SB-220 filament transformer with modifications? Any reason the 3-500 filaments need to be wired in series?
Simply cuz kenwood, in it's infinite wisdom, decide to copy henry radio amps...and wire cathodes in series. The idea was the bifilar fil choke only has to handle 15 amps...and smaller ga magnet wire can be used. Small enough, like 14 ga wire, so they can get 22 uh so it will work on 160m.
If a 5 v @ 30 amp xfmr used, the fil choke would have to be replaced...with a 22 uh choke...wound with 10 ga wire. Ameritron only uses 12 ga wire on their '30 amp' fil chokes.
The other issue, with fils in series is...u then gotta use a 80 uh single layer choke for the B- return for cathode current ( sum of plate and grid current). Next issue is.... with one tube UN plugged, the other tube stays lit, with correct 5 vac across the cathode, BUT the return path is now via that 80 uh, fine wire B- choke they installed..which of course, burns out with 15 amps CCS flowing through it...back to the CT of their oem 10 vac @ 15 amp fil xfmr.
It's a goofy setup, as seen here in their manual.
https://erikarn.github.io/kenwood/TL-922/TL-922_Service_Manual.pdfIt would take some work, but a 5 vac @ 30 amp SB-220 xfmr should work...provided it actually fits.
The 922 fil xfmr also has a 8 vac winding...to run the 6 x lamps, and each lamp has a drop resistor...and runs on AC.
The sb-220 fil xfmr, does not have the 8 vac winding. However, u can obtain almost 6 vdc, by rectifying the 5 vac from the cold end of the fil xfmr...through a FWB rectifier assy. Comes out to 7.07 vdc, but then u lose 1.4 vdc from the .7 v drop across 2 x diodes..... since only 2 of the 4 x diodes in the FWB are conducting at any one time.