Any one know about this Rauland-Borg EL34 monoblock?

bhamham

Super Member
Trying to figure this guy out. Can't seem to find any information or a schematic anywhere. Would like to know how speakers were hooked-up to it?

It's a Rauland-Borg model RA-70

Thanks for any help!
Bruce

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They made PA amps, so it most likely has a bunch of different output options. Guessing its going to be on that un-marked octal socket. Usually stuff like this has 4,8, 25v, 70v, and sometimes other configurations for large distributed sound arrangements. This appears to be a rack mounted thing like you might see feeding music to a store or whatever.

only blip I can find is on pg 48 here

https://www.americanradiohistory.com/Archive-Audio/60s/Audio-1965-04.pdf

It claims 70w output, 20-20k +/- 1 db with 5% THD at full scream. Pretty bloody impressive honestly. At lower output levels it should be even better. Says 25v and 70v outputs, which I'd expect.
 
Wow, thank you Gadget!

How would I go about finding how to connect a pair of 8 ohm speakers? I think I could trace the circuit and post here. I wouldn't mind getting a pair of these cleaned-up and working.


Here's a little history on these amps: I have it on good authority these monoblocks - there are five - were used in the old Alamo Hotel in downtown Austin. There were five tuners/preamps with them but I let them go as they were pretty trashed, much worse than the amps. Each of the tuners were marked with a popular radio station from back in the 1960s. The guy I bought them from said that each room had some kind of panel with a switch where you could select one of five radio stations to play through a speaker in the room.

Here's a write-up on the old Alamo Hotel, long gone for some high rise residential housing. "Cash-strapped travelers and equally down-and-out musicians found sanctuary in the Alamo Hotel and Lounge. Legendary songwriters Townes Van Zandt, Blaze Foley and Lyle Lovett played for tips in the hotel's downstairs Alamo Lounge. Part of the music video for Willie Nelson and Merle Haggard's "Pancho and Lefty" was filmed here, featuring a cameo by Townes Van Zandt, who penned the song." Stevie Ray Vaughn probably played there too.

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The manual or schematic would have spelled out what positions on that plug are for what. Without that you'd probably have to do a little sleuthing to figure it out. Basically feed some known voltage from plate to plate and measure what comes out on the secondary. From there some math would tell the impedance ratios and a reasonable guess could be made as to what goes where. also not impossible that R-B had a standardized wiring config for that plug. Looks like other models exist in Sams, I'll have dinner and see if I have any of their other models in my collection to reference.

A 25v line on a 70w amp is 8.9 ohms impedance, so thats most likely where it would go if it doesn't have a dedicated 8 ohm connection.

Pretty cool history on these too.
 
So Bhamham was kind enough to offer me a pair of these to experiment with. I haven't gotten too far, need to order a few caps but I was able to draw up a schematic. Its not quite a polished one, but it should get the point across. I did not draw in the output socket though, mostly because I forgot. Pinout on that is as follows:
1 - chassis ground
4 - output transformer black
5 - output transformer yellow
8 - output transformer green

Seems to me that 4 to 5 is the 25v output. To tie an 8 ohm speaker to it, connect 1 to 4 and send that to speaker -, pin 5 to speaker +.

Voltages marked on the supply are not accurate, the caps are in pretty sad shape and one started ailing and eating a lot more current while I was taking readings. Once I get caps in it that work I'll give it another go. You know the deal, no matter how many parts are in the bin, the one that is needed isn't there.

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Overall the design looks fairly solid. Split load inverter, extra triode gain stage, power supply that should be reasonably stout. On the list of things to add is a bias level control and test points, but thats easy stuff.
 
Ordered some parts last night. The upper pair of paralleled caps will be replaced with a single 80uf cap, the lower 40+20 will also be swapped with a single 80. Can't find anything in the 60uf range and I would prefer to not stack up a bunch of caps just because it gets messy. The 5AS4 (effectively the same as a 5U4GB) will "see" 40uf, but the 5Y3 will see 80uf. Currently it sees 60uf. Both beyond the datasheet ratings, but not much to be done for it unless I want to risk making the supply filtering considerably worse.

I seriously gave thought to changing those to 100uf since caps in that value are about half the cost, but I didn't want to push my luck too much. Initial plan is to just replace the power supply caps and see what I get. I ordered caps for the bias supply too just for sanity reasons.
 
Wow, Gadget! It's great to see the progress you're making on these monoblocks. Thanks for making a schematic. Maybe these guys might turn out to be decent sounding after all. Who knows?
 
I'm probably going to be spending the next few days on the kitchen remodel project, hopefully by the time that is done I'll have parts. I don't really care for home electrical work. Its just not as interesting as playing with amplifiers.
 
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