter is really handy for a guy like me. I never learned to set the bias with a regular meter and with this thing I can do it in less than 10 minutes start to finish. Taking it to the shop for a bias setting was just ridiculous because the guys around here charge a minimum $60 bench fee and they'll usually take 3 weeks to a month to get it back to you. I'm a working musician and I just can't do without my tools that long.