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Author Topic: An old RCA tube amplifier? Matthew Deppe Film Handler Posts: 4 From: Grinnell, Iowa Registered: Dec 2003 posted 01-03-2005 01:59 PM                     This weekend I pulled out, what seems to be, an old RCA power amplifier from behind one of our screens. It is model #4256-C and requires the following tubes: RCA 59, RCA 2A3 (2), RCA 80, and RCA 83. I have been unable to find any information on-line and the RCA (Thompson) people I have spoken with have been no help. Does anyone know anything about such an amp or know someone who might? I spoke with the local Radio Shack proprietor but he said we would need a repair manual of some sort to begin working with it. The thing seems to be very old. With resistors the likes of which I have never seen.  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 01-03-2005 02:27 PM                     This is a circa 1936-1939 power amplifier[15-18 watts on a clear day] used in RCA Photophone Groups [PG] 90, 91, 91dlx 116 117in those years. It was a good design of the time. Push pull triode output [2A3`s] with fixed bias supplied by the 80 rectifier tube. The 83v was a mercury vapor rectiier supplying the high voltage for this amplifier and its associated voltage amplifier, MI 4228 usually, which had a low impedance output to the front wall attenuators[one on each machine so you could preset the volume on the incoming projector. This is making me feel so old, if you want anything further email me.  |  IP: Logged Doug Eisemann Film Handler Posts: 21 From: Douglassville, PA Registered: Mar 2001 posted 01-03-2005 04:44 PM                        Hello, Fixing amplifiers like this is what I do for a hobby and a job, so I could offer some pointers. First of all, I can't find any literature in my files on this particular model, but I have seen them before, and I know they can actually be very good sounding amps. I have the 1939 model MI-9355, using 2 845 output triodes and 2 866A mercury rectifiers. It needed a smaller power amp (15-20 watt MI-9354) to drive it to full output. Part of the PG-142 system I believe. On your amp, it is likely that any electrolytic capacitors will be beyond useful life at this point, and need to be replaced. Small paper capacitors may or may not be good at this point. Whatever you do, don't just hook up power to it and turn it on. Things may be irreversibly fried that way. The person at Radio Shack is unlikely to know much about fixing it, unless he is used to tube gear, but it should certainly be able to work again with some component replacement. All tubes are still available, except old 2A3s are expensive, but new ones are being made. Feel free to email me if you need some more help Thanks Doug Eisemann [ 01-03-2005, 04:55 PM: Message edited by: Brad Miller ]  |  IP: Logged Ken Layton Phenomenal Film Handler Posts: 1452 From: Olympia, Wash. USA Registered: Sep 1999 posted 01-03-2005 05:28 PM                     Film-Tech member Dick Prather should have tons of stuff on that amplifier. He's got lots of original RCA theater sound systems literature including schematics and service manuals.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-03-2005 07:02 PM                     Remember that if you go to sell it it also has a VERY high value. Some of the old theatre amps have values higher than some very high end power amps cost new. Don't let some one abscond you out of that amp for a song and or a dance! While RCA equipment is not as valualable as some of the Western Electric stuff that particuluar RCA amp is not very commonly found in this day and age.... and 2A3 based amps are VERY POPULAR within certain circles of audiophiles, especially in Asia. If you restore it fo not change or remove anythng from the amp. This will de-value it immensly!! Check out places like www.diyaudio.com for more info(not on values)of 2A3 power amps and other links. Mark @ CLACO  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 01-03-2005 07:19 PM                     quote: The 83v was a mercury vapor rectifier Incorrect. The 83V is an indirectly-heated full wave vacuum rectifier. The 83 (no V) is a directly heated mercury vapor rectifier. I have two of these amps myself (pulled them both from the dumpster of a theatre undergoing remodeling), they are in pretty bad shape but once I get around to restoring them they should be awesome -Aaron  |  IP: Logged Doug Eisemann Film Handler Posts: 21 From: Douglassville, PA Registered: Mar 2001 posted 01-03-2005 07:35 PM                        I wanted to add to Mr. Gulbrandsen's post about replacing components. If you do have to replace components to make the unit functional, I always try to use modern replacements, carefully hidden in the old cases. IE, often with electrolytic capacitors, it is possible to cut open the old case, remove the old internals, and place the new, smaller part inside. When reassembled carefully, it is hard to tell anything was done. It is nice to see these units restored carefully and enjoyed, rather than simply sold off due to their monetary value. Generally, the build quality of old theater amplifiers is better than any new "high end" audiophile tube stuff, and cost a fortune when new, hence the high value even today. It will be interesting to see your progress. Best Regards Doug Eisemann  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-04-2005 12:00 AM                     quote: Doug Eisemann Generally, the build quality of old theater amplifiers is better than any new "high end" audiophile tube stuff I agree with your first part Doug but not the above. Having owned and still own a number of high end tube power amps. There are many, many high end tube amps that far exceed what any of the old stuff can do either single ended or PP. Today we have excellent output transformer and capacitor technology which both are far superior to yesteryears stuff, and then there are the Tube OTL power amps which are amazing and a whole other world in themselves. The main thing about the old stuff is that its nostalgic, sounds nice if peoperly restored, and has an extremely high value.... but from my experience much of todays stuff still surpasses the performance of yesteryears. Happy listening! Mark  |  IP: Logged Doug Eisemann Film Handler Posts: 21 From: Douglassville, PA Registered: Mar 2001 posted 01-04-2005 09:20 AM                        Perhaps I did get a bit carried away saying that old gear surpasses all new high end stuff! There is something to be said for modern gear with component quality far above those crummy old paper capacitors and dried out electrolytics. Not to mention transformers with winding insulation that does not break down and quality tube sockets that actually contact the tube pins! I have some old German made theater and studio amps that give the high end a run for it's money, but they are the exception, rather than the norm, in vintage gear. Sorry to turn this into an audio discussion! There are other forums for that, although it was theater sound systems that got me fascinated with theater technology and tube amps in the first place. Best Wishes Doug Eisemann  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 01-04-2005 12:23 PM                     To add to what Ken Layton posted, I talked to Dick Prather some while ago and he indeed had ex RCA engineer Don Howards material,and Don was meticulous in maintaining it, so you should be able to come up with a schematic. P.S. Aaron sorry about the "V"  |  IP: Logged Frank Angel Film God Posts: 5305 From: Brooklyn NY USA Registered: Dec 1999 posted 01-05-2005 11:02 AM                        I have a number of Dynaco tube amps -- I've got some of the Stereo 70s and the Dynaco mono (I think 35w) units -- the chromed chassis vintage. These were about to go into the dumpster until I said that the next one who lays a hand on one of those amps is going head first into the dumpster. So now I have these tube amps and don't know how to go about finding homes for them. In that same dumpster were two MagnaTech 1024 stereo tape preamps that were used with their the MagnaTech R2R studio tape transports. My recollecton is that they were very quite preamps. Someone with a home setup and a mag penthouse might want to take these off my hands (pay for shipping only) and use them for mag playback. They are 3U rack mounts each -- fairly large for what they do. If you have a mind to, they can also record. Let me know if anyone has any interest in them or the tube amps. Frank  |  IP: Logged Aaron Sisemore Flaming Ribs beat Reeses Peanut Butter Cups any day! Posts: 3061 From: Rockwall TX USA Registered: Sep 1999 posted 01-05-2005 12:40 PM                     Frank, you have mail. -Aaron  |  IP: Logged Will Kutler Phenomenal Film Handler Posts: 1506 From: Tucson, AZ, USA Registered: Feb 2001 posted 01-06-2005 01:01 AM                     I agree with what many people have said on this forum. Usually resistors and wax-paper caps go bad and must be replaced. Just install the new ones exactly as the origonals were. If those origonal can-style electrolytics are bad, and no new ones can be found, a simple solution is just to disconnect the leads, install insulated terminal caps, and then just install a new modern cap between those disconnected leads....or like what was stated, remove the guts from those origonal caps, put the new cap inside, and reinstall onto the chassis. Check out Antique Electronic Supply in Scottsdale, AZ for your needs. You may also take a look at Antique Radio Classified magazine. Changing the subject, I finally aquired a radio at a very reasonable cost that I have been searching for for several years....the 1938 RCA 816K, and am looking forward to beginning restoration!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-06-2005 05:30 PM                     Frank, Those Dynaco trannys are some of the finest output trannys ever made! They can be converted to a high end equivelent amp with the addition of a new driver board and changes to the power supply. Believe it or not brand new Stereo 70 kits are still available . The Stereo 70 is probably the most famous of all stereo amplifiers past and present. Check E-Bay sold auctions for realistic values and this web site: Dynaco Upgrade Boards About 10 years ago I did the premium upgrade to a Stereo 70 and the results were nothing short of incredible... but ots still a small power amp that was too small for the speakers I had at the time..... Mark @ CLACO [ 01-06-2005, 07:17 PM: Message edited by: Mark Gulbrandsen ]  |  IP: Logged Bob Koch Expert Film Handler Posts: 183 From: williams ca Registered: Nov 2001 posted 01-06-2005 09:12 PM                     Everyone seems to missed the point, when he went behind the screen encountering ,who knows how many rodents, he missed one half of the system. The voltage amplifier has to be back there somewhere which is on a separate chassis containing a 57[grid cap] and 2 56`s and an output xformer to make it a complete system. With someone today, trying to drive this to its full 15 watts and applying aN ersatz driver, I think would even displease the the Japanese  |  IP: Logged

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