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Author Topic:  Bias your tube amp
Paul Sutherland


From:
Placerville, California
Post Posted 14 Nov 2017 9:05 pm     Reply with quote

Just a reminder: If you have a tube amp checking the bias is very important. I was playing today and getting very unhappy with the tone I was getting out of all my tube amps. I tried various combinations of amps and speakers and still no improvement. I was pretty sure the power tubes in the various amps were reasonably fresh. I was starting to think my speakers were going bad. I was getting bummed out thinking my beloved JBLs were history.

But then I thought I should check the bias on the amps. All three of my tube amps were running cold with readings well below where they should be. After resetting the bias each amp improved immediately. My speakers are fine and all the amps sound wonderful again.

It's much cheaper to reset the bias than to buy new power tubes or speakers.
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Jim Sliff


From:
Lawndale California, USA
Post Posted 14 Nov 2017 9:40 pm     Reply with quote

If I might add:

For those with "bias tools" - if your tool only reads the bias current - NOT plate voltage as well - it doesn't allow you to set the bias correctly unless you get really lucky..

Bias needs to be checked and adjusted using plate voltage as a part of the "safe range" calculation. A good bias adjustment also involves fine-tuning by ear for good tone, but without knowing the plate voltage it's impossible to know the actual setting.

You can't find "60% dissipation" (or whatever you think you should be using)without it - and arbitrary numbers like "35 ma" are meaningless as standalone figures.

The bias setting itself usually changes with every different brand of tubes - often even among tubes of the same brand ("graded" tubes should still be checked!). The proper method is to check the plate voltage, calculate the safe setting range for the specific tubes (from the tube data sheet); adjust the bias somewhere in the middle of that range; tweak the setting and listen for differences - then set it how you like it as long as it's inside the safe range.

Some advocate setting it however you want as long as the tubes don't "red plate". That can be a fine way to still burn tubes out quickly.

Caveat - if you are not well-versed in tube amp safety DO NOT open the chassis. Many tube amps contain lethal voltages - even turned off and *unplugged*. If you don't know what you're doing - take it to a tech. Bias checking/setting is a pretty cheap operation, and should be done every time you change power tubes.
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Paul Arntson


From:
Washington, USA
Post Posted 15 Nov 2017 8:04 am     Reply with quote

65 DRRI: Built my own harness. First I matched the 6V6s in circuit from my collection of random used tubes. Once I had a pair I then brought the bias up from cold by ear.
There was definitely a spot where the tone was juuust right. They've been in there a couple of years no problem.
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John Limbach


From:
Billings, Montana, USA
Post Posted 15 Nov 2017 11:26 am     Reply with quote

If repair or modify tube amps regularly you can't beat the Compu Bias. Dirt simple and takes all the math and shock hazard out of it. Reads out plate volts, plate current, and plate dissipation for each tube at the same time.




Also, just about any amp that is grid biased can be easily modified to bias the final tubes individually. So no need to buy "matched" sets.

I like mine a lot.


Last edited by John Limbach on 16 Nov 2017 10:18 am; edited 2 times in total
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Paul Sutherland


From:
Placerville, California
Post Posted 15 Nov 2017 12:19 pm     Reply with quote

I understand, sort of, the risks of working on tube amps, and I don't do anything on the inside of the amp. But some amps have the bias pot on the outside, and for the ones that don't, pulling the chassis and carefully reaching in with a rubber handle screw driver seems safe enough.

Regarding getting the correct values, I was advised by my late amp tech what to use on my Twin. His value was also what Tim Marcus recommended as a good ballpark number for my Milk Man. Those settings always sound good, and they are very conservative. I also check visually to make sure there is no red plating.

My point is, if you are not comfortable checking and setting the bias yourself, you should pay someone to do it. It makes a huge difference. Somebody needs to do it.

Tube amps are not cheap to properly maintain. If you don't want the expense, buy solid state.
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Steve Sycamore


From:
Sweden
Post Posted 16 Nov 2017 1:37 am     Reply with quote

Could you adjust the bias using an oscilloscope? The aim is to set the level of the shoulder of each tube's response, where it comes out of hysteresis and becomes linear, to a matching level of its partner tube right?

That way minimal distortion is produced and maximum bandwidth is achieved. At least with an oscilloscope you can easily see the results of your settings on the waveform.
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Bill Terry


From:
Bastrop, TX
Post Posted 16 Nov 2017 5:59 am     Reply with quote

Most techs don't recommend biasing out the crossover notch as a reliable way (by itself) to bias an amp, although it's definitely something you can look at if you have the equipment. Here's an article from Aiken amps on that topic that's pretty good.

http://www.aikenamps.com/index.php/crossover-notch-biasing-why-it-should-be-avoided
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Mike Scaggs


From:
Nashville, TN
Post Posted 16 Nov 2017 6:15 am     Reply with quote

Bill Terry wrote:
Most techs don't recommend biasing out the crossover notch as a reliable way (by itself) to bias an amp, although it's definitely something you can look at if you have the equipment. Here's an article from Aiken amps on that topic that's pretty good.

http://www.aikenamps.com/index.php/crossover-notch-biasing-why-it-should-be-avoided


Yes, Aiken is a great resource.

Also, this is a nice little box to keep non-techs out of the insides where bad things can happen. It does measure plate voltages too which is VERY important to know.

http://www.asharpfretworks.com/5543.html

http://www.tedweber.com/webervst/tubes1/calcbias.htm
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Steve Sycamore


From:
Sweden
Post Posted 16 Nov 2017 7:15 am     Reply with quote

Interesting... How negative feedback affects the results could be a complicated issue, especially if you have, for example, a Mesa Road King or Roadster where the first 2 of the 4 channels use no negative feedback (from what I can see from the schematic) and where the Modern mode of the other 2 channels also drops negative feedback. Of course Mesa amps lack adjustable bias circuits which itself is a unique design choice.
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Richard Tipple


From:
Ohio, USA
Post Posted 16 Nov 2017 10:22 am     Reply with quote

Fellas im in way over my head here since I dont understand any thing about this ,bias, thing your all talking about Confused Are you talking only about amps with power tubes like 6L6s ? or also tubes in a pre amp ?

I use a Mesa Boogie pre amp with 12AX tubes & I didnt want to ask a Tech. if he could Bias my tubes & get a silly look Laughing
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Paul Sutherland


From:
Placerville, California
Post Posted 16 Nov 2017 11:20 am     Reply with quote

Biasing only applies to power tubes. It has nothing to do with preamp tubes.

There are some amps that don't have an adjustment on bias. Mesas are that way. There are probably others.
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Last edited by Paul Sutherland on 17 Nov 2017 7:37 am; edited 1 time in total
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Jerry Dragon


From:
New York, USA
Post Posted 17 Nov 2017 6:41 am     Reply with quote

all you ever need to know about tubes downloadable for free here

http://www.tubebooks.org/technical_books_online.htm
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Harold Dye


From:
Cullman, Alabama, USA
Post Posted 17 Nov 2017 7:44 am     Reply with quote

What does bias do? Is it set for a given amp or is it set for the instrument the amp is used for..i.e..is the bias different for a tube amp used for steel as opposed to 6 string or bass? I have two MM heads that I use for steel only. On one of them the bias can be set and on the other it cannot. They sound different but I think that is just the difference in amps.
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post Posted 17 Nov 2017 7:52 am     Reply with quote

I.m not aware of any tube amps that don't have a way to adjust the bias.
Here is the Music Man procedures:
http://www.pacair.com/discus/messages/6/35.html
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Chris Reesor


From:
British Columbia, Canada
Post Posted 17 Nov 2017 9:15 am     What is bias? Reply with quote

Biasing, in electronic circuits, sets the operating point of any tube or transistor amplifier at an optimum point for linear operation and minimum distortion.
In the case of the voltage amplifiers, like the preamp circuits in any audio amp, this is hard wired into the circuit by the choice of resistor values.
Power amp circuits in tube instrument amps are of two main types, called Class A or Class AB.
Class A power amps may be single ended (e.g. Fender Harvard or Champ) or push-pull ( Vox AC30). The bias is set by the choice of resistor value in the cathode circuit by the designer.
Class AB amps have a separate bias supply to apply a negative voltage to the output tube control grids, usually but not always adjustable.
Mesa Boogie amps at least through Mark III are not bias adjustable, and some Fenders have only a bias balance control which doesn't achieve what most of us think of as "setting the bias".
This is a gross oversimplification of course, and should be where your knowledgeable tube amp tech takes over, folks.
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Jerry Dragon


From:
New York, USA
Post Posted 17 Nov 2017 9:32 am     Reply with quote

most amps I have pulled apart and worked on were all self biasing with no adjustments.
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Paul Sutherland


From:
Placerville, California
Post Posted 17 Nov 2017 10:07 am     Reply with quote

There is so much that I don't understand about this topic. But my ears tell me it's an important topic for anyone trying to use a tube amp that has an adjustable bias.

The Compu-Bias and the ASharp Fretworks bias tools look very promising. A bit expensive (about $220 & $210 respectively) but probably worth it in the long run. There is a somewhat cheaper option in the Eurotubes Pro-One (about $100), but you can only test one tube at a time. Each devise gives readings for both plate voltage and bias current.

My current bias tool is a VHT Tube Tester that gives bias current using the cathode method. I don't really understand what the cathode method is, but I've read that it gives you somewhat higher readings. And it does not tell you the plate voltage, which was Jim Sliff's concern.

The nearest tube tech that I trust is about a two hour drive away, so getting a good bias testing tool makes sense for me. I guess I know what I'm getting for Christmas.
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Last edited by Paul Sutherland on 17 Nov 2017 10:43 am; edited 1 time in total
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post Posted 17 Nov 2017 10:11 am     Reply with quote

Mesa amps are bias adjustable, all tubes are calibrated by a resistance of some sort, to set the current flow through the tube. Mesa sells tubes in different "strengths", that allow you to purchase a replacement without resetting bias. I have heard a few claim that the "strength" numbers vary by a lot, and many Mesa owners check and reset bias when they change tubes. (Replace a resistor).
I always check and set bias during a tube change, whether it requires another resistor,(cathode), or setting the trim pot on a fixed setup.
More info:

https://robrobinette.com/Tube_Bias_Calculator.htm
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Paul Sutherland


From:
Placerville, California
Post Posted 17 Nov 2017 10:21 am     Reply with quote

Bill: Are you saying Mesas are adjustable because you can change a resistor?
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post Posted 17 Nov 2017 10:42 am     Reply with quote

Yes, just as are cathode biased designs. The reason for Leo to use a fixed bias on his 60's designs allowed easier adjustments with a variety of tubes.
That's how techs get the best sound out of them. Once you know the voltage, you determine the correct resistance to get the tube dissipation you are looking for. As Jim noted, playing the amp will determine if you still want to fine tune the resistance to get the sound you want.
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Paul Sutherland


From:
Placerville, California
Post Posted 17 Nov 2017 10:47 am     Reply with quote

Changing a resistor means soldering on the inside of the amp. That's something I'm not going to do. IMO that's more of a modification than an adjustment.
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Bill A. Moore


From:
Silver City, New Mexico, USA
Post Posted 17 Nov 2017 12:31 pm     Reply with quote

When I built my "Twin" clone, I was able to get a set of "Winged C" 6L6's. I had to change the resistor on the bias pot to get the bias set correctly.
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Chris Reesor


From:
British Columbia, Canada
Post Posted 17 Nov 2017 9:15 pm     Reply with quote

I'm inclined to side with Paul here on just what constitutes "adjustable bias" from the point of view of your everyday amp user; they are all adjustable if you can read a schematic, read color codes, understand voltage dividers, take voltage readings safely on live high voltage equipment, do Ohm's Law calculations, trace a circuit, solder, etc. Sure sounds like an electronics tech to me.

The "cathode method" probably refers to the practice of inserting a 1 ohm precision resistor between the output tube cathode and ground; then one reads the voltage drop across this resistor in millivolts to get the total current through the tube in milliamps. Multiply this number in amps by the plate voltage in volts to get your total plate plus screen dissipation in watts.
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