Biasing my amp

[Bogged]

Say guys, I am wondering. My Bogner Alchemist 112 combo is a 40 watt amp. So does that mean each of the two 6L6 tubes be biased to 20 watts dissipation at idle?

I did this formula - 417 plate voltage X .047 ma = 19.6 watts. So this is 65% dissipation at idle. Good so far?

The tubes are SED SV6L6GC and I believe rated at 30 watts.

EDIT: I remember that the recommended value is .035 ma as told to a user by a Line6 tech but that's hearsay. I had it there one time:

425 X .035 = 14.8 watts. So that's less than 50% at idle. Sounds kinda cold.

 

[glpg80]

Say guys, I am wondering. My Bogner Alchemist 112 combo is a 40 watt amp. So does that mean each of the two 6L6 tubes be biased to 20 watts dissipation at idle?

I did this formula - 417 plate voltage X .047 ma = 19.6 watts. So this is 65% dissipation at idle. Good so far?

The tubes are SED SV6L6GC and I believe rated at 30 watts.

EDIT: I remember that the recommended value is .035 ma as told to a user by a Line6 tech but that's hearsay. I had it there one time:

425 X .035 = 14.8 watts. So that's less than 50% at idle. Sounds kinda cold.

40 watt amplifier class AB or class A? push pull or single ended?

im going to assume class AB push pull.

this is personal opinion, but i find it is actually better to bias around 57-62% PD, your cleans will not suffer and you will get better bass response - not as chewy so to speak. this is of course talking high gain amplifiers, blues players will probably prefer a warmer tone, as will low volume players who do not crank their amplifiers much.

to calculate current, use ohms law. a standard 6L6 is 25W max, 22W for calculations, 18-20W to be more realistic with tube quality and tube life.

P=IV, I= P/V

22Watts/417VDC = 52.8mA at 100% PD - do not exceed this for safety reasons. never use the absolute max of a tube as a bias ceiling - that is the point to where the tube will become damaged.

52.8mA times the percentage acceptable, in this case 62%, is around 32.7mA for 1 tube. try it again with 70% which is a much warmer bias is around 40mA.

if you are measuring using the center tap method of the OT, you will need to divide that measurement by 2 because that is for each side of the 60W class A/B amplifier you are using. the same goes for 120W amplifiers biased in push pull class AB i posted a pretty lengthly post about biasing a few posts ago with a few other, more involved methods if you are going for accuracy.

the other method to use is a bias rite - self explained tube socket method. not much to explain here, and no division required.

another method is to do it by ear, and measure it for safety. i can tell you that a few members here do that instead. just do not play the amplifier while measuring with weber bias probes since current draw fluctuates when a sin wave is introduced which can fry your meter causing it to short and bias to run away.

 

[Bogged]

You assumed right, class AB push pull. I got the plate voltage from pin 3 of each tube. Whenever I change the bias the PV changes so I worked back and forth to use the formula.

EDIT: by the way I put it back on .035 ma for now til I learn more about it. That's 15 watts per tube.

 

[glpg80]

You assumed right, class AB push pull. I got the plate voltage from pin 3 of each tube. Whenever I change the bias the PV changes so I worked back and forth to use the formula.

EDIT: by the way I put it back on .035 ma for now til I learn more about it. That's 15 watts per tube.

.035A or 35mA

that will be plenty for that 50 watter if you dont crank it all the time. you have alot of lee way - like i mentioned you can tune by ear and check for stability afterwards, i know alot of people that do that here on the forum.

take care and be safe

 

[Bogged]

Thanks and I stand corrected.

Do you always use 22 watts for calculations no matter what's the max wattage of the tube?

Under this formula I'm at 68% dissipation at idle, would that be correct?

 

[glpg80]

Thanks and I stand corrected.

Do you always use 22 watts for calculations no matter what's the max wattage of the tube?

Under this formula I'm at 68% dissipation at idle, would that be correct?

no, there are what is called datasheets for each tube stating parameters. most state absolute max and max (one meaning tube destruction the other meaning safe max) by engineering rule you always give room to grow - unless you like replacing tubes ever 2 months.

all electronic components have datasheets which help designers match components that fit their needs to their builds. think of it as a standard catalog system adopted by all electronic parts, companies, and engineers.

if you are going to self bias and want the most out of your tubes i would recommend knowing how to find the datasheet for your tube, and how to read them properly.

from there you know how to calculate it out as i have showed you

 

[glpg80]

Thanks and I stand corrected.

Do you always use 22 watts for calculations no matter what's the max wattage of the tube?

Under this formula I'm at 68% dissipation at idle, would that be correct?

nope it is not. read the post one more time, 22W/417V = 100% current maximum you do not exceed

multiply that answer by a percentage to get the suggested bias - the percentage is not the answer that you will get, but the number you choose. you cannot bias an amplifier off of a percentage number, you bias an amplifier off of its current for the mode of class you are using (class AB push pull)

hope this helps