Just picked up a Harmon-Kardon A50K (A500 which was a kit) 7355 integrated amp, for dirt cheap
. Works and sounds excellent, gave it a four hour test run yesterday. Maybe there is one good side to the current economic mess, after all.
I figure that someday the existing 7355s will go TU, and then I'll have to re-configure it for other output tubes.
The A500 uses a weird mix of catrhode and fixed bias for the 7355s.
Basically all four 7355 cathodes are tied together, and it uses three 12AX7 filaments connected in series, with a series connected 330 and 47 ohm resistor connected parallel across them.
The cathode voltage indicated on the schematic is 29V (which seems low for 37.8V worth of filaments. The 47 ohm resistor is connected to ground, and the junction between that and the 330 ohms resistor is then fed to the "cold" end of the 7355 grid resistors, thereby impressing about +3.5VDC on the grids, which are then 25.5V negative in relation to the cathodes. This may also offer a bit of DC feedback in order to partially regulate the combined cathode currents. Whatever it does, it seems to work well... FWIW, the three preamp tubes with the 29VDC filaments take a LONG time to warm up, as compared to the "downstream" AC-filament drivers and 7355s.
Power supply is the usual SS rectifier full wave voltage doubling scheme, main B+ is 370VDC, then various lower voltages are R/C tapped off as needed for teh preamp / driver / PI stages.
This amp was cheap enough so I wouldn't feel bad about turning it into a "parts donor" if it wasn't up to snuff, or if the 7355s die off and are too expensive to replace. Cosmetics are a tad "rough" with some scuffs and scratches on the front panel, but overall it is very clean and not too much rust / corrosion, jus a few specks on the top of the chassis.
. It's a "keeper".
Looks like pin 4 and 1 of all of the 7355s were used as tie points