Originally posted by

**scole**View PostYou don't need a ten turn anything with this.. a regular old 50 cent 1/2 watt linear trim pot is fine.

Actually a 1/4 watt trim pot ought to be OK, but I'd look for a 1/2 watter.

Look at it like this....

A given PT has a 50v bias tap.

50vac x 1.414 (reverse rectification and filtering) = about -71vdc

With a traditional 1/2 wave rectifier, that is the max -DC voltage your bias supply could make if it was not grounded out.

However even with the supply built up correctly, that -71 volts goes somewhere.

With a good bias filter cap, that "somewhere" eventually has to go to ground through all your bias control resistors... why? Because the grids of the power tubes (UNDER NORMAL OPERATION) won't be feeding bias voltage anywhere except as a static charge on the grids so the resistance and the filter cap in the bias supply are the only path to ground.

A good 47uF to 100uF @ 100v cap ought to last for many many years.

Lets say you have a 1/2 watt, 25K trim pot sitting on top of a 1/2 watt carbon film, 10K resistor.

That is a total of 35K

Ohm's law for current: E/R=I

-71v/35000ohms = 2.03ma

Ohm's law for wattage: P=I^2*R

.00203a^2 * 25K (trimpot) = .103 watts

Just a bit more then a 1/10 of a watt is used in the trim pot.

Now look at the 10K resistor.

It also has to be able to handle the 2.03ma of current in the bias supply, so you can solve for it's wattage ratting the same way.

.00203a^2*10K (fixes limiting resistor) = .0412 watts

Less then 1/20 of a watt in the 1/2w setting resistor.

So lit looks like the wattage of the resistors and resistance in the whole bias supply (not including the actual R of the PT winding) is less then 1/4 watt.

So I'd use 1/2 watt resistors and or trim pots to be ultra safe.

You need say -45v?

The trim pot now offers a variable and lower -DC resistance path to ground for the -71v, lowering the total amount of -DC bias voltage.

The 10K resistor limits how low you can go with the total DC grounded resistance so there will always be some kind of -VDC on the power tube's grids, ...least you launch them into outer space with high current by having none.

Regardless, what ever -DC voltage is left over is always applied as a static charge to the grids of your power tubes for bias voltage.

10K might not be much voltage but for the few moments in time that it happens the tubes will just get super hot and not blow up.

It "might" allow them to draw enough current that the mains fuse could sag an pop.

If not too preachy and wording... I hope that was helpful..

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Of course there are much more sophisticated ways to build up a bias supply but this is the basis for all of them, unless they are using lots more parts like transistors and precision current sensing resistors in the power tubes for tracking power tube current and auto adjusting bias voltage...etc

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