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RE: 26 tube filament ripple

Posted by BenM on January 27, 2017 at 11:03:40

In Reply to: RE: 26 tube filament ripple posted by sony6060 on January 27, 2017 at 09:25:01:

Andy is referring to filament bias. In filament bias, the bias resistor is connected in series with the filament, and the filament power supply is connected with the positive at the free end of the filament and the negative at the ground of the bias resistor. In this situation the capacitor across the bias resistor can be eliminated.

To power the filament and to provide the bias, a higher filament supply voltage is needed. For instance, in the case of the 26 with a filament voltage of 1.5v and currrent of 1.05A, and say bias of 9v and .007A current, a total of 1.5v + 9v =10.5v and 1.05A + .007A = 1.057A need to be provided. The bias resistor needs to drop 9v so the resistor needs to be 9v/1.057A = 8.5 ohms. Power rating required would be 9v x 1.057A = 9.5 watts times two for safety equals 20 watts.

I have a 26 preamp and with filament bias, it is very quiet. Previous to filament bias, even with a regulated filament supply there was still some hum. So for me, filament bias solved my hum problem and it also eliminated the cathode capacitor which is another plus.

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