Technical help with McIntosh C-4 preamp DC vs AC filament supply

[noah300g]

I have a pair of C-4 preamps that I use with MC-30 power amps (using the supplied cable to connect the two).
My issue is, I opened up the C-4 after seeing in my Sam's photofact that there are additional filter caps (electrolytics) in that unit. I had already recapped the power amps, but never the preamps. When I opened up the C-4, I was surprised to see a solid state rectifier and additional filter caps not shown in the Sam's schematic or photos. It is definitely factory original, and appears to be a power supply for 6.3 V DC filament voltage. However, the schematic in Sam's shows that the preamp gets 6.3V AC voltage for the filaments via the connecting cord between the two units. I'm concerned that there is both AC and DC 6V signals being sent to the filaments in my preamps. Seems like it should be either one or the other? Any help appreciated.

 

[tcdriver]

I know that this thread is more than three years old and that a reply will be of no use to the original poster. I am posting because someone else, in the future, may have the same question.

The C-4 was designed to get its supply voltages from either an amplifier (e.g. MC-30) or a D8A or some similar external power supply. In either case the filament voltage at the C-4 input socket was expected to be 6.3VAC. Inside the C-4, a voltage doubler circuit was designed to rectify and double the filament voltage to 11.7VDC, to be applied to the three tubes using the 12.6V filament connections (i.e. pins 4 and 5). The diodes and capacitors noah300g describes make up the doubler circuit.

Edit: Looking at the Sam's Photofact it would appear that early C-4 preamps used 6.3VAC for the filament supply. Later units used the voltage doubler.

 

[dcgillespie]

Sams invariably always developed their Fotofact Folders fron early production units. As production changes were made within a given model number, Sams had no way of knowing that unless the model number was updated with (for example) an "A" suffix, or the manufacturer notified them of the design changes. The type of improved design you're describing that Mac made would be a classic example of an improvement that the Sams folder would not include. Did the Sam's folder get updated with the change?

Dave

 

[tcdriver]

...The type of improved design you're describing that Mac made would be a classic example of an improvement that the Sams folder would not include. Did the Sam's folder get updated with the change?

Dave

Great question. My copy of the Instruction Manual McIntosh Model C-4 and C-4P Audio Compensators is for Serial No. 6600 and above. My actual unit is Serial No. 6197. There is a copy of the Instruction Manual here on AudioKarma that shows a schematic for Serial No. 6092 and up with the voltage doubler. I wonder how many significant changes there were from version to version?