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Setting Bias

7/12/1998 5:58 PM
BIG Dave
Setting Bias
For years I've been led to believe that bias needed to set by an experienced bench tech that a had a signal generator, osciliscope, load devices, etc. Now I've seen these devices popping up that measure mA at the power tube and you simply tweak the bias pot until the meter reads within the range prescribed on the chart that comes with these devices. My question is, is this all there is to setting bias correctly and should I feel confident I won't burn out my tubes?
7/13/1998 12:40 AM

That's not quite all there is too Dave.  
A chart is OK for generic work... but not for fine tuning or tweaking for tone.  
If you use those charts you'll be stuck with what that person thinks is safe but still sounds good.  
I bet they wouldn't put it in writing if there was a chance you'd blow they tubes using their bad recommendations.  
There are plenty of techs out there that would have you believe there is some black magic, monkey paw mojo crap going on with "biasing tubes".  
Most of the time, with factory built amps, there is no need for anything but a good DVM set to a few hundred miliamps and some sound experience.  
It is so easy to do, that I frequently have people stop by the shop and chat about it and I end up doing it for free if the chassis is out of the amp.  
It only takes a few minutes.  
It also gives me a chance to hear the tonal differences of different tubes in different amps and at different bias levels, etc etc.  
I do it for free every time if they've bought their final tubes from me.  
I'm sure others do the same but might not want to admit it.  
7/13/1998 1:31 AM

never ending question...once i had my marshall biased at 70-90ma! no red plate, and it sounded incredible. but after a couple weeks i got paranoid and rebiased to 40-50ma. i'd be curious to get opinions on how others bias marshalls and such, and what is too much?
7/13/1998 9:26 AM

It depends on what tubes you are useing in your marshall,EL34s, 5881s 6L6s 6550s KT88s, the EL34s I usally set at 55 ma.if the plate voltage is low about 60 ma. never anymore than this, on 5881s and 6L6s, 35 ma. to 40 ma. never anymore than 40 ma.this sets it at optimim performance,I had a 50 watt plexi come in with the slide bias control and the plate voltage was 400 volts and they were running at 110 ma. ! when setting by the resistor method be sure to always check your resistor because if its off [1 ohm] it will change your reading quite a bit. I also go back and check the crossover distortion with the scope.for fine tuning.But your should be in the ballpark with the resistor method and have no problems with tubes overheating, If you do you may have some other problem causing it.Also I'll tell you another problem that you might come across,some people install the resistor so they can check the bias when they change their tubes ,If your amp has screen grid taps on your transformer don't install these pemanetly or you will run into a problem when you play loud, it will take about 10 mins. to find this out.[RICHIE]
7/13/1998 10:07 AM

Can you explain further? I can't see what problem would occur in an ultralinear output stage by having a small (usually 1 ohm) current sensing resistor permanently installed in the cathode circuit.  
7/13/1998 4:07 PM

I wish I could explain it, but I don't have an answer either,But something causes it to oscilate,I've only had this happen on marshalls, I just feel safer to not install this 1 ohm resistor on these.If you come across a marshall with screen grid taps try this yourself,play loud, and about 5 mins later you will see what I'm talking about.It will loose power and distort,if you just take out the resistor and tie it to ground it won't do it.Maybe has something to do with the marshall transformers being bipolar wound or something.I know the marshalls are very sensitive to impedance mismatch, so maybe they are sensitive to this too.I'm just guessing,because I don't know why this should happen.All I can say is try it yourself, let me know what you find. I'm curious too.[Richie]
7/13/1998 7:43 AM
Try reading the Tube Amp FAQ at; there is a whole section on biasing, covering why and the multiple how's, as well as some discussion of what it does to the tone.  
This kind of question is what the Tube Amp FAQ is for.  
You can indeed set the bias by meter and be assured you won't burn out your tubes; however, there is much more you can do.  
The bias setting affects the tone of the amp, as well as the static power dissipation, and probably tube life as well. The colder (lower static current) you bias your outputs, the more low level crossover distortion products you have, and the lower level you hear them. The hotter (higher current) you bias, the more you reduce the distortion products, but also the more power you burn in the output tubes just sitting there.

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