the real RCA ba71?

[lewilson]

I am going to put in some nte transistors today and see what happens. I will inform.

 

[lewilson]

That was it. The 2n404 germ transistor was noisy. I went to my local nte outlet and bought a nte128.They only had one and I wanted two. Anyways it made the preamp nice and quiet. I need one more for the other pre thats noisy,I suspect the same transistor is bad. These both came as a set from a tv station and were wired from the inside with a cable for in ,out and power so they must both have been through the same abuse. Moisture or heat or whatever that made them noisy. I went ahead and put in 2200 uf output cap. It made a noticable difference in the low end.

 

[lewilson]

oops corection I used a nte102 not 128. I gotta wrap some presents now

 

[emrr]

Excellent; a PRR tear-down:

http://www.groupdiy.com/index.php?topic=28210.msg341628#msg341628

 

[emrr]

A few schematic and pic links. 

BA-31A schematic

BA-31A layout

BA-31C schematic

comparison of BA-31B and BA-71B packages

 

[opacheco]

For the record:

BA-31A and BA-71A are the same 6 transistor all germanium amp circuit, if still stock.  RCA pushed very hard to sell replacement 4 transistor circuit boards when they did the revision in 1961, so very few 6 transistor circuits are still out there.  You will find many 'A' versions with the replacement 4 transistor boards.  I think the 'A' circuit was only sold for about a year. 

The 'A' versions two extra transistors are simply an extra paralleled output stage, which was done away with when the later silicon design proved capable of same performance with only one output stage.  This is somewhat similar in concept to having a push-pull pair of 2A3 tubes being equal in performance to  four 45 tubes in push-pull-parallel mode.

BA-31 has PSU, BA-71 does not. 

BA-31/71 B and C are basically the same 4 transistor circuit replacing all but the central driver transistor with silicon. 

The 'B' version uses a 2N2270 input transistor.

The 'C' version goes to a 2N3391A input transistor for lower noise, though in practice I've heard no noticable difference. 

Then comes the BA-72A revision, which re-arranged the package layout and changed one circuit value - the output cap goes from the previous 100 mfd to 1500 mfd, so there's your low end difference.  Keep in mind that cap values in those days were to a degree dictated by size, and 100 mfd fit comfortably, and then some years later 1500 suddenly fit.  I usually rebuild these with 2200 mfd and a bypass film cap for better high's.

The BA-41A is a BA-72A with separate PSU in one package like the BA-31.

The BA-72A also has a slightly higher frequency spec printed on the transformers, which I suspect are really the same as the previous.  You tend to see Kenyon rather than UTC on the BA-72A's, though I have seen UTC's on them as well.  Strangely, every BA-41A package I've personally seen has had Kenyon's, while loose BA-72A's from consoles have more often had UTC's.  Probably a coincidence in my experience.

I don't think I've ever encountered Kenyon's on the 31's or 71's. 

These amps have an input overload level of -22 dbm, which was something of an unofficial standard for about a decade.  If you mic up a rock band without input pads you will likely overload them, or any other vintage pre for that matter.  Wiring for 600 ohm input will of course give you a few more db of input headroom (about 3-4 db I recall) over the 150 ohm winding, while wiring for 37.5 ohms will give even less.  The +6 db strap also lowers the input headroom by another 6 db. 

Veering slightly off-topic, keep in mind that these have a bridging load on the secondary, so the actual input impedance is roughly 14.66 times higher than the rated 150 or 600 ohms, or roughly 2200 ohms and 8800 ohms.  This is a vast simplification of what's really going on here, but does inform the basic electrical situation.  There are other interactions here that are beyond my comprehension (some rocket scientist please step in here; I've yet to see a discourse addressing AC impedance issues with mic transformers).    Even with a 1 to 1 transformer (say, for mic splitting purposes) you will get a different sound with 150:150 tapping versus 600:600 tapping when connected to a standard 150 mic input due to the varying AC impedance load of the 1:1 transformer.  The DC impedance difference of these winding options will usually look something like:
150 ohm tapping = 20 ohms DCR
600 ohm tapping = 60 ohms DCR
This is obviously different from strapping a 20 or 60 ohm resistor across the mic and preampo combo. 

The input transformer secondary value is quite on purpose.  I don't go for this crap POV that companies the size of RCA in olden days fudged things in their desire to use up old stock of transformers.  RCA was huge industry, far larger than anyone existing today in audio, and maybe larger than all of todays audio companies combined.  They would just sh*t-can or surplus old iron and redesign for their new products, maybe set it aside for service work and replacement orders.  Looking back at their amps all the way to 1930 you find constant and countless revisions of transformers.  Many in cases where they could easily get away with using the same iron in the next product.  During the mid 1930's you find amps that were made for only 3 years that have 3 different sets of transformer revisions, all in the quest for best operation. 

The 6 transistor 'A' versions have less low end (smaller output caps) and a slightly less linear sound than the 4 transistor versions.  They are a little smearier/browner.  They sound a little more in the direction of  lower feedback tube circuits, though that's a ridiculously general comparison.  I keep some around (with larger output caps) for some guitar tracking, as the difference is worthwhile.  I use the 4 transistor version for drums all the time, and bass pretty frequently.  I acquired these some years after having worked on a Neve with 33114 and 33115 modules, and have to say that they remind me of that sound more than any other amp I've used, and to me sound more like Neve than pieces like the Vintech clone.

Here's a response comparison between the 'A' and the 'B' type, both having 2200 mfd output caps rather than the stock values:

http://farm4.static.flickr.com/3174/2606251950_c103fe86cf_o.png

I also wrote a bit on the history of the BA-33/43/73 program amps over here:

http://www.groupdiy.com/index.php?topic=15372.msg180766#msg180766

emrr,

Do you have a Schematic copy for the RCA BA72?....and I would like to check for the another BA schematics too if this is possible.

Thanks
Opacheco

 

[mjrippe]

Hi Opacheco,

Again, I have a full manual.  I can put it on dropbox for you.  I also have 4 BA-72 on hand if you need photos or details.  Feel free to PM me for more info about discrete RCA modules.

Mike

 

[opacheco]

Hi Opacheco,

Again, I have a full manual.  I can put it on dropbox for you.  I also have 4 BA-72 on hand if you need photos or details.  Feel free to PM me for more info about discrete RCA modules.

Mike

Yes Mike I got your file but this one doesn’t have a schematic for the preamp!!
I read this BA72 preamp is similar to another RCA preamp, the question is which one??

Maybe someone have a schematic in hand and could post a copy for me.

Thanks

 

[mjrippe]

Hi, sorry I was not clear - I have the full BA-72 manual as well    They are nearly identical to the BA-71 but with some higher value caps and sometimes Kenyon transformers.

Mike

 

[opacheco]

Hi, sorry I was not clear - I have the full BA-72 manual as well    They are nearly identical to the BA-71 but with some higher value caps and sometimes Kenyon transformers.

Mike

Great news!!...Could you send me a copy please??

Opacheco

 

[emrr]

BA-72 is in the tech docs section.  Many of these are online all over the place, if you read between the lines in what I wrote. 

 

[opacheco]

BA-72 is in the tech docs section.  Many of these are online all over the place, if you read between the lines in what I wrote.

emrr,
Sorry I got it there!!

Thanks
Opacheco.

 

[shoonbassjp]

got my BA-71 powered up and tested, as many said, this is really good amp
thank you all for all the info !!

 

[emrr]

The front end is 2N2270's in the BA-31B/71B.  The BA-72 update moved to 2N3391A, and a lot of BA-31/71 have been updated in the field.  I took measurements of a 2N2270 BA-31B that had just been recapped, then again with a 2N3391A as Q1; noise improved 12dB below 500Hz, and about 6dB 1-16kHz.  This after same result in a group of five BA-33B's.  All had a grainy hash and shot noise before, totally gone.  Well worth it.    Not as big a deal with the lower gain of the 31/71, but an improvement. 

Measured response on these again for the first time in years, looks like a straight line until you zoom in and go wide.  Looking at 192kHz I get -1dB points of 10Hz and 32.4kHz.    1/2dB points of 12 and 21.2kHz. 

 

[aaquilato]

Hi all. Can I revive this thread?
I’ve just purchased a BA-71a with 6 germanium transistors. It needs some TLC.
The noise floor is quite high.
A hissing type noise. Not 60cycle.
More like air escaping from a tire.
Any thoughts?

 

[emrr]

Replace all the caps first, see what you get.

 

[aaquilato]

Replace all the caps first, see what you get.

OK. I’ll check them.
I’m a bit out of my depth here.
Thanks for replying. It’s appreciated.

 

[aaquilato]

OK. I’ll check them.
I’m a bit out of my depth here.
Thanks for replying. It’s appreciated.

I tapped around on the pcb and found that R2 was microphonic. We’ll start here and see if that is the culprit.

 

[emrr]

I can't begin to express how terrible those caps are now with age. I've redone many that were noisy AF before a recap. Many that weren't but sounded pretty bad, that then didn't sound bad. The first transistor is the most likely culprit after the caps are dealt with. Resistors a distant 3rd.

 

[aaquilato]

I can't begin to express how terrible those caps are now with age. I've redone many that were noisy AF before a recap. Many that weren't but sounded pretty bad, that then didn't sound bad. The first transistor is the most likely culprit after the caps are dealt with. Resistors a distant 3rd.

Noted. It’s going on the bench tonight.
I’ll keep you posted.
Thanks again for your insight.
It truly is appreciated.