Bias Question

custom53 · 11-20-2009, 07:43 AM

Okay, I know this has probably been discussed before (many times)... How can I bias my JMP 2203 (changing from 6550s to EL34Ls) without the bias probe/tube socket...
I am an electrician and have many multiMeters (Digital and Anaolog)... Can I just measure across the pins or do I need to install a resistor...?

Wilder Amplification · 11-20-2009, 02:31 PM

The way I do it is to set the meter up for DC current, then connect it between pin 3 and the HT fuse on both valves (or both pairs of valves for 100 watters). The current meter function creates a short across each 1/2 of the primary since the meter has a 0 ohms input resistance when set to DC current mode, so the current flow through the valve will see the meter as the easier path.

Before reading current though, measure voltage at pin 5 and adjust the bias pot for the max negative voltage, then measure your pin 3 voltage. Perform this equation -

(Max Rated Plate Dissipation / Plate voltage) X .7 = Max Bias Current in Amps

Example, max rated plate dissipation for an EL34 is 25 watts. Let's say for instance you measure 460VDC at pin 3. The equation would look as such -

First divide max rated dissipation by the plate voltage -

25 Watts / 460 VDC = 0.054

Now find 70% of this value -

0.054 x .7 = 0.038 Amps or 38mA

In this scenario, your max bias current would be 38mA. For 100 watters, you would multiply this value by 2 since you would be looking at the current draw for two valves when measuring each side of the OT primary. Then you can measure current across both halves of the OT primary, which is effectively what you're doing when you measure from pin 3 of both valves to the HT fuse.

Also, when adjusting bias, first find out which pair is drawing the most (one valve or one pair of valves will be drawing a couple more mA than the other). Whichever valve (or whichever pair) is drawing the most will be which ones you want to set to this value.

Then adjust bias for what you're ears want to hear WITHOUT going above your max bias figure you obtain via the equation above. You can go BELOW that all you want, but NEVER above. Going above will overheat your power valves as well as create the potential for other problems as well.

Hope this helps.

TheGummy · 12-01-2009, 07:19 AM

Quote:

Originally Posted by Wilder Amplification

The way I do it is to set the meter up for DC current, then connect it between pin 3 and the HT fuse on both valves (or both pairs of valves for 100 watters). The current meter function creates a short across each 1/2 of the primary since the meter has a 0 ohms input resistance when set to DC current mode, so the current flow through the valve will see the meter as the easier path.

Before reading current though, measure voltage at pin 5 and adjust the bias pot for the max negative voltage, then measure your pin 3 voltage. Perform this equation -

(Max Rated Plate Dissipation / Plate voltage) X .7 = Max Bias Current in Amps

Example, max rated plate dissipation for an EL34 is 25 watts. Let's say for instance you measure 460VDC at pin 3. The equation would look as such -

First divide max rated dissipation by the plate voltage -

25 Watts / 460 VDC = 0.054

Now find 70% of this value -

0.054 x .7 = 0.038 Amps or 38mA

In this scenario, your max bias current would be 38mA. For 100 watters, you would multiply this value by 2 since you would be looking at the current draw for two valves when measuring each side of the OT primary. Then you can measure current across both halves of the OT primary, which is effectively what you're doing when you measure from pin 3 of both valves to the HT fuse.

Also, when adjusting bias, first find out which pair is drawing the most (one valve or one pair of valves will be drawing a couple more mA than the other). Whichever valve (or whichever pair) is drawing the most will be which ones you want to set to this value.

Then adjust bias for what you're ears want to hear WITHOUT going above your max bias figure you obtain via the equation above. You can go BELOW that all you want, but NEVER above. Going above will overheat your power valves as well as create the potential for other problems as well.

Hope this helps.

sorry to revive a bit of an old thread.

just wanted to ask if this is how biasing works with all valve amps?

Wilder Amplification · 12-01-2009, 09:20 AM

Quote:

Originally Posted by TheGummy

sorry to revive a bit of an old thread.

just wanted to ask if this is how biasing works with all valve amps?

It works for all Class AB push pull amps (all 50 and 100 watters). Make sure you fully understand the instructions that I've provided before attempting this.

Also -

When setting the meter up, remember that the meter essentially becomes a dead short when set up to read current (Amps). DO NOT REFERENCE THE METER TO GROUND (THE CHASSIS) WITH THE METER SET UP TO READ CURRENT!!! ALSO DO NOT TOUCH THE BLACK LEAD WHEN THE RED LEAD IS CONNECTED TO THE HT FUSE!!! ELECTRICAL SHOCK WILL OCCUR!!!

TheGummy · 12-01-2009, 09:38 AM

it'll be a while before i actually get stuck in, but i want to start to learn what goes on. usually when you ask what biasing is people say "if you don't know, get a pro to do it" so it's nice to actually learn a bit about it.

thanks yet again.

Wilder Amplification · 12-01-2009, 10:10 AM

Quote:

Originally Posted by TheGummy

it'll be a while before i actually get stuck in, but i want to start to learn what goes on. usually when you ask what biasing is people say "if you don't know, get a pro to do it" so it's nice to actually learn a bit about it.

thanks yet again.

What is bias?

When you're biasing the amp, you're adjusting how much electrical current (Amps) the output valves are drawing when no input signal is applied. This will also set how much current they can draw while playing the amp at full crank as well. Too much current and the plates will overheat and turn red (known as "cherry-ing out" or "red plating") while not biasing hot enough will cause the amp to sound "cold" and thin, although it won't hurt anything to bias cold.

By connecting the meter across each 1/2 of the transformer primary with it set to DC amps, the meter shorts out 1/2 the transformer primary and the valves draw current through the meter instead because the meter is essentially a dead short, so plate current sees the meter as the easier path and flows through the meter instead of flowing through the DC resistance of 1/2 the output transformer primary. Thus you're able to see how much current the valves are drawing and use the bias control to adjust this current draw.

TheGummy · 12-01-2009, 10:17 AM

makes pretty good sense, with the equations you gave earlier too.

Wilder Amplification · 12-01-2009, 10:21 AM

Quote:

Originally Posted by TheGummy

makes pretty good sense, with the equations you gave earlier too.

Just basic Ohm's Law really.

Voltage x Current = Power in Watts

Dividing the max plate dissipation power rating in watts for your valve type (25 watts for EL34s) by the voltage on the plate gives you the amount of current draw that will give you max dissipation.

Multiplying that by 0.7 gives you the current draw that will give you 70% of the max plate dissipation at your plate voltage, which is the maximum you should bias a Class AB amplifier at.

MajorNut1967 · 12-01-2009, 11:01 AM

You know what Jon shares all this accurate info and theory for free! And none of you, well most of you do not really appreciate it! If you guys have any real appreciation for Wilder you throw some work his way or tell your fellow Musicians about him. Cause personally I hint and beat around the bush with you guys, I'm not sharing all my experience and the time & dues I put in with anyone; much less for free!

And you blokes in the UK support PaoloJM too, he's bloody brilliant also. Point is they are both good gents and deserve a little credit. And you wannabe Internet gurus that question them willy nilly with out any grounds, go get stuffed. Now bugger off till the next time.

TheGummy · 12-01-2009, 06:49 PM

Quote:

Originally Posted by Wilder Amplification

Just basic Ohm's Law really.

Voltage x Current = Power in Watts

Dividing the max plate dissipation power rating in watts for your valve type (25 watts for EL34s) by the voltage on the plate gives you the amount of current draw that will give you max dissipation.

Multiplying that by 0.7 gives you the current draw that will give you 70% of the max plate dissipation at your plate voltage, which is the maximum you should bias a Class AB amplifier at.

yea the ohms law bit makes sense, i just wasnt sure on how it applied itself in practise, but now i do.

is there a particular reason that you bias at 70%? is it related to RMS running levels at all?

Wilder Amplification · 12-01-2009, 11:47 PM

Quote:

Originally Posted by TheGummy

yea the ohms law bit makes sense, i just wasnt sure on how it applied itself in practise, but now i do.

is there a particular reason that you bias at 70%? is it related to RMS running levels at all?

All has to do with load line, plate load and class of operation, average power output/dissipation, and B+ voltage. In Class B operation the valves are biased at or near "cutoff" (i.e. where no plate current flows) and pass current for 50% of the AC cycle, whereas with single ended/Class A you bias to 100% dissipation as one valve conducts for 100% of the AC cycle. Class AB falls in between the two where each valve conducts for more than 1/2, but less than the full AC sine wave. With Class AB, you have what's called "overlap", where unlike Class B there is a period in the AC sine wave where each valve is passing current at the same time as the other.

Class of operation also has to do with how much electrical current draw the valve can handle at the plate voltage it runs at.

5er driver · 12-02-2009, 02:57 AM

Quote:

Originally Posted by MajorNut1967

You know what Jon shares all this accurate info and theory for free! And none of you, well most of you do not really appreciate it!

Jon, thanks for sharing the bias procedural info. I'm to the point where I want to bias my amps myself and in the future before I actually get my hands into them I feel I know who to contact. You Rock!

Thanks again,

TheGummy · 12-03-2009, 11:25 AM

Quote:

Originally Posted by Wilder Amplification

All has to do with load line, plate load and class of operation, average power output/dissipation, and B+ voltage. In Class B operation the valves are biased at or near "cutoff" (i.e. where no plate current flows) and pass current for 50% of the AC cycle, whereas with single ended/Class A you bias to 100% dissipation as one valve conducts for 100% of the AC cycle. Class AB falls in between the two where each valve conducts for more than 1/2, but less than the full AC sine wave. With Class AB, you have what's called "overlap", where unlike Class B there is a period in the AC sine wave where each valve is passing current at the same time as the other.

Class of operation also has to do with how much electrical current draw the valve can handle at the plate voltage it runs at.

ah i see, that makes sense. again, thanks!

11-20-2009, 07:43 AM	#1 (permalink)
custom53 Senior Member Join Date: Sep 2009 Location: Ohio Posts: 126	Bias Question Okay, I know this has probably been discussed before (many times)... How can I bias my JMP 2203 (changing from 6550s to EL34Ls) without the bias probe/tube socket... I am an electrician and have many multiMeters (Digital and Anaolog)... Can I just measure across the pins or do I need to install a resistor...? __________________ 2004 Gibson Les Paul Flametop 1995 Hamer Duo Tone Sunburst 1991 Ernie Ball EVH Trans Black 1981 Hamer Special Trans Green Quilt Sustain Block Bridge with Checkerboard Headstock 1975 Ovation Breadwinner Black Marhsall JMP 2203 Half Stack Marshall JTM 45 Reissue Half Stack 2ea 370/301 Acoustic Bass Amps Peavey Classic 100 Half Stack Tweed Soldano HR50+ Half Stack Black Peavey Butcher 120 Watt Half Stack

12-01-2009, 09:38 AM	#5 (permalink)
TheGummy Member Join Date: Oct 2009 Posts: 45	Re: Bias Question it'll be a while before i actually get stuck in, but i want to start to learn what goes on. usually when you ask what biasing is people say "if you don't know, get a pro to do it" so it's nice to actually learn a bit about it. thanks yet again.

12-01-2009, 10:17 AM	#7 (permalink)
TheGummy Member Join Date: Oct 2009 Posts: 45	Re: Bias Question makes pretty good sense, with the equations you gave earlier too.

12-01-2009, 11:01 AM	#9 (permalink)
MajorNut1967 Senior Member Join Date: Jul 2009 Location: South Pacific Posts: 1,020	Re: Bias Question You know what Jon shares all this accurate info and theory for free! And none of you, well most of you do not really appreciate it! If you guys have any real appreciation for Wilder you throw some work his way or tell your fellow Musicians about him. Cause personally I hint and beat around the bush with you guys, I'm not sharing all my experience and the time & dues I put in with anyone; much less for free! And you blokes in the UK support PaoloJM too, he's bloody brilliant also. Point is they are both good gents and deserve a little credit. And you wannabe Internet gurus that question them willy nilly with out any grounds, go get stuffed. Now bugger off till the next time. __________________ MJN1967 aka BA (Bad Attitude) 1958 Fender 5E3 Deluxe 1968 model 1987 50watt. A Baby Major (Half-Power) I built from the ground up. JTM 45 Clone combo Ground up build. JCM 800 Clone Ground up build, with Wilder Mods. Custom Built Warmoth Strat 75' Fender Tele-Deluxe '70 style strat custom build w/Custom Shop 69's PU's

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