PICTURE TUTORIAL: Biasing a Tube Amp

greenplastic · #1 10.13.06, 6:12 AM

After endless looking on the internet I was so frustrated to find no resources anywhere that clearly described the process of biasing a tube amp WITHOUT a bias probe. After deciphering the gibberish, I have decided to post a tutorial complete with pictures on how to bias a JCM 900. This can still be applied to other amps, only the layout and look of the parts will change, not the method.

DISCLAIMER: I (A. J. Campbell) WILL TAKE NO RESPONSIBILITY FOR ANY INJURIES OR DAMAGE SUFFERED BY ANYONE AS A RESULT OF THE MISINTERPRETATION OR MISAPPLICATION OF THE INFORMATION CONTAINED IN THIS ARTICLE. PROPER HIGH-VOLTAGE SAFETY PROCEDURES AND PRECAUTIONS MUST BE PRACTICED AT ALL TIMES WHEN SERVICING VACUUM TUBE AMPLIFIERS! IF YOU ARE NOT *COMPLETELY* FAMILIAR WITH THESE PRECAUTIONS, SEEK GUIDANCE FROM SOMEONE WHO IS!
I have limited electronics experience but I am by no means an amp tech. FOLLOW AT YOUR OWN RISK

Additionally, if your amp has multiple Bias Controls, you will need to repeat the process by the number of trim pots you have, ie. 1 control: only do it to one tube, 4 controls: do it on 4 tubes. My amp has 1 control that affects all of the tubes therefore i modify only one tube socket.

OK lets do it!
http://i20.photobucket.com/albums/b2...n/DSC01860.jpg
1. Lets look at what you will need:
-Soldering Iron and Solder
-Solder wick or a Solder Sucker
-Digital Multimeter w/ aligator clips
-Medium phillips head and small flathead screwdrivers
-2W 1-Ohm resistors

2. Now take the guts out of your amp. Mine has a grille on the back which is help in place by 4 screws. Whip them off, remove it.

3. Lay the amp on it's front and unscrew the four screws underneath that hold the amps chassis inside. Take hold of the large transformers on either side and carefully remove the circuitry and place it on your bench.
http://i20.photobucket.com/albums/b2...n/DSC01863.jpg

4. Remove all of the large Power Tubes and flip it over onto the transformers.
http://i20.photobucket.com/albums/b2...n/DSC01865.jpg
CAUTION! Do not touching anything on the right side of the amp. The large capacitors (Blue Things) store over 500V WHICH WILL KILL YOU! Also, DO NOT TOUCH THE TUBE PINS WHEN ON!

5. Desolder the Ground wire from PIN 8 and replace it with the 1-Ohm resistor. Reattach the resistor to the ground wire by twisting the resistors wire around the ground's exposed end.
http://i20.photobucket.com/albums/b2...n/DSC01871.jpg

6. Turn your amp on, but leave it on STANDBY. Set your Multimeter to the highest DCV scale, ground (attach) the black probe to the chassis, and take a reading from pin FIVE of any power tube socket. The numbers of each pin are written on the underside on the tube socket.
A negative voltage of -35 to -50V should be detected.
http://i20.photobucket.com/albums/b2...n/DSC01868.jpg

7. Adjust the bias control until you have MAX NEGATIVE voltage on pin FIVE. The bias control is a small trim pot (often a 3-legged component with a flathead hole in the center for adjusting). I got a voltage of about -80V.

8. Install your tubes (the amp is still on STANDBY, remember) and wait a few minutes for them to warm up. Flip it over again and rest the transformers on a couple of yellowpages or thick books as the tubes are taller than the transformers.

9. Plug your amp into a cab and take the amp off STANDBY. Make sure your multimeter is still set to the highest DCV scale; take a reading between the chassis (ground) and pin THREE on any power tube socket. Remember, the BLACK probe always goes on the CHASSIS. Write this voltage down; you'll need it later.
http://i20.photobucket.com/albums/b2...n/DSC01869.jpg

10. Set your multimeter to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor. Adjust the bias pot till you get a reading of around 35-40mA.
http://i20.photobucket.com/albums/b2...n/DSC01870.jpg

11. MULTIPLY the voltage you read on pin THREE earlier by the reading you just obtained from the 1-ohm resistor, Compare with specs to see if it is operating within capacity (usually 20 watts MAX but each tube varies. Look up the specs for YOUR tubes.)

12. Take another reading from pin THREE (remember to set your meter on the HIGHEST DCV scale before you do!) and write it down. This new reading should be LOWER than the first reading you took, because the tubes are drawing more current now and the plate voltage will drop somewhat. Multiply this new reading by the value you measured across the 1-ohm resistor(s); this will give you the idling (static) wattage. The cooler you run the tubes, the longer they'll last. The higher the wattage, the shorter they will last. Feel free to plug your guitar in and play each time you adjust the control to see what sounds best.

13. When you have adjusted the control to how you like it, and you have cheacked they are operating within their specs turn off you amp and remove all multimeter probes. Remove the resistor and solder the ground wire back the way you found it and finally put all the guts back in the box.

FINISHED!

I Hope this made sense and help a lot of people out. Biasing I have found is a really easy process, certainly not worth the 200 bucks plus tubes you could pay for an amp tech to do it.

I took some of the instructions from http://www.duncanamps.com/technical/lvbias.html
Give it a read, it's very helpful.

Happy Biasing!

NoFroBro · #2 10.13.06, 8:18 PM

Quote:

Originally posted by greenplastic
<SNIP>10. Set your multimeter to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor. Adjust the bias pot till you get a reading of around 35-40mA.
http://i20.photobucket.com/albums/b2...n/DSC01870.jpg<SNIP>

You probably meant to write lowest DCA scale (usually 200mA) here.

Nice work otherwise.

olddawg · #3 10.13.06, 11:24 PM

(You probably meant to write lowest DCA scale (usually 200mA) here.)
Not necessarily, if I remember correctly, you're measuring the voltage drop across the 1 ohm resistor, so you would use the DCmv scale to calculate the current in miliamps through the 1 ohm resistor? I=E/R Since the resistor is 1 ohm the current in miliamps will be numerically the same as the DCmv measurement. You want to know the current through the resistor. Putting a miliamp meter across the across the resistor won't tell you much. It would have to be in series with the resistor which usually doesn't work well.

greenplastic · #4 10.14.06, 4:26 AM

Exactly, remember Ohm's Law!

If you measure the voltage across a 1-ohm resistor, V=I. The resistor is used as only very expensive multimeters have a current setting.

NoFroBro · #5 10.15.06, 6:59 PM

Could someone sticky this?

greenplastic · #6 10.16.06, 5:25 AM

Come on guys give me some feedback!

I was hoping someone would sticky this.
I believe it will help hundreds of people who want to or who are curious about biasing.

What do you guys reckon?

gollywog · #7 10.16.06, 9:23 AM

Quote:

Originally posted by greenplastic
Come on guys give me some feedback!

I was hoping someone would sticky this.
I believe it will help hundreds of people who want to or who are curious about biasing.

What do you guys reckon?

a very helpful tutorial, indeed.
the only thing i thought necessary were larger images.
other than that, good work.

nickwarhead · #8 10.27.06, 12:23 PM

"Set your multimeter to the lowest DCV scale (usually 200 mV) and take a reading across the 1-ohm resistor. Adjust the bias pot till you get a reading of around 35-40ma."

I'm confused. Are we reading DC Volts at this point? Are we looking for a reading of 35-40 volts DC or 35-40 millivolts DC?? Millivolts correct?

NoFroBro · #9 10.27.06, 5:21 PM

Quote:

"Adjust the bias pot till you get a reading of around 35-40ma."

I'm confused. Are we reading DC Volts at this point? Are we looking for a reading of 35-40 volts DC or 35-40 millivolts DC?? Millivolts correct?

I think it would be sensible to edit the text to read "Adjust the bias pot until you get a reading of around 35-40mV (Note: You are reading the voltage across the resistor to determine the current required. A 1 ohm resistor accomplishes this because V=I/R becomes V=I/1 which is of course V=I).

Now I am aware of the arguement that says if you can't figure this out for yourself you have no business in the back of a live valve amp. So I leave it up to the original poster.

If an edit is made I would also recommend adding that you should check the 1 ohm resistor is at least close to being 1 ohm before using it for this purpose.

nickwarhead · #10 11.01.06, 2:46 PM

I’ve been looking at the specs sheets for different brands of tubes(EL34), and cannot figure out where the idling wattage (Static Dissipation wattage) is listed, Does anyone know how to read these?

The document from Duncan amps says 20 Watts is a standard maximum for EL34s but generally true from brand to brand?

brokenbabyrobot · #11 01.08.07, 11:06 PM

I use a weber bias rite for biasing my amp. I like it a lot because there's no soldering and desoldering involved when I need to bias my amp. It's a real conveniant tool for those who want to spend a few bucks more. It makes biasing so much easier to do. it roughly takes 5-10 min. to bias my amp

Check it out: https://taweber.powweb.com/biasrite/br_page.htm

cannibal animal · #12 03.13.07, 4:36 PM

what if your amp has 2 little (+) and (-) clips on the back (i assume theyre for connecting a multimeter) and these two turn knobs above each? how do i bias that?

mapofyourface · #13 08.31.08, 3:16 PM

What if your amp doesn't have any trim pots?
And is a combo

frood · #14 01.28.09, 8:51 AM

i found that really interesting and the instructions were clear. i want to say it's helpful and i'm sure it is but i'm the clumsiest **** in the world and i'd probably drop the whole lot on my foot or electrocute my girlfriend or something.
still, something to bear in mind....