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Vintage threads from the first ten years

Re: Blackface BandMaster

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2/24/2000 3:40 PM
Doc	Re: Blackface BandMaster The volume on the Vibrato channel of a Bandmaster is usually a little lower than its Normal channel, due to the loading of the tremelo circuit.

2/24/2000 3:43 PM
Trace	Re: Blackface BandMaster..Master Volume? Nils; You could also install a post PI master volume as well. This will not change the tone of the amp like many masters tend to do. It's pretty easy to do and when you crank the master to "10" it's out of the circuit. Trace

2/24/2000 6:00 PM

Nils, I've got one that I've been messing around with. Does your's have a "Showman" chassis ie. Does it have 2 octal holes that are covered? If you want to go hog wild, then you could wire it for 4-6V6's. This will increase distortion, yet you have to do Bruce's zener diode trick as well to get the B+ down. I put 2-12V/5W zeners in series with the PT Center Tap to drop the voltage about 22Vdc on the plates.

Troy

2/24/2000 6:12 PM
Bruce	Troy, did you put the 135ohm resistors in the HV AC leads to the diodes like we talked about? What is the actual B+ from cathode to anode on the 6V6s now and what happens to ratio between the grid bias voltage (-Vb) and the B+ on the plates (Ep) or (Vp) when you crank the amp up now? We need to all agree on an the abreviations for this stuff!! Ha Bruce

2/24/2000 9:06 PM

Bruce: I initially put the 2-130ohm 5W resistors on the AC lines going into the rectifiers and this worked fine until one of the resistors opened up on me (may have been a fluke since I calculated about 2W across each resistor at idle; can't remember the voltage drop). I then pulled the resistors and put 2-5W-12V zeners in series from the PT CT to ground. This has worked so far with about 424Vdc on the 6V6's plates (415Vdc on the screens). I haven't measured while cranked; I've been trying to chase a hum that I believe to be in the PS (still hums when PI tube is pulled).

Troy

2/24/2000 6:58 PM

"Does it have 2 octal holes that are covered? If you want to go hog wild, then you could wire it for 4-6V6's."

...or you could use the octal spaces to add a couple of tube rectifiers, leave the diode rect. and switch between them. I would use the defunct ground switch, since it's not necessary with a modern three prong plug! I'm pretty sure this can be done with a SPDT as described in Kevin O'Conner's book. And if you wanted to get real crazy, the other pole of the switch could be used to switch to cathode bias. (This would also give you a means to compensate for the required bias adjustment needed with a lowered plate voltage) One position would give you higher voltage fixed bias for stronge clean sound, and the other would give you lower voltage tube rectification with cathode bias (more voltage drop) for a "squishier" sound. With the rectifier sockets in parallel you can choose to use only one if you want. The big drawback to this, though, is that the tremolo will be unusable in the cathode bias mode.

-steveR

2/24/2000 11:27 PM

Ha ha...Ok Steve.
I wonder where the rectifier filament voltage would come
from, since the transformer is wound only for solid state HV supply and there is no 5v@2a winding.
Now that's OK if you wanted to go through the extra work of installing a seperate 5v tranny and all that rot.
A rainy day project.
Tremolo... actually there are a few here who've been able to make a semi-weak vibrato circuit work with a cathode biased PA.

Bruce

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