Tube Amps: Why Fixed Bias Amps Cannot Self-Bias

Tube Amps: Why Fixed Bias Amps Cannot Self-Bias

September 08, 2016 18 Comments

Tube Amp

In response to our recent article about replacing tubes in your amplifier, we received the following question from a Carvin Audio enthusiast. 

“Great article. I would like to see an article that describes how the power tubes are correctly biased. Ex: I own a Carvin Audio V3M head, but it uses Electro-Harmonix power tubes. If I want to use Sovtek EL-84s what must the voltage for the output tubes be set at? Lastly, I hope Carvin Audio can develop a power amp with a self biasing output section like my Mesa Tremoverb had. All I had to do was select EL-34, 5881, or 6L6…It self adjusted to whatever power tubes I decided to use.” 

Is there a specific bias voltage? The short answer is no.
The first thing is Electro-Harmonix and Sovtek are brands of tubes and in fact some Electro-Harmonix tubes may be Sovtek tubes. You will not see a switch on an amp for tube brands. To answer the question, “What is the correct bias voltage?” We need to look at what is being set with the bias voltage. There are bias voltage ranges for the different types of output tubes like 6L6, EL34 or EL84, but inside of that range you need to set the actual bias voltage for the particular amp and for the new tubes being installed. There are rule of thumb formulas using the tube’s wattage, the voltage of the particular amp and a percentage of operation that help a technician know the bias voltage starting point. But there is not a specific single bias voltage for a particular tube type that magically biases your new power tubes. In most cases, it is safe to say that if your bias voltage is adjusted to the factory setting for the same type of tube, you probably won’t have catastrophic failure and the amp will function. But it is equally unlikely that your new tubes will just happen to draw the same current as your old tubes. When it comes to your output stage, you want lots of power, but you don’t want unflatteringly crossover distortion or to fry a tube! Simply being in the middle of the adjustment range doesn’t mean your amp will sound and perform properly.


Let’s look at what’s happening inside your amp’s power tube. The bias voltage is like the idle adjustment on a classic car. If you went to your mechanic friend and asked, “What is the correct idle setting for this model?” Chances are they will say, “When it’s right.” Every motor runs a little differently, so you can’t set it to the same spot on every car. There may be a start spot that puts you in the range, but it must be fine tuned for the right performance. The bias voltage on your amp energizes the input control grid inside the tube. This is a negative voltage which sets the flow of electrons or current through the tube. It must be fine-tuned for each tube, because the voltage input controlling the resulting current flow varies greatly in tubes. Out of 100 tubes 10 may have similar current output for the same input bias voltage. And this is why tube amps like matched tubes. As mentioned in the previous article most amps have one bias adjustment for all of the tubes, so you can’t adjust for each tube. If you buy premium tubes that are matched by current-rating (or ‘hardness’) then you can have your amp adjusted professionally for the first set. Then if you keep using tubes from the same supplier with the same current-rating, you can just put them in without re-biasing, although it is still good to have it rechecked at some point to make sure it is still at right setting. Otherwise every new set of tubes will need to be biased. This may be a hassle, but getting it done properly is the only way you will know your tubes are dialed-in correctly.

Tube Type Selection Switches:

Why does the amp have those little switches on the back that are supposed to ‘bias’ the amp for one tube type or another, if you have to adjust the bias every time anyway? For example, the Tremoverb amplifier mentioned in the comment, the Carvin V3 and Legacy amps all have a selection switch on the back to adapt the general output circuit for EL34 or 6L6/5881s. This is where the bias voltage range for the different tube type is adjusted. These two (or three- 6L6 and 5881 tube are basically the same tube) tube types can be designed to work in the same amp, but the bias range is very different, so the switch places the bias adjustment control range for the tube type selected. Think of it as a rough adjustment before you do the fine bias adjustment.

Self-Biasing Amplifiers: Yes they do exist

The typical tube amp is a fixed bias amplifier. These may have an adjustment pot or a fixed resistor for the bias voltage setting, but either way they are called a fixed bias amplifier when there is no cathode resistor on the output tubes. The bias current is set purely by the input bias voltage. A self-biasing amplifier, also know as a cathode bias amplifier, has a cathode resistor that sets the bias current of the tube. Typically these are in the 30 watt and under power range, but not all lower wattage amplifiers are cathode biased amplifiers. In these amps a cathode resistor creates feedback and corrects the bias of the tube. It is still better to get matched tubes, but it is not as critical as with the fixed bias amplifiers. Of course, if your amp has a single output tube; it is running in full class A mode and does not apply to our article here. You can’t get a matched set of one tube.

If your amp is a fixed bias amp, you will need to bias it. If you are qualified to do it yourself, one reliable and relatively safe way to do this is with a specialized ‘bias probe’, but you will still be dealing with lethal high voltages to make adjustments. This method allows you to read the current draw in the circuit with a simple voltage meter. Carvin Audio uses the standby switch and current meter method that shows the current through the main voltage supplying the tubes. With either method the voltage can only be adjusted for all the tubes, so you still need to get matched sets to know they are all working the same. The only alternative is to buy a big pile of tubes and test them all for current to get a matched set. You either have to pay for the pile of tubes or pay somebody else to match them for you. As a wise mentor once said, “You pay... or you pay. How do you want to pay?” In the end, no matter how you choose to bias your amplifier you will always be rewarded with great sound, lots of power and long tube life, and that makes it an important consideration each time you re-tube your amp.

Carvin Audio offers matched sets of EL34, EL84, and 6L6 power tubes



18 Responses

R_F
R_F

February 24, 2018

Robert Tomer in his 1960 book was adamant that fixed bias is a mistake. “It provides no margin of safety in the event of tube or circuit malfunctions. It magnifies the inevitable minor differences between tubes. When trouble does develop the results are typically quite destructive.”

Jeffrey Fowler
Jeffrey Fowler

September 26, 2016

There is a point when you are going to need an amp tech. Current, voltage…all of them can be adjusted to cause a tube characteristic all its own. I owned a Laney that just would not sound good clean. It was the way the former owner had it set up. I thought about having it set up but instead I traded for a Twin that sounded good clean and was set up that way by my amp tech. I don’t that you can ever get all your sounds out of one amp…..perfectly.

If you find a good amp tech, treat them good.

Ronald Mullins
Ronald Mullins

September 09, 2016

I agree great article

bob mitchell
bob mitchell

September 09, 2016

Yes, that’s all fine and good… now back to how do you want to pay. I want an engineered design that allows the owner to bias the tubes without opening the case. A digital meter that is switchable into the circuit to set the bias. Yes you will have to open the case to insert new tubes but the bias would be set case closed, no lethal voltage … for about 20 extra bucks, meter, pots, and a new pcb design. You could then check the bias every time you turn it on instead of wondering does that sound weird, which is the other sign that the tubes are not biased correctly….

Ed Sullivan
Ed Sullivan

September 09, 2016

Robert Jensen makes a very important point. A self-biasing amp saves cost by not needing an extra negative-voltage supply, and saves the consumer time and money by not needing professional servicing and adjustment of such. It is, however, a compromise in that the optimum bias setting for maximum power and lowest distortion is not really possible without, as the article suggests, trying hundreds of tubes. And an amp with fixed (negative-voltage) bias but no way to adjust it, does a disservice to the consumer by forcing them to buy specially-selected (expensive) tubes, or a big box of cheap ones, hopefully finding a couple that will work ideally. Better to have a technician install an adjustment pot, or two if there’s room.

Any amp can be constructed to be self-biasing, but there will always be a certain amount of audio power lost to the cathode resistors, especially as the design power goes up and larger resistors are needed. If high power is the goal, then this type of construction is somewhat self-defeating.

David
David

September 09, 2016

Agree completely with the article. Have 2 tube testers, one made in the 60’s and a modern tube tester with automated and more through testing. They concur.

Tip on the v3m, try some 9-pin extenders, so the bias probes, similar to the ones for a Weber type bias meter, will fit. The probes will not fit in through the chassis into the tube socket. The 9-pin tube “saver/extenders” bring it just high enough to work perfectly.

I have one of the 1st ones and is a fixture in my studio. Gets use every week! The “iron” and circuits of the v3m really respond to different tube brands and strengths, shows the quality of the components. Keep it up Carvin!

Bruno Mariano
Bruno Mariano

September 09, 2016

Still, 100mA through the standby switch, as printed on Carvin Legacy PCB, isn’t too low for a quartet of EL34? It’s only 25mA in each one, max.

Bill Greene
Bill Greene

September 09, 2016

Excellent article!

Jim Lahman
Jim Lahman

September 09, 2016

Your article is a little misleading. A cathode biased amp can be either fixed or adjustable. Part of the cathode resistor can be made adjustable.
Grid biased amps can be fixed or adjustable. Mesa boogie, on some models, have no bias adjustment, so you could say they are fixed and cannot be adjusted without changing parts.
So with grid bias or cathode bias, either can be adjustable or fixed.
A single tube class A amplifier can be equipped with adjustable bias as well. It can also be grid biased or cathode bias. Even self biasing amps, (Cathode Biased) need to be checked for plate current draw to be sure the output tube is operating within it’s safety zone.

Gene Oates
Gene Oates

September 08, 2016

Great article and thanks for the explanation. I’ve changed the output tubes on my V3 and found the instructions to be be great, assuming you have an electronics background. The only issue on my V3 is the thumb potentiometer position on the circuit board is obstructed making it impossible to simply use a plastic screwdriver to properly adjust it in the screwdriver slot. This makes it difficult as you have to use the side of the pot wheel to adjust it. A better layout should be considered.

Biasing the new tubes, the amp still sounds sweet…..

Brian Strasmann
Brian Strasmann

September 08, 2016

I have heard that Fender Blues Jr. are self bias-ing amps. I just put a new set of tubes in my Blues Jr. and it corrected the “ringing” effect I experienced on some chords and notes. I bought a matched set of output tubes. This is a 15 watt amp. Does this sound right?

Barnet M. Schmidt
Barnet M. Schmidt

September 08, 2016

This is a useful but fairly simplistic explanation of bias. I realize this article is for musicians who are laypeople in the area of electrical engineering but instead of the analogies to car engine idle speeds, simply say that the grid or cathode bias adjusts the gain balance (DC balance) between the two halves of the output stage. The bias adjustment is to ensure that there is no zero gain or output at the point where the output signal changes polarity from positive to negative or vice versa (which is the cause of the crossover distortion you mention). Of course, in engineering terms the bias sets the operating point on the tube’s characteristic curve (plate current vs. grid to cathode voltage, etc).

The mention of a “tube drawing current” is a little ambiguous in this context. More precisely, the plate circuit of the tube draws current from the plate voltage source proportionally to the grid voltage and supplies this current to the output transformer primary, thus providing power gain.

I love your amps by the way!

Very truly yours,

Barnet M. Schmidt, Ph.D., P.E., SM IEEE
Managing Partner
Kaplan/Schmidt Associates, LLC
Consulting Communications Engineers
West Orange, NJ

David
David

September 08, 2016

Thank for the solid advice. I have worked as a bench tech, mostly repairing solid state devices. This info has totally informed me or biasing tube amps, and I do own a couple three Carvin tube amp. I am retired but do get calls to repair amps, so I am set to tackle tube devices . Thank you Carvin. Your the best.

Kevin
Kevin

September 08, 2016

Not to be rude but I think you might’ve missed the premise of the question. I own a Peavey Ranger 212 120W amp which uses 4 6l6 power tubes and 4 12ax7 preamp tubes. The amp is cathode bias and it’s a breeze to change tubes. Mesa Boogie is also cathode bias and a breeze. So, if the technology exists to create high power amps that never need to be biased then why do companies still make fixed bias amps and why do people buy them? It seems like an unnecessary hassle. Maybe you could explain.

Roger Bergen
Roger Bergen

September 08, 2016

Great article, I learned a lot! I use a bias probe and volt ohmeter to adjust the bias in my amps. I have my tech install a mini bias pot in my old amps that don’t come with a bias pot from the factory…he installs it inside the chassis wiring so he doesn’t have to drill a hole in the amp’s frame and hurt the originality and value!

Cameron S
Cameron S

September 08, 2016

right!!!…on tubes I found out by keeping up with carvin is… its like tuning your motor on your car or what have you…there is no set range…just tune it till it puuuurrrrrzzzz… lol…love the info from carvin!!!!!

Larry Scudder
Larry Scudder

September 08, 2016

My power tubes just went out on my less than year old v3 head. If I buy a set of el34s from you do I need to take it to a tech to be biased?

Robert Jensen
Robert Jensen

September 08, 2016

Mesa amps have no bias adjustment. This is why you’re forced to buy their output tubes. These have been tested to fall within a bias current range that works for their amps. Again, using the carburetor analogy, this range is narrower than just a starting point but doesn’t allow complete optimization that a “fine tuning” might allow.

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