Click to enlargeDoes my amp really need
the bias adjusted?

adjust bias Does my amp need to have the bias adjusted?

In a single word - yes. All existing amps, with no exception, will benefit from having the bias set correctly for the power tubes.

There is a huge amount of incorrect information traveling the world about "fixed bias" and "automatic" or "auto" bias.

Fixed bias, if the term is being used correctly, refers to a negative voltage being fed into the control grids of the power tubes via a negative voltage bias supply.

This supply runs at very little current and 25 to 60 negative volts.

Many current amps have a truly fixed bias - that is, the voltage is permanently set by a resistor in the bias supply - to provide just one fixed voltage.

Better Amps have an adjustment pot somewhere on the amp chassis.

For economy some amps do not have an adjustment pot, possibly because of cost, or possibly because the manufacturer does not have confidence in you to adjust it right.

NO- it can not be made to accommodate all tubes - totally impossible.

The Automatic Bias name is often applied to a cathode biased amp.

In this circuit the tube's bias is set by a high wattage resistor connected to the tube's cathode.

This looks pretty permanent!! - but, the bias can be measured and can be adjusted by changing the resistor.

WRONG FACT. Cathode bias does not, in any way, automatically make an amp Class A, no more then a fixed bias prevents it from being a Class A amp. Either one can be either Class A, or Class AB.

CLEAR THINKING. Overcome the wrong concepts about fixed and automatic bias by doing a little logical thinking.

Why do we have "matched tubes" and how are they matched?

They are matched by measuring their characteristics, current draw, transconductance, and finding others that have the same character.

The matching process DOES NOT go inside the tube and change the grids, plates and cathodes to match them to another tube, the tube is already manufactured and sealed in a vacuum.

The very fact that they have to be matched clearly shows you that they are not able to be manufactured to extremely close tolerances - there are considerable variations between individual tubes, and even larger variations between brands.

You could not possibly think that the manufactures make the tubes to all these hundreds of different specifications on purpose - an absurd concept.

SO - since the tubes are obviously NOT scientifically consistent, how in the world can ANYONE make an amp that knows about these variations and adjusts itself automatically?? and with just a 10 cent resistor?

Your car is a very good example of the same variable science

You can buy new tires all day long and NEVER get a set that is automatically balanced - impossible as the tire manufacturing process is not that consistent.

You must have the wheels balanced or the car will shake itself to pieces.

Even a million dollar race car has to have the wheels balanced.

The power tubes in your amp, even if they are a matched set most likely have very different characteristics from the previous ones - the bias needs to be adjusted so they run in the correct operating range.

The power tubes that came with the amp will all have been different EVEN IF the company supplied matched sets, each set will have different specs from some of the others.

It is impossible to order 1000 sets of exactly the same match specs - that would require testing and sorting of maybe 150,000 tubes.

Taking this in mind, a simple look at the bias supply will show all have the same bias resistor (if it is not adjustable) - how could it be right all the time? - it can't and it is absurd to think it can be.

If you see it has been changed, as it may be with a very high end amp, there will have been an effort to bias the amp correctly, but this does not prepare it for the future - unless you have exactly the same matched set of tubes with exactly the same specs from the same manufacturing batch you still would benefit by setting the bias.

Even companies supplying their own brand of tubes change brands over the years, again resulting in a different set of specs even if their color or number coded labeling is the same.

Overall summary. there is no magic - everything is subject to the normal electrical and physical laws. no one can bypass reality.

So - set the bias!



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