Oui, bon nombre d'amplis n'ont pas de résistance ajustable, d'ailleurs c'est expliqué sur ton lien comment les trouver.
Citation:
TYPES OF (FIXED) BIAS CIRCUITS
Many amps which use "fixed" (negative grid) bias have provisions for adjusting the negative grid voltage upward or downward. Making the grids LESS negative will cause MORE current to flow through the tubes. Some amplifiers don't have a bias-adjusting control (pot) but instead use a fixed resistor to set the voltage. If you encounter one with a fixed resistor, the best thing to do is convert it to an adjustable type. Most of the time, the fixed resistor will be in parallel with the bias capacitor; the lower this resistor's value is, the lower the bias voltage will be. If you can locate and identify this resistor, you can replace it with a simple network consisting of a (lower value) resistor in series with a potentiometer. What you'll be shooting for is a range of adjustment that goes from LESS voltage to MORE voltage than is set by the (existing) fixed resistor. Take the value of the fixed resistor and divide by two; pick the closest standard value to your result, and put it in series with a pot which is as close to the original resistor's value as you can find. Example: the existing resistor is 33K; use a 15K resistor in series with a 25K pot to replace it. The original resistor was 33K; you now have the ability to adjust the value from 15K to 40K. This should provide you with sufficient adjustment range to set any plate current you wish. If not, use a different value pot or resistor.
Some amps have a "balance" type bias adjustment, which allows you to vary the negative grid voltage between the two halves of the output stage; this makes a "matched" set of tubes less crucial to good performance, although it can't compensate for tubes that are wildly different. If you encounter this circuit, the easiest way to adjust it is to simply "tune" the control for minimum 120Hz hum on the output. This type can be modded to the *best* type, which is not only variable from side-to-side, but adjustable up-and-down, too. Usually, this circuit will have the "balance" pot's wiper connected to a resistor which is grounded at the other end. You can replace this resistor exactly as outlined above (half the value, add a pot, etc.) and have the best of both worlds.
If the simple mods outlined above (and the reasons for making them) don't seem perfectly clear to you, DON'T TRY THEM. A schematic (and the expertise with which to interpret it) will go a long way towards helping you do them correctly. You can have the mods performed by a tech, and then do your own biasing from then on, if you wish.
