finished another RCA rack: BC-2B consolette modules
Aug 27, 2017 18:56:15 GMT -6
jsteiger, ragan, and 12 more like this
Post by EmRR on Aug 27, 2017 18:56:15 GMT -6
Finished another rack that's been staring at me awhile. This is a set of mid-'50's BC-2B consolette modules, a two channel preamp and a pair of program amps. Note the console type shock mounts. Rear switches are 20dB pads. All use the same input transformer. 12AY7 front ends in both types, RCA had a part # for specially selected low noise 12AY7's - unobtainium now.
PGM amps are 4 stage single ended with a 6V6 output, good for about +24dBm out max, gain pot after the first stage. Same output trans as BA-3. Actual output Z appears to be roughly 100 ohms. Mic pad on the input causes a +1dB rise around 14K.
PRE gain is a mod, variable NFB which I've documented elsewhere, I've changed the NFB cap to to get a range from -15 to +8 relative to stock. They run 10dB NFB in the stock position. Same transformer set as BA-11. 600R output load gives flattest response, and causes a 7.7dB loss relative to a 10K load, predominately due to the 440 ohm DC resistance of the output secondary 600 ohm winding. I've yet to see anyone explain the logic of the high resistance winding in these and some other RCA/Langevin/Western Electric transformers. I suspect it may flatten the overall impedance curve. These have a 2.5dB rise at 16kHz with 600 ohm loading, and it's 1.5dB higher with a 10K load. A mic input pad causes an additional 1dB rise at 600R, and 1.5dB into 10K. Plug a ribbon in, and you have what RCA called a equalized system, preamp compensating for ribbon droop.
All new caps, front end plate and cathode resistors replaced with metal film, all others checked. Rack connector bar is my machining work, also had to drill the rack shelf for mounting standoffs. Tested and measured, still working on final tube selection for lowest noise. External PSU, AC heaters. Last pic is mounted next to earlier RCA generations from the late '30's and '40's.
PGM amps are 4 stage single ended with a 6V6 output, good for about +24dBm out max, gain pot after the first stage. Same output trans as BA-3. Actual output Z appears to be roughly 100 ohms. Mic pad on the input causes a +1dB rise around 14K.
PRE gain is a mod, variable NFB which I've documented elsewhere, I've changed the NFB cap to to get a range from -15 to +8 relative to stock. They run 10dB NFB in the stock position. Same transformer set as BA-11. 600R output load gives flattest response, and causes a 7.7dB loss relative to a 10K load, predominately due to the 440 ohm DC resistance of the output secondary 600 ohm winding. I've yet to see anyone explain the logic of the high resistance winding in these and some other RCA/Langevin/Western Electric transformers. I suspect it may flatten the overall impedance curve. These have a 2.5dB rise at 16kHz with 600 ohm loading, and it's 1.5dB higher with a 10K load. A mic input pad causes an additional 1dB rise at 600R, and 1.5dB into 10K. Plug a ribbon in, and you have what RCA called a equalized system, preamp compensating for ribbon droop.
All new caps, front end plate and cathode resistors replaced with metal film, all others checked. Rack connector bar is my machining work, also had to drill the rack shelf for mounting standoffs. Tested and measured, still working on final tube selection for lowest noise. External PSU, AC heaters. Last pic is mounted next to earlier RCA generations from the late '30's and '40's.
