Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Bugman3183 on June 15, 2015, 07:20:44 pm
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Hello all, wanted to order some parts for a 5e11 build and had a couple of questions. First, I wanted to use 6sl7 preamp tubes and wondered if that would have any effect on tremelo. (I'm guessing no). The other question I had was as far as substituting in4007 for the selenium diode. The couple of amps I've built thus far have been cathode biased and I can't say I really understand how fixed biased works. Any info on this subject would be greatly appreciated. Thanks.
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About the preamp tubes, I'm not sure, less gain, which may or may not effect much.
The bias is simply a negative power supply with a diode (replaces selenium just fine) backwards, so it's output will be negative with regard to ground. Remember to put your electrolytic bias smoothing caps with positive end to ground, as ground is more positive than a negative voltage.
Cathode bias uses a resistor to allow the cathodes to go positive with regard to ground. This means the grids, referencing ground via the grid leak resistors, are negative with regard to the cathodes, same as fixed bias. In fixed bias, the cathodes are held at 0v (ground), and the grids are supplied a negative bias voltage. two different ways of achieving nearly the same situation functionally. The only real difference is that with cathode bias, the cathodes are allowed to sort of flop around on their resistor, while at certain AC frequencies, most, are held somewhat static by the cathode bypass capacitor.
Hope this is helpful.
Jim
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Very helpful. Thanks.