I've been working on a Bandmaster to Bassman/Marshall conversion, and I'm nearly done (it's really more Marshall than Bassman - but that's OK).
All I need to do is convert the pots in the tone stack to Bassman/Marshall values, and drop the B+ down in the preamp (there are 400v on the plate of the second stage in the cathode follower - seems awfully high).
My Bandmaster right now has a 4.7K/1K combination in the power supply, with the 1K following the choke. What I want to do is put a 4.7K after the choke, and then follow that with a 10K.
Or should I leave the 1K following the choke, and bump up the 4.7K instead? Or just leave it the heck alone? I'm a little concerned about the high voltage on the cathode follower...
Does this sound feasible? Any caveats, "gotchas", etc.? Would anyone suggest any other values besides the ones I'm proposing?
Thanks in advance!
400v on the cathode follower is only 17v more than the 5F6A RI - High? Yes, a little. Unheard of? No.
I know of RI & repro 5F6As that run more B+ voltage than the RI schematic lists and rarely hear of blown V2 tubes. I wouldn't use the Sovtek LPS here though, fine for V1 & V3.
If you increase the value of the 1K, you will drop the voltage at the cathode follower. Also, you will drop voltages at the PI slightly, like a Bassman (4.7K) or Marshall (often 10K).
Have you taken your voltages with tubes or without?
I took the voltages with tubes installed. Should I not do it this way?
If you want it more Marshall than Fender, use 10K's in both positions. I don't think 400V on the Cathode follower Plate is necessarily a problem. Read the voltage from Cathode to plate to find out what the tube "sees". Also read the Cathode voltage with respect to ground and calculate the power across the Cathode resistor. Many Cathode Follower circuits are running this resistor too hot. Power disipation should be held to 1/2 the rating of the resistor or less. If necessary, use a higher power resistor.
What you want to watch is the voltage on the actual cathode of the follower.
Most modern made 12AX7s freak out and can go open with more then about 220vdc to 250vdc from cathode to ground.
Denver, CO. 80022
Perfect if you've put all the tubes in, as they draw current and drop down the voltage a smidge.
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