RCA SA-3

Brendan

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Hi Folks,

This is showing up on my local CL after not selling on ebay.

http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... 8pr6c%253D

The newest ad claims it's a '57, but it's definitely the same amp from what I see in the pics. He's asking $100 for it. He also says it's clean when dimed which surprises me for a 6V6 amp. Could that be the 6CG7 preamp tubes? I found some detail shots of the same model here:
http://www.antiqueradios.com/forums/vie ... 03b8fa11a1
I'm trying to find more information on these tubes and it appears that maybe the 6cQ7 is equivalent. Haven't been able to determine if the 12A*7 type tubes can work here. It's set up for RCA jacks and I would need to convert to a 1/4" jack. This could be a fun project. Thoughts?

Looks to be all original RCA in good shape. Speakers and amp tested good. Sounds great! 1-12" RCA speaker and 2-3" RCA speakers.

6V6 based amp model SA-3.

Tube layout 1-RCA 5Y3GT rectifier, 2-RCA 6V6GT power, 2-RCA 6CG7 preamp

Front mounted controls are Volume, Treble, Bass, Selector (stereo,tape,phono,tuner)

RCA type inputs for tape,tuner,phono

Front mounted power indicator lamp
 

Brendan

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Looks like the 6GC7 is triple diode vs twin triode for the 12ax7. Oh well. Another clean machine, but I already have that in my Thunder 1. I want something that can get a bit of "hair" without pedals.
 

capnjuan

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Hi Brendan; well ... that's as nicely-dressed up as 6V6 amps get ... :wink: Link to the 6CG7 layout/connections and to the 6CG7 tube data sheet. The 6CG7 is a twin-triode, and because of the 6CG7 pinouts, a 12A_7 is a drop-in replacement for it.

If the amp is mono and has a phono input, then the first 6CG7 stage/half is the phono stage which works a little differently than a conventional gain stage; it has an RIAA equalization network (read more) resistor/capacitor circuit that is necessary for playing back LP recordings. Not quite sure what's intended on the Selector switch; 'stereo' and 'phono' inputs. If the amp supports stereo inputs, then the first 6CG7 is split; one half handling Left and the other half handling Right. Without a schematic I'm just guessing but the other half of the first 6CG7 is a gain stage for the phono section. The line amp starts with the first half of the second 6CG7; it sees the tape or other line-level inputs and would be followed by the tone controls, and then the phase inverter/driver ... the other half of the second 6CG7.

The amp has a cap in the supply to the tweeters that blocks low frequencies from mudding up the tweeter output. How much hair this thing might have is usually governed by the cathode bypass coupling caps, inter-stage coupling caps, and the OT's ability to handle saturation; smaller OTs not doing this as well as larger ones. There are other little bits and pieces that control hair but the design intent here was the 'hi-fi' sophisticated experience; you know ... sitting around with a martini, smoking a pipe, wearing a tweed sports coat ... reading Esquire magazine ... :wink: and not listening to distortion ... subsonic squeal ... oscillation ... the (good) gritty stuff.

What are you after; a little bit of hair on something of a budget?
 

Brendan

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Hi Capn,

It was an even split between getting a bit of hair and being able to do so on a budget. I'm always looking for nuggets of old Americana like this. The only schematic available seems to be on a subscription site. The Thunder 1 covers the tame edge of things, although she's got quite a kick with a heavy dose of reverb and a Jaguar attached. I've never had a 6V6 powered amp and the price is right. Do you there's a chance a pair of 12ax7 tubes could warm it up? I'd be happy with some light breakup at half throttle. I guess I'll have to get that schematic first. I'll keep searching for it.

Thank you!
 

capnjuan

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Hi Brendan: below is a comparison of the pinouts of the 6CG7 and the 12AX7 and the preamp of a Gibson GA18; a golden, crunchy brown twin 6V6 amp from the early 60s ... what 6V6s are really good at:

brendan01.jpg


Pinouts: as you can see and except for how pin 9 is drawn, they're the same. Whether you can drop in a 12AX7 depends on whether the 6.3VAC heater supply has a center tap and how the heater wiring is done. If there's a center tap and provided pins 4 and 5 are jumped together on the socket and the other leg of the heater wiring hits pin 9, you are good to go. To save money, some mfrs used transformers that didn't supply a center tap; if that's the case here, then some fiddly-diddly would be in order. I mean a 12AX7 would still work but you'd have to mess with the 6.3VAC wiring. Assuming you could get 12AX7s running, you can manipulate the values of the bypass cap ... affecting tone ... and the dropping resistor ... push the preamp tube towards distortion ... see below. Respectfully, just swapping in 12AX7s without messing with the circuit won't get you there ... but it's a place to start.

GA18 12AX7 preamp: the pieces in the green box ... left to right .. are the cathode resistor, cathode resistor bypass cap, and the power dropping resistor. The cathode resistor (between pin 3 and ground and pin 8 and ground of either tube type) sets the idle current in the tube stage. The bypass cap effects the gain of the section and is frequency selective ... the larger the cap, the bassier or woodier the tone .. the smaller the cap, the 'gainier' or more 'trebly' the tone. The dropping resistor sets the voltage at the plate; in the case of the GA18, it drops the raw supply from approx. 245 volts to +/- 125 volts.

If you want a 6V6 'hi fi' amp to behave like a 6V6 guitar amp, then it has to be wired more or less like a 6V6 guitar amp; I mean that if you left the 6CG7s in there and wired the rest of the circuit like a GA18, you wouldn't have to change the preamp tubes (and maybe their heater wiring) at all. If the amp handles phono, then there's an RIAA circuit hanging on the first preamp tube somewhere. If you were to try and get a guitar amp out of this, you'd probably want to bypass the first preamp tube and cut up one of those other inputs (or see if you can find a male 1/4" to RCA adapter); tape ... whatever ... and you be looking at a circuit with an input resistor, one gain stage, tone controls, and then the phase inverter/driver. I'd also expect to see relatively large uf coupling caps between the PI and the output tubes; stereo gear ... and clean amps like your Guild, are notorious for large value caps; .15uf in Guilds and up to .5uf in lots of stereo gear; .01uf in Silvertones and .02uf in Gibsons and Fenders.

Getting the amp turned in your direction depends mostly on how much Indiana Jones you have and how much soldering-slinging you're willing to do. Good luck!
 
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