RCA Ba2A Pre amp qestions & answers

[Purplenoise]

I am having a strange issue with my build. I keep blowing the 10uf cap ( c7). The unit works the same even with the cap blown…maybe one of the resistors is bad? ( r3,r10). Any ideas? Thanks

 

[dmp]

Is it smoking like an overvoltage failure? What's the voltage rating of the cap? Probably want it to be at least 250v if your B+ is 200v

 

[Purplenoise]

Is it smoking like an overvoltage failure? What's the voltage rating of the cap? Probably want it to be at least 250v if your B+ is 200v

Yes exactly…wow that high? I was under the impression this was a 50v cap like the 47uf ones

 

[rock soderstrom]

Yes, this cap should have a minimum voltage rating bigger than B+

 

[Purplenoise]

Great, thank you so much guys. I really appreciate it.

 

[craigmorris74]

I've been curious to try building this preamp for a while, and was wondering if any of these transformers would be suitable:
input:
Stancor A-4408 500/333/200/125/50:78,000
Stancor A-4708 200:57,000
output:
Stancor A-3250 20000CT:500 15ma max. primary dc

Thanks,
Craig

 

[emrr]

Try’em!

 

[craigmorris74]

Try’em!

I know just enough to get by on a project like this. I've read all this thread, so I was wondering if the 3250 output would be suitable for handling the DC required.

 

[Purplenoise]

I've been curious to try building this preamp for a while, and was wondering if any of these transformers would be suitable:
input:
Stancor A-4408 500/333/200/125/50:78,000
Stancor A-4708 200:57,000
output:
Stancor A-3250 20000CT:500 15ma max. primary dc

Thanks,
Craig

Input transformers need to be very well shielded…preferably in a mu-metal enclosure

 

[dmp]

Yes, "15ma max. primary dc" would be more than enough.
I think the plate current in this is less than 10 ma

If you can find an old catalog with specs on them, the frequency response is worth looking at. If they roll off in the audio range, you might be less satisfied. The inputs are both pretty high ratios. You want shielding on the inputs and a catalog might give details.

 

[craigmorris74]

I own each of the transformers. The 4408 input is a huge, shielded beast. My only concern was the secondary impedance was higher than specified for this circuit.
I could test that output and see what kind of response it has

 

[dmp]

The 4408 will have a turns ratio of sqrt(78,000/500) = 12.5, which gives you 22 dB of gain from the transformer. In a tube preamp circuit like this, more transformer gain is good. The problem is winding higher ratio transformers with good response is difficult. So you most often see 10:1, which is 20 dB of gain. So there is no concern with the higher turns ratio (or the impedance specs) but you may find the transformer in use will roll off the highs or lows.

 

[craigmorris74]

Thanks for all the replies! Sounds like it's time for building and trying things out.

 

[DrRick]

Hi
Im looking for some UTC M 8366A RCA part 205326 mic transformers for my RCA BC 3 consolette!
Anybody !!

How many do you need? I may have some extras.

 

[toniwave]

This might be a stupid question, but could I add a DI by connecting a jack to the grid of the 1620 with a couple of resistors like this:

Hey, I'm a bite late to the game. But did you actually implement the DI like @lassoharp explained?
Sounds like you just need a TRS jack with a switch function that connects 1M to ground.

 

[emrr]

Hey, I'm a bite late to the game. But did you actually implement the DI like @lassoharp explained?
Sounds like you just need a TRS jack with a switch function that connects 1M to ground.

Yes, as Lassoharp explained. Even better, pull the grid cap connector off and attach the jack and 1M with SHORT clips to the grid and to ground. The ground can be longer. You want all of this, with switch or not, as short as possible as you can introduce other noise or treble loss the longer the wires get. You want the jack as close as possible. Consider the distance from transformer to tube, and then that you're proposing introducing additional path length.

 

[toniwave]

Would this be proper solution? Just a TRS stereo jack (for mono TRS plug) where the 1M is between ring and tip. So when the mono plug is connected ring and sleeve/ground shorts. Tip goes with a shielded cable to the gain pot where the original grid wire goes. When no TRS plug is connected, it's just a dead wire to the jack.

Talking about a real BA-2 here. I don't need the mounting wholes on the side so I can use them for very short cable runs to the TRS jack outside right next to V1.

 

[beatnik]

I don't see why you would need a TRS jack since you are connecting a mono unbalanced signal.

I think this needs some spdt switch like on the above OP-6 schematic so you can send to V1 grid either positive end of input transformer primary, or DI jack tip.

If you don't want the additional switch you could get jack sockets with integrated SPDT or DPDT switch like this one

https://www.musikding.de/63mm-Stereo-jack-with-2-SPDT-switches
Probably it's best using an insulated jack and connecting the sleeve to 0V but it might work just fine with an uninsulated jack with the sleeve going to chassis

 

[toniwave]

Oh jeez I see. My sketch is wrong, I went for the wring grid cap at V2....

I get it now. Thanks everyone. Also the hint with the SPDT switch jack is nice!

 

[My3gger]

A friend told me BA-2 i built sounds best to him with 20dB pad in and wide open interstage pot. Description of the sound and levels made me think he likes color of the output stage with gapped iron core transformer, although i'm not sure how it could be driven any harder this way. Input transformer is fairly clean 1:10 Cinemag very similar to Jensen 115-KE.
P.S: I was overthinking this because of previous problems when high nfb tube preamps were driven very hard, BA-2 works fine with only 20dB input pad.