You may not need the choke at all.
I notice from the
AB763 Twin Reverb schematic that the preamp is fed from a 410vdc supply node, with the reverb drive tube fed from nearly 460vdc. Let's say 420vdc is our target power supply voltage.
If you follow every tube for the Vibrato channel and calculate the tube currents by dividing cathode voltage by cathode resistance, the entire vibrato preamp draws just under 10mA.
The existing Vibrato channel has 4x 12A_7 tubes, which draw 1.2A from a 6.3v filament winding. You
might need an extra cathode follower to drive output cable capacitance and/or provide a low output impedance to feed the input of your solid state power amp. So add another 300mA to allow for another tube; 6.3vac @ 1.5A.
No telling how much plate current the cathode follower may need, so we could allow another 5-10mA for it from the high voltage supply, or you could cheat and use a MOSFET source follower.
Now our requirements are laid out: 420vdc @ 10-20mA, 6.3vac @ 1.5A
420v/1.414 = ~297v, so a 350v PT winding would give some excess voltage to be dropped in filtering. For maximum weight reduction, skip full-wave rectifiers in favor of a solid-state bridge. So you want a 350v winding at 20mAdc or a 175-0-175v winding which will have a center-tap which you will not use.
350v * 1.414 = ~495v peak.
Let's pack a bunch of capacitance in the power supply and have a 100uF cap (or bigger if you like), followed by a resistor and another 100uF cap. Maybe even 2 pi-filters. The circuit may not draw enough current to excite the choke enough to do its job (I don't know the minimum current for a Fender choke), so resistors are fine. You'll have to stack at least the first filter stage or two with at least 300v caps to get the needed voltage rating.
75v / .02A / 2 = 1875Ω
A 350v @ 20mA transformer is not a common part, but Edcor has a
350v @ 60mA PT with a 6.3v 2A winding. The output voltage will likely be a bit high due to your under-loading, so you'll probably have 2kΩ for the first dropping resistor (455vdc at this node) and 2kΩ for the next dropping resistor (415vdc at this node). You can bump those up if needed (2.2kΩ, 2.7kΩ). 3w ratings for these resistors should be plenty.
There is also no need to feed so many preamp stages from the same supply node, so you could add a couple extra nodes, which will further reduce ripple on the way to the input tube.
Edcor lists the weight for the transformer mentioned as 1.6lbs. You could likely reduce total weight further if you don't mangle the original Twin chassis, but put this in a different rack chassis. You should be able to get everything in a package smaller than 4U.
Topic: fender twin as preamp only? (Read 5783 times)