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Bias controls for fixed-bias tube amps
Hi Guys

Readers of our books will know that we follow a universally-applicable definition for bias conditions:

High bias = high heat caused by high current

Low-bias = low heat caused by low heat

A "fixed-bias" tube output stage can be confusing to some hobbyists and builders inasmuch as the bias should be adjustable. The "fixed" term is with respect to whether the idle point shifts with signal present. The first attempt to control this facet of performance was with cathode-biased amplifiers, where a very high-value bypass cap is added across the bias resistor. The typical 22uF to 50uF seen in guitar amps barely has an effect as anyone who measures Vk will know.

Cathode bias tends to be inefficient and has unnecessarily high heat at idle for higher-power applications. The fix is to ground the cathodes and apply negative voltage to the control grids. Remember, tubes are cathode-centric and to control them the grid voltage must be negative with respect to the cathode NOT negative with respect to ground - the tube does not know where ground is but it knows where its cathode is Smile

The negative grid supply is referred to as the "fixed bias voltage". The circuit is equivalent to a cathode-biased circuit with an infinite Ck value, so this new bias method achieves the goal of "fixing" the idle condition regardless of the signal amplitude.

If Vg = Vk there is no control over idle current and the tube draws as much current from the supply as the circuit allows, destroying itself and the output transformer in the process. obviously, we should do what we can to keep this from happening. For self-bias, Rk is the limiter, but in fixed-bias, we have to restrict how close to ground Vg can get. This is achieved very simply by placing a "range resistor" in series with the bias adjustment pot-X end. This resistor is often left out in generic "concept" or "application" drawings, as the writer wants to make the drawing clear, or they are having the quite common issue that drawing software is not as easy to use as it should be. The result is that uninformed readers will take this simplified drawing as the recommended approach and build circuits lacking bias-restricting resistors.

Ironically, many people who should know better, present their information using the simplified circuit AND their own products use it too, making it possible for a user of either the product or the information to damage the tubes or expensive OT.

Some designers or builders believe that if they use a multi-turn trim pot for bias control, that there should never be such a situation as described above. However, this assumes that the person doing the biasing knows enough to preset the grid voltage as negatively as it can be right at the tube socket. Most people do not think of this and need to be directed.

Following our bias condition definition, we also approach the wiring of bias set-pots in a universal manner:

Fully-clockwise = hot = high current

Fully -counter-clockwise = cold = low current

In a tube amp, the raw bias voltage should be applied to pot-0 end (CCW) and the range resistor to ground ties to pot-X (CW). This gives the same response as a Level or EQ control, where clock-wise equals "more", which in this case, means "more current".

The range resistor value depends on the bias pot value and whether there are multiple bias pots in parallel for individual tube control or for control of the two halves of the push-pull circuit. Generally, the range resistor is about one-quarter the pot value, or one-quarter the net parallel pot value.

Ideally, there is a safety resistor across each bias pot from pot-0 to the wiper. This resistor is usually ten-times the pot value and comes into play if the wiper opens providing an alternate path for the bias voltage.

Obsolete texts will use a grid-voltage bias reference and thus present an incorrect notion of bias. Grid voltage is the most important voltage in a tube amp BUT its value is unimportant except that it should have sufficient range to properly control every tube sample.

All of the above info and much more can be found in the TUT volumes (The Ultimate Tone book series)

As TUTs show, single-ended amps can be fixed-biased, too.

Have fun
Hi Guys

As TUT3 (The Ultimate Tone vol.3) shows, bias pots should be actual panel pots NOT small trimmer pots. The reason for this is two-fold:

The life cycle of most trimmers is 200-cycles (rotations). Yes, you can find a few with higher ratings, but panel pots are still better. The least expensive panel pot that does not even provide a life-cycle rating is typically good for 15,000-cycles. You are not likely to approach this number even if you change tubes constantly.

The ergonimics of trimmers is exteremely poor. A tool is needed to manipulate most of the available styles; some have thumb screws on them. Bourns offers a chassis mount holder for some of their models, and this fits into the same hole-size as a 1/4" jack. You still need a tool to set the trimmer and there is still not as good life expectancy as a panel pot.

Per TUT3's recommendation, the bias pot should have the same accessibility as the tubes. In most cases, this means the pot should be externally accessible with the best mounting position being on the tube plane of the chassis. The pots and tubes then have the same protection and the same accessibility.

ideally, there are meter jacks on the rear apron of the amp for easy monitoring of the idle condition.
Thanks for the great post! I'm in 100% agreement with you about trimmer pots. I work on tons of amps that have them---they adjust poorly and often placed too close to high voltage to comfortably adjusted. After reading TUT 3 all my amps got pots for each tube and repairs and adjustments are a breeze. It also a mod I do to vintage amps so players can use NOS power tubes without constraint.

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