Tube Amps: Why Fixed Bias Amps Cannot Self-Bias

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. 

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