RCA BC-3C

[rotation]

Hi!

I'm thinking of making RCA BC-3C preamp for my next project. I chose it because i never had any RCA gear, because it's triode based pre and simple to make.
I really searched for any information about in/out trafos for this amp and nothing showed up. Well, i managed to find out that they were custom made for RCA, that's all. Anyone knows more about this (ratio)? I will buy very good transformers for this project because i found out it's worth paying some money more. Any opinion on possible transformers will be very welcome.
Also, output 1u cap seem to be on very low voltage because output stage is cathode folower. But part list says it is 200v part. I know i can use anything i want, but isn't voltage at that point really low, not bigger than max possible signal? Or is there something else i don't understand?

Miha

Schematic:
http://binauralaboratories.net/lab/consolette/manual/page6.jpg
it's second version..

 

[PRR]

> output stage is cathode folower. isn't voltage at that point really low, not bigger than max possible signal?

It has 2K cathode bias resistor and another 56K load resistor. Move the 56K around to the plate. Standard self-biased plate-loaded stage. You expect the plate to sit at 1/2 to 2/3 of the supply voltage. Put the 56K back under the cathode and 2K resistor. The DC is the same. You expect 1/3 to 1/2 of supply voltage to appear across the 56K. 90V to 140V. A 200V part is quite correct.

The doubled-up caps on the upper plan look like paranoid design. Maybe they were afraid of leakage current causing fader-scratch when levels were adjusted live on-air. Maybe they thought cap failure rate was too high for commercial broadcast operation. Maybe the amp-rack was crowded and HOT, reducing calculated reliability. Maybe they had too many caps around the factory. Surely a good 0.5u 200V cap will be ample in DIY operation.

1C3 is sized for some bass-rise, only partly to compensate the 1C2-100K pot loss, probably to compensate some bass-loss in the original iron.

> i never had any RCA gear

If you don't use RCA iron, and preferably fresh 1953-technology capacitors, then this is just some tubes and stuff, NOT "an RCA".

 

[PRR]

me> You expect 1/3 to 1/2 of supply voltage to appear across the 56K. 90V to 140V.

Well, hell. The voltage is right ON the plan you linked, albeit on page 20.
http://binauralaboratories.net/lab/consolette/manual/page20.jpg

RCA says 75V-93V there, measured with 20K/V meter (which would be 2Meg at 0-100V range and about as negligible as a 10Meg VTVM/DVM).

They don't say why it isn't "really low", but I covered that.

You could argue that a 100V cap would stand 75-93V. You could be right. If it were -my- radio station depending on a cap working that close to rating, when failure means loss of Billable Commercials, I'd like to see plenty of up-rating.

Other text explains the double-cap network. It's not a magic cure for switch-click, but I guess it was good enough for the purpose.

> not bigger than max possible signal?

The DC "must" be greater than peak AC. For maximum output it must be greater by about the ratio of 56K DC resistor to 3K(??) summing resistor. And what is "max possible signal"? There is a TON of gain in the total box. Rational level-setting will pad the output line-amp to the line, have lots of loss between program amp and line amp, so the channel faders may use most of their rotation. But say you have a weak talker and a loud talker on the same mike. You may need to gain-up to get the weak talker well-modulated. You may have to let the master limiter trim the loud guy. But ideally the system will not clip the loud-guy before the limiter: that's nasty. So if nominal mike-amp output is 0.5V, and LOUD is 20dB up or 5V, the system should pass it. But I don't think this plan will pass 5V into the 3K mix network cleanly.

Looking at the PCBs and stuff, this must be late-1950s into early 1960s.

 

[emrr]

PRR nailed the doubled up cap purpose; it's there in the description. 

Introduced in '56, still in the '67 catalog.