Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Curranm on November 25, 2018, 01:57:09 pm
-
I am planning an AB763 build using Doug's single channel schematic and layout. I am curious IF making the bias section more like a marshall bias section is a good idea OR if I should even bother with it. I am talking about adding a 2nd Smoothing cap to the bias section. and moving the bias pot AFTER the smoothing caps. Just like how a 2204 is done. BUT using the 470R/5 Watt before a 1N5804 Diode. then leaving all the other components the same as a marshall? (2) 10uF/150v Sprague's with a 15K between, and popping off the a 47K into a 10K Bias pot? Is this a good idea or just dumb? I am NOT interested in vintage "vibe" or adhering to any kind of "make it look like a fender" mentality. I am just curious IF there is a benefit to two smoothing caps in the bias circuit, and if doing what I am thinking is going to work for a set of 6V6's with the tube tremolo idea Doug is using on this board/project. I have other build and MOD questions BUT I will wait for answers to this question before posting more.
-
Either bias circuit will accomplish the same thing. The AB763 single 47µF cap probably provides just a much filtering as the Marshall 2 x 10µF caps. I prefer the Marshall circuit for one reason... If the pot wiper fails the bias voltage will default to max negative voltage and just run the tubes cooler. If the Fender pot wiper fails the bias voltage defaults to zero voltage and will likely ruin the output tubes. There is a simple failsafe resistor mod to eliminate the Fender dangers. Please note that the Fender uses a good quality chassis mount bias pot whereas the Marshall uses a rinky-dink trimpot. If you follow the Hoffman AB763 layout and use a trimpot I recommend the failsafe resistor. It's a cheap insurance policy.
Having said that, if I'm building a Fender amp I will use the Fender bias circuit (with the failsafe resistor) and if I'm building a Marshall amp I will use the Marshall bias circuit.
-
thank you very much for that information!