various RCA tube preamps - progress and experiments

[emrr]

I implemented stepped gain control in the BA-11's, via NFB manipulation.  For whatever reason it is not working as well as it did previously, in that the noise floor moves with gain setting in an obvious way, when going for higher gain.  I'm still experimenting with different situations, and haven't determined yet what I consider to be the highest practical gain from these.  It'll likely be less than I implemented, and it's perfectly fine at the stock gain setting.

I'll say this:  large use of NFB lets the assembly workers be a lot looser in how they lay out and wired a point to point piece.  If I were to try and get noise lower for high gain situations, I'd be trying to clean up a lot of the wiring paths and lengths. 

I went with the following:

remove 22 pfd
10 stops give 3 dB steps, 27 dB gain sweep. 
R      total R    dB change
22k   22k   -12
11k   33k  -9
14k   47k   -6
21k   68k   -3
30.9k 98k9 0
44.2k 143k1 +3
64.9k 208k +6
95.3k 303k3 +9
154K 457k3 +12
249k 706k3 +15

 

[emrr]

Finished another rack that's been staring at me far too long.  This is a set of BC-2B console 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.  Tested and measured, still need final tube selection for lowest noise.  External PSU, AC heaters. 

 

[CJ]

cool and a half!  time to open up your own studio,

where did you get the soldering iron tip cleaner?

 

[emrr]

Studio in 23rd year.....Mouser, I think.

 

[Gene Pink]

where did you get the soldering iron tip cleaner?

Brass is great, doesn't cool the tip like a wet sponge.

I DIY'd mine, brass scouring pad from the grocery store. You'll need to weigh it down, a chunk of steel, or an automotive wheel balancing weight embedded in it, so it stays put.

Gene

 

[CJ]

and it's powered by a UTC transformer for vintage sounding solder joints, 

 

[fazer]

Great work and the preamps see a thing of beauty.  I would love to hear those with a ribbon mic. 

The brass pad is great for solder tip cleaning I'm enjoying mine. Got one with a hakko desoldering gun.

 

[rackmonkey]

Fantastic, Doug! Thanks for documenting these projects!

Hey CJ, if you were talking about the brass garbage can tip cleaner, they sell those on Amazon for about 10 bucks.

https://www.amazon.com/Hakko-599B-02-Solder-Cleaning-Holder/dp/B01MXMH47L/ref=sr_1_3?ie=UTF8&qid=1503407889&sr=8-3&keywords=solder+tip+cleaner

BT

 

[emrr]

It's been quite a journey learning the secrets of these pieces over the years, and hopefully these reports add to some sort of knowledge base.  The old dead guys who designed this stuff surely knew the details inside out, but their notes are long gone.

I should add here that my experience servicing the complete BC-2B consolette (along with many other various 1940-1960 modules) is what made me go ahead and replace the front end plate and cathode resistors.  Noise floor at the 1st stage is the most critical element, and while the purist may object to metal film, those two are still a very small percentage of the overall sonic signature, and can make a big difference with the results coming from a 62 year old amp.  You may not notice the noise floor difference from a single 40 dB preamp, but you sure will once you passively mix it into an 84 dB line amp. 

 

[duffharrisnyc]

Here’s a picture of the side.