RCA PA-138 Noise Reduction Amplifier as Microphone Preamp

[LeRoyLimited]

Hello All - long, long time lurker, but a new member and first time poster. I have successfully built an La2a and recapped/reduced noise on some vintage gear with the help of everyone here, so thank you, especially EMRR. It is a testament to the forum that I have spent a decade reading posts and have never had to ask a question due to the volumes of information already covered. That being said, I am still an amateur, so here we are:

A few years ago an old RCA Photophone noise reduction amplifier walked into my life, and though I don't have a use for an NRA in my little project studio, I kept it because pieces of this vintage, rarity, and high quality manufacture don't come around very often. So, it got put on the shelf in the corner as a display piece.

I knew the schematic was in Tremaine's, but only after looking at it for the hundreth time did it pop into my head that I might be able to easily repurpose this into a mic-preamp if the transformer after the 6V6 cathode follower was suitable. I tested DCR on the secondary, came up at 79, so I knew I was in the ballpark. Spent the morning tracing wires to the Cannon multi-pin, labeled and disconnected the leads from the output/interstage transformer, hooked up power and XLRs, and finally brought her up slow on a variac: She worked, sounded good and was dead quiet.

So, my question is, can I run it like this? I'd prefer to not unhook the unused parts out of historic preservation and laziness. Is there any harm in leaving them all connected to power? Should I pull the NR rectifier and DC gain tubes? Any preventative maintenance in the power supply or audio path I should look into? Someone before me put a few electrolytics in underneath, so this is not 100% original, but all of the multi-cap cans on top look to be.

I am used to cheap Gates program amps, where, in my experience, pretty much everything needed to be redone to get the noise floor low enough for modern use. Having everything work right off the bat without 60 hz hum from the power supply on the same board is blowing my mind. I would also guess there are a hundred m5576's for every one of these, so I want to tread respectfully. Would love to hear the thoughts and opinions of everyone before I get crazy in the woodshop making a nice case for this with proper xlrs and power cable. (unless someone miraculously has a spare RCA rack mount, pull-out slider case that this would slip right into...)

 

[moamps]

You can remove the vacuum tubes 3 and 4, but still check after that if all the power voltages are ok. It can happen that the filament voltage and B+ voltage are too high if the power supply is not nominally loaded (situation when tubes 3 and 4 do not work), which largely depends on its design.

 

[Bo Deadly]

I wouldn't disconnect or unplug anything. I don't understand how that thing works but if it implements "noise reduction" as I know it, there's probably some bias setting that makes it not reduce level much or at all.

These sorts of circuits are all balanced to have a certain load on each stage and on the supply. So if you start disconnecting and unplugging things, you're going to throw that balance off and possibly cause bad things to happen.

 

[LeRoyLimited]

Thanks for the input! I agree, Bo, my first inclination was to not pull anything so as to maintain equilibrium in the entire system. I don't mind having a couple 6H6 and 6V6 tubes sitting in there for preservation purposes.

Anything else I should look out for?

Setting aside some cherry for a nice box to re-home it and put it back into service for another 75 years!

 

[emrr]

You can definitely leave the 6h6 out

 

[emrr]

Any preventative maintenance in the power supply or audio path I should look into? Someone before me put a few electrolytics in underneath, so this is not 100% original, but all of the multi-cap cans on top look to be.

If you don't have a cap meter and don't want to replace electrolytics, now's the time to get one and check the cans out. Maybe someone parallelled new ones on the cap sockets? If so you can pull the orig and set them aside, that's what I'd do. It'd be a small miracle if there's not at least one bad section in those cans.

I am used to cheap Gates program amps, where, in my experience, pretty much everything needed to be redone to get the noise floor low enough for modern use. Having everything work right off the bat without 60 hz hum from the power supply on the same board is blowing my mind. I would also guess there are a hundred m5576's for every one of these, so I want to tread respectfully.

Not really comparable, you don't have a ton of gain here like those Gates, probably half or less. But it is also probably quieter. I'd want to look at ESR on every cap and replace any that are obscenely high, even if capacitance is right. That can roll off treble and/or cause a generally smeary sound.

With that gain pot you are true 600 ohm in, so that's at least a 3dB loss of S/N to any mic. If it's not a problem....it's not a problem!

(unless someone miraculously has a spare RCA rack mount, pull-out slider case that this would slip right into...)

Those rack mounts are common, but this version with the rollout bits is not something I'd ever expect to find loose in the wild. Very very few of these or anything from this series still out there. Just about no one needing the noise reduction aspect.

 

[LeRoyLimited]

Thanks, Doug, I was hoping you would chime in!

I do not have any capacitor test equipment, it's on the short list of things for the bench, but every time I go down that rabbit hole I end up coming to the conclusion that it's cheaper just to replace the electrolytics and move on. That being said, this is the first piece of gear that I have worked on that isn't a dime a dozen, so I wanted to get other opinions before putting my mitts in. I think I will recap the electrolytics, and because it's going into a dedicated case, I can easily find a place for the old cans inside for history's sake.

Will start there and let you know how I make out. Thanks again!

 

[LeRoyLimited]

Well, I finished her up this morning and am happy to report that it now looks as good as it sounds and should be good to go for a long time to come.

I was lucky with the resto-mod/preservation job: the can capacitors were tabbed plug in types and all of the other capacitors were housed in brackets underneath. I simply soldered the new filter caps into crimp tab connectors and they fit very snug in the sockets. The other originals I snipped the leads off at the insulation and tucked them behind the bracket the best I could and then soldered the new capacitors in. All in all, about an hours work with the soldering iron and any competent tech could put it back to original in the same amount of time.

Haven't had a ton of time with it yet, but seems to have a respectable amount of gain. I needed to use everything it had speaking into a ribbon, but a Telefunken copperhead was at around 2/3 gain close miking an acoustic guitar. Sounds good to my ear, very quiet wide open, and I am excited to properly put it through the paces.

Thanks again for helping me out on this one!