by EWBrown » Mon Nov 01, 2010 3:43 pm
I have a quantity of both GE and RCA 12B4As, and indeed, pins 6 and 8 are truly "no connection". So they could be used as tie points, or other wise disregarded.
I got the little 12B4A SET "Darling Killer" r amp unning and it sounds pretty doggone good, with TF103-48 OPTs, and about 250V B+. I wired the filaments for the 12B4As and driver as 12 V, so I can also use 12DT8s, which BTW sound very good, as well, for a cheap "plinker".
Its ouptpu section uses nearly identical basic circuitry and plate voltages / curent as the 1626 "Darling", but with 2X the output power and lower 2HD and 3HD. With a bigger chassis I could have made it 12B4A / 1626 "convertible".
12B4As also make for a really nice sounding linestage, I've seen and heard Gary Kaufman's design, and it is very nice. He gave me one of the B+ regulator PCBs and two of the plate load CCS boards (10M45S). so I have the major parts, including two of the Hong Kong 100K ladder step attenuators from E-bay. The rest of it is ppretty simple, just run 12B4A "grounded cathode" VA stage on Tube CAD with B+ of 250V, and IP of 10 mA.
/ed B
Real Radios Glow in the Dark