Classic Hifi Care

Greetings RCA fans,

I'm normally a Motorola devotee, but a friend of mine who finds, upgrades and sells MCM consoles wanted a USA classic that was all-original. When he bought this he was pretty sure he wanted to keep it. I started a years ago and its finally coming together after numerous parts needed to be replaced.
While it is not unusual to see lousy original wax-paper capacitors in these otherwise expensive and well designed chassis, almost every resistor was far out of tolerance for being banded as 5%. The electrolytic capacitors were tested and seemed to pass both ESR and voltage breakdown, but I replaced them as this was a full restore not a repair

This model is a three-cabinet console built in the 36th week of 1957. Its head unit and speakers are heavy walnut louvered cabinets with heavy duty spun-aluminum legs. The SHF-1 was the first console to be equipped with stereo preamplifier and two (RS147 C-D) power amp chassis. These mono block amps are also under a separate Sams folder as SP-20 and, although its a full stereo unit, only the reel-to-reel tape unit is stereo and only as playback.

I was able to successfully retro-fit a stereo cartridge onto the original RP-215 record changer and have it track at 2-4 grams. The AM-FM radio is also mono and probably would not be worth finding a tubed MPX unit. Below are speaker legs getting new feet to replace the lucite (?) originals. These weigh close to 75 pounds each.

That's an impressive pair of amplifiers you have there Dave. When you have time I would love to see the fronts of the cabinets. Does it have 5 speakers total?

Thanks,
Bill

I had no idea RCA built anything like that. I'm impressed! Both with the instrument and with the workmanship. Thanks for sharing.....

Photofact by Sams 418 cover shows the whole "family", with RK-213 speakers each having 15" woofs, 6" solid back mid-range driver and 3.5" standard RCA tweeters facing forward, not on the wedges like almost all 1-cabinet consoles RCA offered.

This was a fantastic top of the line console and as a "set", allows speaker placement beyond 6 feet apart. This was a main reason CU recommended component systems and basically panned the one-cabinet consoles. I favor Magnavox and RCA products of this late-50s era for repairs, once the lousy parts are all replaced.

My present challenges with it relate to tight resistor tolerances in the amp's 6AU6 drivers to quad 6V6 outputs. class B biases are tight, most P-P or 2xPP
configurations like this amp have a single voltage amp first and phase splitter second feeding the grids 180 out of phase. This amp on the other hand has the voltage amp and phase splitter - 12AU7 , then two 6AU6 pentode stages to drive each pair of 6V6. These two stages on each amp chassis have a voltage divider to control the bias on each 6AU6 grid.

Instead of racing right into the weeds, Ill conclude by offering this conundrum:

How can voltages on grid and cathodes in identical circuits be different when the input voltages and component values have been verified as with 5% (per schematic) after replacement of all resistors and caps?

That's one impressive 3 pc console!
RCA made some very impressive audio in the '50s and before. They made much of the amplification and monitoring equipment for recording, theaters and radio stations, some of which are extremely desirable today and sell for huge prices, like the BA series amps and the Harry Olsen designed LC-1A 15" speaker which easily brings 4 figures these days if you can find one. Here's an example of the RCA BA amps. I had a similar one years ago that I sold to a recording studio guy who uses all tube recording and monitoring equipment. Look at the military-grade construction underneath and the fleet of top mounted caps. This stuff was designed to run 24/7/365.

https://www.ebay.com/itm/402963135472?h ... 7675.l2557

The mid-late '50s SHF consoles were some of the last-best RCA consumer audio products, as were SP-10 and SP-20 amps.
Unfortunately RCA went rapidly downhill starting in the mid '60s, putting more effort into TV, aerospace, and cmmunicaion, where the big $$$ was by then.

Help out some with RCA nomenclature. The amp in the link are ID'd with a number beginning "MI-". I've got a couple little RCA integrated mono hi-fi amps, 6V6 P-P, ID'd as "SVP-10" and also "MI-12198." I figured that it meant something interesting.

And on those big amps, the black cardboard-covered tubular things that look like electrolytic caps have octal bases. Are they something like input transformers, or maybe plug-in electrolytics, or what?

Chris Campbell

Wow, that is one impressive RCA Dave, I think I need to add that to my want list, but probably will never find one.

Chris, I have the same questions you have. What are the black things, and how do RCA model numbers work?

Bill

Chris/Bill: the MI-xxxx was used to indicate RCA commercial/industrial products. The SP amps were a consumer version sold as separate components (painted chassis) by big dealers like Harvey Radio on "Radio Row" lower east side NYC, or unpainted mounted in TOL consoles, the MI versions came with some ruggedized/upgraded passive components. The black tubular things on the BA-4C, BA-14, and other RCA amps (all these also had MI nomenclature, I used to have some of the manuals that went with them when sold) were plug in electrolytics that typically are replaced in restoration with more common twist-lok type. Mono SP10 pp 6V6GT amps are very nice well designed and built vintage units. Good (and easy) to restore and hook up to some similar period efficient speakers!

Roger has elevated the post considerably on this amplifier, considering it was RCA's best that year and when they dominated markets in TV and broadcasting equipment. The iron is impressive, circuits simple but the beastly RK-213 speakers must accompany them. They are large and heavy or, in other word, efficient.

I have been playing with both amps and found a few NOS resistors that were actually more than 10% high. I grabbed all and tested 1.8 K resistors' placing them in order of low-high ranging from 1700 ohms to 1900 ohms (binning is LED-speak to get the right color temp among a batch) to approximate an equal bias network on both 6AU6 drivers. Most driver circuits were 12AX7 or 6CG7 in RCAs, so this pentode circuit seems more tricky to get the bias correct for class AB operation.

The schematic shows 195 v on each 6AU6 plate, I was able to get 195 dialed-in with both amps but only on one 6AU6 each. The other 6AU6 on each one had 215 v on plate pin 5. At least all the cathode at 7v and grid voltage at 5v on those driver tubes was equal at -2 volts difference. Sound from the radio has improved but a record player is the real test of this console, up next for set up.

IIRC these SHF-1 had a weird moving coil ? cartridge and preamp circuit for it that make parts replacement difficult nowadays. I got most of a SHF-1 a few years ago from member (wayfld) here that ended up being beyond my severe space limitations and restoration (in)competence. So I gave it to a retired former TV-radio tech I knew out in Oneonta (NY, about 2 hr away), knowing he'd be able to do it justice. Then his wife died and he followed a few short months later. I've since had no luck in trying to contact his one son but I hope it has been saved as it was TOL for that time, RCA was really at it's peak then.