Film-Tech Forum ARCHIVE: Old RCA Theater Amps

This is topic Old RCA Theater Amps in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Louis Metzman (Member # 8035) on 01-17-2014, 06:50 PM:

Gentlemen,

I'm looking for some help on my pair of RCA theater amps - model MI-9351 - they use a pair of 2A3's in PP mode, and also RCA 59, RCA 80, and RCA 83.

I'm hoping someone has a schematic for this exact model, and perhaps for some insight on the sound quality if I rebuild these.

Thanks!
Lou

Posted by Louis Bornwasser (Member # 3063) on 01-18-2014, 06:40 AM:

That amp looks great. I MAY have a drawing, but I will have to search for it. Serviced several of these back in the 70s; even in a drive in. Surely repurposed.

Sound quality: probably a bit worse than a 6L6 amp because those amps were after the 2A3 and were a considerable improvement. (Yeah, i know there are "fans" of the older tubes like the 300B and i found those sadly lacking.) Louis

Posted by Robert Koch (Member # 3728) on 01-18-2014, 07:28 PM:

This was a power amplifier used in PG 116 and PG91DLX systems. The rectifier tube was the 83. The 80 provided the fixed bias for the 2A3 TRIODE output tubes. The preamp for the system had a 57 and 2- 56`s in it. I must respectfully disagree with Louis on the sound quality of this amplifier compared with later amps with 6L6 finals.Although the amplifier in question had less power [35dbm] I think the sound quality was excellent. Think about it 2 TRIODES with fixed bias. It really doesnt get any better than that. On very large [for the time]systems the amplifier would drive a pair of 845 triodes. with a plate supply of around 1200 volts.

Posted by Steve Matz (Member # 1890) on 01-18-2014, 09:19 PM:

KEN LAYTON can probably fix that Amp! [thumbsup]

Posted by Mark Gulbrandsen (Member # 72) on 01-18-2014, 10:57 PM:

Check the insulation very carefully in that amp before usng it or leaving it on for long stretches. 98% of all RCA amps suffer from insulation dry rot. Quite a few either went up in flames or developed shorts in them due to insulation break down. RCA Transmitters of the same era also suffered the exact same problem. I removed one of those amplifiers from the Teton Theater in jackson Hole man years ago. I donno what ever happened to it...

Posted by Robert Koch (Member # 3728) on 01-19-2014, 12:40 AM:

Don`t exclude me from from the list of you could fix that amplifier. Mark is absolutely correctly correct about the wiring. I was remiss for not stating how poorly this amplifier was wired.

Posted by Louis Metzman (Member # 8035) on 01-19-2014, 09:17 AM:

under the hood view - thanks for all your replies so far!

Anybody know if these will be rolled off at the high end if properly restored, and any filtering removed - how good are the OPT's?

Lou

Posted by Will Kutler (Member # 770) on 01-19-2014, 10:08 PM:

When restoring a piece like this...there are several excellent resources. For example, I am a member of VRPS - http://vrps.org/

There are several sources like Antique Radio Forums http://antiqueradios.com/forums/index.php that might have either the tech info or a link to a resource that does.

A rewire might be in order. Antique electronics used rubber coated insulation that had either a solid color or striped woven cloth overlay that was lacquered. Antique Electronics Supply in Tempe/Phoenix, AZ is an excellent resource and this material is inexpensive.

Do not turn on without recapping...if the electrolytic capacitors are bad they can ruin other components like your transformers. Do not discard the top mounted capacitors. These are part of the aesthetics. Rather, they can be gutted and stuffed with a new modern cap. Purists will also do the same with the large paper-tubular sub-chassis mounted caps.

Any selenium components will require servicing and a few other safety-related things need to be done.

Cheers

Will Kutler