RCA SA 354 PA.

[DummyLoad]

got this thing on a trade and it's in really nice condition. power tubes are at end of life but i do have some spares. there is a schematic affixed to the inside of the bottom plate: i took a photo of it and converted it to a PDF.

can't find much on these on the web: a few pics but no schematic. there is a link to the PDF i created below. i'll notify doug and request he add it to the archives.

https://www.dropbox.com/s/6851jnwusbfnu1i/RCA-SA354-PA.pdf?dl=0

ckt/ is 2x 7025 for 4 input channels preamps with four MI-12399 lo-z balanced input transformers, 4 mic volume wired to 6AU6 summing amp/james tone stack driver, 6C4 for tone stack recovery & master volume, 6SL7 split as gain stage coupled to a self-biased cathodyne. 7027 P-P outputs are running in self-biased with around 30V on the cathodes. B+ is ~400V with a 5U4GB rectifier pushing a 5H choke load. haven't fired it up as the power cord is totally frayed. all the transformers test good. i'll probably flip the four input matching transformers on fleabay. they make great phono input matching transformers.

RCA 7027 datasheets show a cathode-bias conditions with 36W output with 380V B+ - i guess that where the model # comes from 35W with 4 channels, e.g .SA-354. 

was thinking 5f6a type mods - at least that seems like the easiest path:
1) bypass the input x-formers.
2) install 1/4" jacks in first two MIC pot holes .
3) bond the 7025 triodes & ditch the grid-leak bias in favor of cathode bias.
4) rewire one 7025 pair as the "bright" channel the other 7025 pair the "dark".
5) discard the phono input and MIC/MAG EQ. ckts.
6) rewire NFB loop as "presence" control.
7) optional: rewire 6SL7 as LTPI as in 5f6a ckt.

the channel count is reduced to two. the phono volume pot becomes presence knob. the MV stays since it's outside the NFB loop. so the front panel would be MIC 1 & MIC 2 are 1/4" input jacks MIC4 and MIC4 pots are the channel mix volume pots, phono pot is presence, bass, treble, master vol/power switch. i would wire input jacks so that plugging into just ch1 bonds channels and plugging into channel 2 splits the channels.

link to some hi-res shots. amp pr0n guys: have a peek under her dress.

https://www.dropbox.com/sh/2qgcgtr79rd6ulg/AADUVErrM8-0NYwiS8ktqYyza?dl=0

--pete

[trobbins]

and it's in really nice condition.

Did that come from a museum - top notch.  Great start for an amp.  You may even get in to listening to LP's again with those mic transformers.

[eleventeen]

That looks simply great. Early-mid 60's gear is fun and my favorite. I'll bet the RCA engineers thought it would be neat to use a 6U8 for V3 & V4 but the accountants probably said "but we have 25 acre-feet of 6C4 and 6AU6 sitting in Joisey City! (I grew up in NJ and once went on a HS school field trip to the Harrison, NJ plant where they were making toobs.)


One kind of nice thing about modding a PA Amp vs an older semi-classic hi-fi amp (which are obviously more common) is that you *could* want to preserve a hi-fi amp but the chances of your using this PA amp for a PA amp are probably very small. At the same time, you have pots, which means ROOM for pots. Great numbers of rescued hi-fi power amps are awkward when it comes to drilling in a row of pots. Silliest example: Where would you put a row of pots on a Dyna MkIII?

[mresistor]

Wow! That's a really nice PA amp, and in super condition. I like your plans for it as well.   

[DummyLoad]

thanks guys. after cleaning off the dust and mold reveals an amp that it is superlative condition. it had a broken fuse holder and is getting a 3 wire AC cord for now.

i am having second thoughts about cutting it up. i'm considering making up a pair of the hi-z 9pin plugs with a pair of 1/4" phone
 to XLR adapters to bypass the mic transformers to see what it sounds like before taking the dykes to it.

--pete

[eleventeen]

You're being smart not rushing to chop. That's really in nice shape and it's very well built for a piece of commercial, definitely not broadcast-grade gear. RCA broadcast gear from the 50's and 60's was considered the cat's meow and was built to withstand armor piercing rounds fired at it. Super overbuilt. Sync generators requiring 30" of rack space. 


But interestingly (well, for me) while tube afficianados can appreciate the build quality of old RCA bdcst gear (http://www.pharis-video.com/p4627.htm this is actually a guy I knew when I worked in LA in TV Land) that stuff is insanely useless and enormous and heavy while an old RCA PA amp built like they built their TV sets is heavy but incredibly useful!

With that in mind, if it were me, I'd snip the wire between the entire preamp and the PI input at the top of 250K R26 and call that point "power amp in", and then breadboard my dream preamp on an external chassis using the 6SL7 > 7027 output section as guinea pig. (also useful if you are into building preamps) That way you can drill and punch and solder away to your heart's content without bodging up your neat amp. Pull out B+ & fil on an umbilical cord and you're done. Plus you fiddle with a much smaller chassis (eg; and external preamp box/breadboard) Just a suggestion. 

Even assuming your arrival at the ideal guitar amp design, I would still look to build in final form my preamp on a small external chassis or box that can fit within the existing cage. Rather than drill and hack. So that it can bolted in and connected with 6-7-8 wires. You have room to do this because your cage is tall, for octal tubes.


And if you perfect such a thing or get it close, then you can duplicate it for the next tube amp find. Fun.

I am sort of doing this right now with some amps I am building but external events have shoved any amp building to the side for the time being.