R*C*A BA-43 Program Amps

[gar381]

Hi Gang

Just got my hands on some R*C*A BA-43 Program amps from 1965 .

Are these worth restoring or should I just rip the Trafos out of them??

Input trafo: RCA #3464718...150/600:20k??
Output trafo: RCA# 3463498...very low??:150/600 (30 dbm!!)

Any Idea who made the trafos for RCA?? (CJ...?????)

I'll post the schmo if you all are interested.

GARY

 

[CJ]

UTC!

 

[rafafredd]

Hey post the schematics! I think it might be well worth the effort to restore this one.

 

[gar381]

pardon the camera pict..my scanner is on the fritz

GARY

 

[rafafredd]

mmm. What the hell is that strange resistor in the output (R11???). It´s labeled <<something>> TEMP...

 

[mikep]

a few years ago I racked up an RCA BA?? as a mic amp for a friend that looked like it came from roughly the same era as this piece. I can't remember the model, but it has some of the same transistor types, the UTCs, and made about 45dB of gain. anyway it sounded really cool. Id say definately try this thing like it is before ripping it up.

mike

 

[soundguy]

[quote author="gar381"]
Are these worth restoring [/quote]


ab soooo lutely.

dave

 

[DOX-EMF]

[quote author="rafafredd"]mmm. What the hell is that strange resistor in the output (R11???). It´s labeled <<something>> TEMP...[/quote]

It's for bias compensation/stability with temp rise. Probably mounted with the output pair.

Bill

 

[gar381]

rafafredd..
kind of hard to tell in the schmo pict but R11
is actually notated as RT1. It looks like a little round
black disk about 8mm in Dia. but there not caps.
There is also one in the PSU notated as RT2.
They are both on the PCB and no where near the output
sand. Another interesting note: Q8 (40250)is a TO66 case and
Q9(40050) is a TO3 case.

Soundguy..Should I replace All of those 40+ year old
Sprague caps with BCs??

GARY

 

[soundguy]

replace them with whatever you want.

sprague still makes those caps.

dave

 

[analag]

RT1 and RT2 are thermistors

analag

 

[emrr]

definitely worth restoring. May not need anything though new signal path caps and film bypasses will improve greatly. These are the cleanest sounding of the RCA SS program amps, and they work great for high gain mic pre's.

 

[ulysses]

RCA made the BA43 to replace the earlier BA33. The BA31 and BA33 are older, and I believe they're germanium based. They both have built-in power supplies too. The 31 is much smaller, only about 1.5" wide. It's pretty much intended as a mike preamp or line amp. The BA33 is essentially the same circuit, but it has an input attenuator and it has a much more robust output. It's intended as a program amplifier. Back in the days when this was state-of-the-art broadcast gear, the program amps had to have tons of headroom because the signal feeding the transmitter got pushed into much higher voltage levels, and remember this was in the days of 600 ohm terminations. In other words, the BA33 was a little power amp. The BA43 is the same deal, but it's a little newer and I believe it's all silicon, though I haven't checked. In any case, these are robust preamps that can do lots of different things, most noteably serve as mike preamps. Since they have so much headroom, you can drive them pretty hard (turn up the input attenuator) and get the transformers working, and then pad the output (it'll also get you better noise performance than if you turn down the input attenuator). This headroom would also make these units very appropriate for makeup gain after (or before) a passive equalizer.

These units (BA33 and BA43) were solid-state replacements for the all-tube BA23, which did pretty much the same thing. The tube BA21 was the same size as the BA31 but didn't have an internal power supply. I've never seen anything about a BA41, and I don't think such a thing was ever built.

 

[gar381]

GREAT INFO ulysses!

I got a set of sprague caps on order for one of them.
If its sounds GREAT I'll recap the rest of them and build
some 48V boards for them. Thanks all for the info and
advice. :grin:

GARY

 

[emrr]

I just wrote a history on the BA-31, BA-41, BA-71, BA-72 just the other day, at the bottom of page two:

http://www.groupdiy.com/index.php?topic=2867.msg180431#msg180431
There is a BA-33A that is all germamium, and rather rare. The BA-33B changes the front end transistor of each stage to silicon. The BA-43A is all silicon.

The BA-33 types IMO have the most useful amount of gain (strappable for 55/61 or 70/76 db), being lower than the BA-43A's 82 db and significantly lower than the BA-73's 92 db. The additional gain in the 43 and 73 seems to do little other than give a higher noise floor.

The BA-43 has a regulated PSU section, while the BA-33 does not. The BA-73 has no power supply, and came as part of consoles that had an overall regulated PSU. The 73 is a similar circuit that for some reason was manufactured to a much later date as an all germanium piece. The final version at least (73E, not sure about 73D) made small changes to accommodate silicon transistors in the first 2 stages, and updated the germanium types in most others.

The BA-31/41/71/72 are significantly different circuits from the BA-33/43/73 group, with the 'x1 and 72' varieties being low level fixed gain 40-46 db preamps having +18 dbm outputs and the 'x3' series being program amps with roughly 1 watt outputs and adjustable total gains on the order of 55-92 db. The input stages before the attenuator may be similar to the 'x1' series, but the output stages are basically low level power amps. The 'x3' series can all be used for mic preamps, though the tube BA-23 requires attenuator modifications due to an attenuator loaded input transformer secondary which reflects too low an impedance back to the mic and leaves one with the full noise figure of the amp at all times.

 

[DrRick]

I have a bunch of these, along with some compressors that match and a pair of BA-31's from WNBC-TV in NYC. There are matching rack shelves that hold 3 units. Some of mine were modded by the tech shop at NBC for phantom power.

I let them sit for about 10 years, because they were solid state. I recently tried a pair of them and was really pleased with the sound.

I also got a bunch of the xformmers from the same source, a surplus house that was stripping them for scrap!


Peace,
DrRick

 

[adrianh]

Gary;
If you tear those amps apart to steal the transformers
we will banish you to silvertone sound land.

Use them. They will blow you away.
Look for bad caps.

 

[ehrmann]

Thanks for the picture of the RCA-Ba 43 schematic.  Any chance to get a scan of it?  There are a few spots I cant make out.  Thanks so much.

 

[gar381]

ehrmann

If I get a little time this weekend I'll see about scaning it.  I took that picture when my
scanner was on the fritz!!  I have rebuilt 2 of my 4.. somewhere I got a good mouser parts
list of replacement caps if you are interested.

BTW this maybe a little better for now.

http://www.groupdiy.com/index.php?topic=287.msg356232#msg356232

GARY

 

[ehrmann]

That's great.  Thanks again.