Setting the Bias

John Knight · From: Alaska

I have a 81-82 Fender Twin and it has an adjustable bias and my question is how do you go about setting it. What do you listen for or what readings do you take???

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C Dixon · From: Duluth, GA USA

John,

There is a missconception about that bias setting. Some, erroneously believe it is to adjust it for the least amount of hum. I am told on good authority this is wrong. Although I have to admit I did just that all the years I serviced them in Ft Lauderdale.

Having said that, I accept the fact I was doing it wrong, but the only problem is, NO body has ever given me a satisfactory answer as to the correct way. At least I have not perceived it as such.

Some say to set it for so many milliamps of current flowing through the ouputs stages. Others say to set if to a given voltage, etc.

My question has always been since I found out I had been doing it wrong,

"Where did you get your information?" I had, at one time, every schematic that Fender ever put out, and I have never seen one word concerning the correct procedure for adjusting that pot.

This is NOT atypical of manufacturers. Having worked for RCA for 34 years, it was common practice to leave our own technicians in the dark on many many things,

If you find out the correct procedure would you please let me know? I need to do it on my old Fender Twin reverb.

God bless you,

carl

Bill Terry · From: Bastrop, TX

John, this is a site with a fairly good description of alternate ways to adjust bias, some better than others:
http://www.people.cornell.edu/pages/mt24/Amp/mm/mm_main.html

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[This message was edited by Bill Terry on 08 May 2001 at 02:13 PM.]

Glenn Austin · From: Montreal, Canada

The purpose of the bias adjustment on a Fender or Marshall amp is to get both halves of the output section to amplify exactly the same, that's to say that you want both halves to clip at exactly the same point. To set the bias you need an oscilloscope and a sine wave generator. Inject some 1000Hz tone into the input jack. The scope should be connected to the speaker output. Raise the level of the sine wave until you see some clipping on the output waveform. You want to see power amp clipping,not preamp clipping, so some experimentation is in order. Tweak the bias pot until the waveform clips evenly on both sides. You are done. You'll probably want to wear ear plugs when you do this! or attach a 50 watt power resistance instead of the speaker.

Jay Ganz · From: Out Behind The Barn

The way later 70's early 80's Twins are
set up, you CAN'T really set the amount of current draw thru
the 6L6's. You can only balance them so
each is about the same. The bias adjustment
pot would have to be rewired similar to the 60's models to fully control the bias (actual amount of current draw in each tube). The output transformer "shunt method" is alot easier way to measure that current draw & accomplishes the same result.

Matt Farrow · From: Raleigh, NC, USA

Man, what a can of worms this topic is! "Bias" in a tube amp output section is just that - it is a fixed DC voltage that is either applied to the control grids of the tubes or developed across a resistor and capacitor in the cathode circuit. It is a means of determining the operating class, linearity, and output level of an amplifier system. Bias also has a tremendous effect on the "tone" of an amp.

There are a lot of different methods about setting the bias on your tube amp, but the main idea is that 1) you get the tone you're looking for and 2) you don't bias your amp so hot (low bias voltage, lots of plate current) that you damage it.

Some amps are ridiculously easy to bias (SVT, some Twins,) and some are terribly difficult (Ampegs with capacitive voltage dividers in the bias section, some Magnatones.)

If you really want to get your amp sounding great, take it to a tech you trust. This is not to say that you can't bias your own amp - in fact that's how a lot of techs got their start! But if you ask nicely, the tech might show you how he does it.

For me, the "proper" way to set the bias in a tube amp is done in this way:

1: measure the plate and screen voltage of the output section - knowing the output transformer impedance is also handy.
2: using an appropriate tube manual, find out the operating characteristics of the particular output tube in the circuit in question.
3: using either the "transformer-shunt" method or the "series ammeter" method, measure the plate current in the circuit - then adjust the bias until the tube is square in the middle of the operating class that it is supposed to be in for that circuit. This can vary by amp type and design, so a schematic of the amp is helpful.
4: play thru the amp, and fine tune the tone with the bias adjustment until it produces the desired results. I also make a note of the bias setting, both in terms of plate current and bias voltage so that I can get close the next time I service the amp.

Now, don't monkey with the innards of your amp unless you take proper precautions, ie discharging the filter caps etc. Tube amps produce lethal voltages and currents and can store these deadly charges even when unplugged! Respect electricity.

I don't know which Twin you have, but some of the red-knob Twins have a set of test points on the back of the amp that can help you set the bias voltage. Email me if you want some help on it.

Matt Farrow
Pharaoh Amplifiers

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Marlen 9-string 6+2
Kustom K150

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Donny Hinson · From: Glen Burnie, Md. U.S.A.

I have 3 different schematics for the "AB-763" Twin Reverb amps. They all say to set the output bias voltage to -52 volts. (I run mine at -48 volts.) Looking at most amps that use the 6L6's reveals that they call for a setting somewhere between -44 and -55 volts. If you want to "noodle" to try and change your sound, I would suggest you stay within these limits.

C Dixon · From: Duluth, GA USA

John,

Very respectfully, I rest my case! I still do NOT know how to do it!

well maybe I do!!

When I used to service amps, I used an oscilloscope and dummy load resistor (100w's) along with feeding a 1KHz signal in until the ouput stages began to clip to determine maximum power handling capability. I never used it to set the bias. Not saying I shouldn't have. Just never did.

But this afternoon I been doing just a whole lot of thinking about it. And while I don't service amps any more, I believe I may know how the factory did it even though (to my knowledge) they never told us exactly how.

Most quality power amps use Push-Pull stages in some configuration. The Fender Twin reverb uses four tubes in a parallel push-pull manner. That is by using two tubes in each half they doubled the amount of current which doubled the power (minus any inherrent losses).

A known problem with ANY push-pull setup is a thing called "crossover distortion". That is a distortion caused by the fact that when a P-P tube starts conducting current it is NOT linear for the initial part of the signal.

IF something is NOT done to counteract this, it causes a major and hearable distortion. In other words LOoooooow fidelity!

The bias on output stages determines the point where a tube operates. In P-P circuits both tube are cutoff with no signal by the bias setting. As a signal comes in to these tubes one of the tubes(s) is driven further into cut-off and the other one is driven to drawing current. As the signal changes polarity, the opposite is true. The bias setting establishes the point where they crossover so to speak.

As I look back now, I serviced amps before getting my real electronic's education. I feel now, I was over looking the obvious. If I had to do it over again. I would use the same setup and adjust the bias for the least amount of "crossover" distortion.

Note: this distortion is also minimized by special feedback circuits as well. But that is fixed and has no bearing on this post.

If you are thoroughly confused, welcome to the club,

carl

PS. Very respectfully, with the lone exception of the 110V AC power line section of the older Fender amps, I know of NO lethal voltages in the amp. If it was the case, I would have been dead a long long time ago. I admit when you do touch DC plate supply voltages you will wish you were dead (for a few moments

), but they are still quite safe relative to the 110VAC circuit where we are talking very high amperage capability versus a very LOW DC amperage capability in plate voltage supply circuits.

Remember it is the current that kills, NOT the voltage. The current is what goes through you. NOT the voltage. The DC plate supply section would load down very fast trying to send enough current through a reasonably healthy person to be lethal.

NOT so with 110VAC. That is a horse of another color!! And if it was 220VAC, you usually do not get a second chance!!

Let me leave you with an example that should spell it out quite clearly. In most TV's the Picture tube anode voltage is extremely high. in some cases 25,000 volts or higher. I have accidently touched this many many times. I agree it will wake you up in a hurry. But I have never known it to kill any body in over 34 yrs with RCA. And our techs got hit with it all the time!

On the other hand, people get killed all the time with simple 110VAC!!

What's the big difference? Simply this. The 110VAC can and WILL shove 15-30 amps through you if you are well grounded! That my friends will kill you!!

Plate voltage supplies even those with 400V + to 600V + DC capability and Picture tube anode supplies in the 25,000 DCV capability just do NOT have the ability to deliver even an amp of current. And milliamps (which is all the capability they have really) is not sufficient to be lethal.

If it were, there would have been many a dead amplifier repair technician in the last 50 yrs. Fortunately that has not happened because of the above. When you work on it, you get hit with it all the time. NO way around it.

[This message was edited by C Dixon on 08 May 2001 at 09:08 PM.]

Hamilton Barnard · From: Oro Valley, Arizona (deceased)

I'm with Pharoah on this, setting the bias is really quite simple...but a big pain to explain. Here's another take :

Borrow (free), build ($7) or buy ($30) a bias probe, this fits between one power tube and it's socket; hook this up to a meter that reads milliamps.

The newest Fender schematic I have is AA270 and there is no bias adjusting pot, only balance. If the bias is off, a change in the bias resistor will need to be done...or have a tech 'blackface' your amp and you will have a bias pot for your convience.

Bias probe kits can be bought from Hoffman Amps. I set my Fenders to 15 watts :
Cathode current = 15 (watts) / plate voltage.

If you decide to venture into this, and by all means do, ask lots of questions because you will be probing for voltages that can be from 400 to 600 vdc. And I know about that first hand...I took a 'hit' a couple of months ago. Yea, I'm still here, but I could tell it was a good one because of how hard my glasses were bouncing up & down on my nose. (hey! no laughing, that was a definite ouch)

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Blake Hawkins · From: Florida

Yes, we have all been hit with the voltage a few times.
Early on, I learned to keep one hand in my pocket and wear rubber soled shoes when I was poking around in those things.
Blake

Patrick Smith · From: Shreveport, LA, USA

John, I didn't have time to read all the above posts so this all may have already been covered.

I have two twins, a 70's SF twin and a 65 BF twin.

The 70's twin:

These have a set bias, determined by the value of the bias resistor on top of the bias pot.....that's not a "real" bias pot, however, it's a balance pot and shifts current either to or away from each pair of power tubes. This is VERY usefull because it allows you to use missmatched pairs of tubes and adjust for them. You cannot set the overall bias current going to the tubes. You can rewire it, but what I did which works beautifully, is to simply replace the bias resistor on top of the balance pot with a 50K trim pot...now you have the best of both worlds, your balance pot still balances if needed and you can adjust the current going to the tubes.

60's twins:

These have a true bias pot which will adjust the current going to the tubes. Fender is notorious about biasing these amps cold, however. I changed the value of the bias resistor to allow for more of a range. Now I can set the bias where it's supposed to be. Hope any of this helps.

BTW-Ohm's law says to bias these amps to around 32 mA....I tend to back off about 4mA and bias around 28 mA....still in the proper range but easier on the tubes. My 65 twin would only bias to around 20 mA, which is why I changed the value of the resistor...it now purrs at 28 mA

PMS

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