how often to bias?

[nomad100]

how often should i have to rebias? seems like as long as i've had it, i've had to rebias ever week. One tube will drift, biasing a pain the pots are so touch!

 

[m0jo]

Which amps and what tubes are you using?

Also: at what value do you bias them?

 

[nomad100]

I have the rm100, and i'm using 2 x KT77 and 2 x 6l6's all JJ. Yesterday one of my kt77 had jumped from 37mv to 44mv. Seems like it's the tubes on the left side that drift up the most.

 

[tung]

one of my kt77 had jumped from 37mv to 44mv. Seems like it's the tubes on the left side that drift up the most.

yes, it does seem that way. I run 6L6s and E34Ls quad too. I try to check the bias before a gig, but sometimes just don't have the time. If a tube has drifted significantly, I can hear the difference in tone, so I'll take care of the bias right away.

 

[nomad100]

Thanks, it's just getting old i guess. I also keep going threw pretubes in v1, i swap them around but the seem to go microphone quickly.

 

[GuitarGod87]

Thanks, it's just getting old i guess. I also keep going threw pretubes in v1, i swap them around but the seem to go microphone quickly.

what do you mean by "microphone"?

 

[kc2eeb]

Microphonic i.e. behave like a microphone.

 

[nomad100]

yep

 

[eyeball987]

I don't seem to have any bias drift issues in my RM100. I usually only check it when I swap the PI from a 5751 to a 12AX7 or vice versa.

For the V1, are you talking about the modules or in the amp itself?

I burned through a couple tubes in the V1 spot of some modules when I was hot swapping between two RM50 combos to A/B them but otherwise all has been well.

If you are talking about the amp, what are you hitting the front end with?

 

[nomad100]

v1 on the amp itself, and i talked to sweetwater i'm going to take itin some day soon.

 

[Mike P]

I don't seem to have any bias drift issues in my RM100. I usually only check it when I swap the PI from a 5751 to a 12AX7 or vice versa.

Huh?

Biasing only effects the power tubes, not the preamp, effects loop, etc.

 

[kc2eeb]

The PI can affect bias.

 

[Mike P]

The PI can affect bias.

I've never heard that before. Interesting.

 

[Mike P]

I've just spent the last 20 minutes scouring the web to confirm because no one has ever mentioned to me, in more than 30 years of owning tubes amps, that the power tubes have to be re-biased if the PI tube is changed. Not Red Rhodes (RIP), not Jerry at Amp Crazy - no one.

From another forum:

Re: phase inverter tube - different bias?

Postby Vortexion on Sun Dec 13, 2009 7:18 pm
No.

...and that's the long answer.

In 99.999% of instrument amps, no DC voltage is allowed to leak through from the phase-splitter to the grids of the output valves. It's blocked by the coupling capacitors.

http://forums.vintageamps.com/viewtopic.php?f=4&t=88503

 

[kc2eeb]

Mike
Unfortunately, when I posted, I was a work and could not elaborate.
I certainly didn't mean to send you on a quest. No, there would be no reason that a PI would have anything to do with output tube bias, per se.
What I never got back to type was that changing the PI tube to either a different type or a simple replacement may change the point where one would want to set the bias point and , oddly enough, I've seen bias voltages
change a ma or so with the PI removed. Why? Circuit noise, heater current draw, plate current draw, take your pick. When I have some time, I'll look into it. That fair?

 

[Mike P]

Mike
Unfortunately, when I posted, I was a work and could not elaborate.
I certainly didn't mean to send you on a quest. No, there would be no reason that a PI would have anything to do with output tube bias, per se.
What I never got back to type was that changing the PI tube to either a different type or a simple replacement may change the point where one would want to set the bias point and , oddly enough, I've seen bias voltages
change a ma or so with the PI removed. Why? Circuit noise, heater current draw, plate current draw, take your pick. When I have some time, I'll look into it. That fair?

No worries, Dude! I've had amps biased over the years by some pretty renowned amp builders and repairmen and that's never been mentioned, so I searched while enjoying breakfast this morning.

I purchased a Weber Bias Rite last year and haven't needed change any of the tubes or needed to re-bias to date but it would be nice to know if re-biasing needs to occur if the PI were to fail and be replaced. Thanks!