RCA BC-2B

[Gus]

I started to build a two channel RCA tube preamp from the BC-2B schematic. I am using LL1680 outputs and I have LL7905s, jensens and a LL1636 to try as inputs.

I seem to remember a thread about this circuits power supply voltage. I did a search and did not find what I was looking for.

I think the ps B+ was questioned being marked 363V at the site http://www.larryseyer.com/rca.htm. I THINK PRR stated that older tube consoles ran at about 250V. Has anyone built this and tried the different voltages?

 

[NewYorkDave]

275-300V was a typical B+ for amplifiers inside mixing consoles. 363V sounds unusually high.

For what it's worth, somewhere online I saw an original BC-2B preamp schematic with someone's notes added to it, and the B+ was noted by this person as 280V.

 

[Bauman]

There was some notes about making this unit some dirty sounding lowering the B+, to 80V I think. I recall seeing a schemo with the transformer options for that, I'm after that one but can't find.

Do you think it can be done with a JT-115kE as input and a 10k:600, 1:4 output transformer???

cheers!
Fabio

 

[Gus]

I am going to try a 115. The LL1680 is gapped for 5 ma

FWIW I was thinking of building a tube microphone using a LL1680.

 

[NewYorkDave]

A guy on Usenet measured the input transformers in his BC-2B console preamps and found that they're 1:15. So you're going to get less gain with a 1:10 input transformer.

The same fellow claims the output transformers are 12K:600, which works out to 4.47:1. And of course it must be able to handle the unbalanced plate current being drawn through the primary.

 

[Gus]

Thanks for the ratio. 1:10 is Ok with me. The lower ratio should help with high output condensers.

 

[Bauman]

Cool, thanks Dave!

Gus, let me know about the 115k on input, I'm sure you gonna do it before me

cheers!
Fabio