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AINCENT RCA THEATRE AMPS

N

nightfly61

Guest
I don't have much knowlege of electronics, but here goes. I ripped out 3 very old amplifier chassis from a very old condemned movie theatre projection room that was tore down back in the early 80s. RCA MI 4236A, an RCA M9377A, and something else with no # or brand on it-just a (looks like) volume pot on the front. Only 1 of the 3 are rack mounted & all 3 have just 1 pot on the front. I'm thinking 1 was for the lows, 1 for highs & 1 was a standby unit?
Surprisingly I tested the smaller tubes on 1 of 'em & they're still good, but I have no schematic & the harnesses had to be ripped bigtime. It was the type where the wires sent to a plug that screwed into the front of the things. Would collectors have any interest in something like this? I went on the web & found nothing even close. Thanks.
 
Wow...looks like I got screwed...I just sold 2 new & 1 used 2A3 to some dealer & only got $90 each for them.
 
Alan Fletcher said:
I would have kept the amps intact. Could have made a nice sounding pair of mono-blocks. :(

-A
Click to expand...
Oh, the amps are intact...those 2A3's were from something else(my old station remodeled & were gonna trash the tube cabinet & I got 'em all.. The one amp I have is missing all it's tubes though. The transformers have rust on the outside of them...would they still work?
 
Ah, good to hear you still have them.

Rust on the outside of the iron transformer cores should not pose a problem, it's the windings inside that do the work. I suggest, if possible, to power up the units slowly, gradually increasing the line voltage over a 1-2 hour period using a variac. This will help to ease the unit and its associated capacitors, rectifiers and tubes into use, rather than just surging the components into use after a long period of non-use. Old dry capacitors have a tendency to fail over time, especially after long periods of non-use.

This procedure will help you detect any problems before they become catastrophic failures. Keep a close eye on the unit as you increase the voltage and look for any smoke or burning smells.

I'm not suggesting you should expect any such problems, but 'better safe than sorry!'

Also, I know you stated that you're not that knowledgeable about electronics, but if you're feeling up to the task, I would suggest 'shotgunning' all the capacitors in the amp. That is, replacing all the caps with equivalent or near equivalent value modern electrolytic caps.

I'll try to dig up a site that will give you more info on restoring old gear.



-A
 
Thanks Alan-
BTW...the power cord was cut off. Judging by the schematic (the one you sent me the link to) for the M9377A, does it take just a regular old wall outlet or a 220 plug? I can't really tell by the schematic.
 
I would suggest you look for the ratings label on the amp, I would expect it's 120V, but of course it could be 240 seeing as it was in commercial use.

I'll PM you with my phone number in case you want to talk specifics.

-A
 
2A3s and 6A3s are directly heated triode output tubes. They are the sweetest sound ever.
I settled on 2 1936 Phicos with push-pull 6A3s for my "stereo pair" for the living room.
Only 15 watts per, but the finest sound ever out of any audio amp.....

By all means restore the amps!
 
I am actually a Cinema Service and Install Tech by trade, and a Dolby labs certified sound tech. I used to work for the company that was origionally the theatre service division for RCA. Maybe the tubes are worth money and maybe it would be a neat "Antique" and a fun project to restore it but using it for anything, I would not especially in a theatre set-up. Those amps are not designed for the type of dynamic range and power that is needed today. If someone were to try to use those amps with a Dolby Spectral Recording Digital (Dolby Digital or SR-D) system it would fry them pretty quick. Those amps were designed for optical academy mono sound which is very very inferior to any sound format in use today.
 
Wow...thanx for the heads up...to be honest I wouldn't know where to start-I guess by getting some rubber gloves & putting an outlet on it & doing what Alan said...plug it in & ease the pot up.
 
I have to agree with NSPUNX here. I would not try to use them for a theatre setup. However, with a reasonably efficient horn-loaded speaker system in a medium sized room, you could get that beautiful triode sound and it would play at a reasonable level.

As an aside, I'm not currently running any hollow state gear here. My preamp is a Mackie Big Knob, feeding an Ashly XR-1000 active crossover in a fully balanced setup. Amps are Hafler Trans-Ana P1000 and Trans-Nova P3000 respectively, driving Klipsch CF-1's with the passive crossovers bypassed. The ports make a great power entry point. You'd be surprised at what giving a high quality, high-current amplifier direct access to a driver will do for your dynamics. No phase shift, no 90 degree out-of-phase problems, no Zobel networks and no reactance problems except those presented by the driver itself. Heaven.

If anyone is ever in the Danbury, CT area I'd be happy to demo the system. You'd be amazed!

-A
 
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