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Scratch building a RCA SP20, help with transformer and OPT needs.

Ken Boyd

Ken Boyd

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Morning AK members. I want to try my hand at building an amp just by the drawings, and I want to try to build a small PP amp using the RCA SP20 drawings. I think sourcing all the parts shouldn't be a problem with the exception of the needed power and OPT trannies, I am not real comforatable with that. I don't mind using used off eBay like Tango or other good brands I just don't know exactly what the requirements would be.

Anyone want to make some recommendations?
 
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No specific recs, but here are the specs from Sams as reference.

1783602411912.png

I will admit to an amount of laziness here and might consider trying to find these specific units if they are available for not-stupid money, or at least the datasheets for more detailed specs to shop for.
 
gadget73 said:
No specific recs, but here are the specs from Sams as reference.

View attachment 3797887

I will admit to an amount of laziness here and might consider trying to find these specific units if they are available for not-stupid money, or at least the datasheets for more detailed specs to shop for.
Click to expand...
Thats good, I think I can figure it out with this. I think those tango's will work as I see they have a switchable primary on the OPT and one of them is 5K ohms.
 
The circuit is interesting

1783604615689.png

No global feedback. Its using local feedback from output tube plate to 6AU6 screen.

Should make the output transformer selection less fussy since there won't be a bunch of feedback re-tuning to do, though you'll want something good here. The only obvious frequency tailoring I see is the cap+resistor across the first 12au7 cathode resistor, and that will be providing some HF boost. Can tweak those parts as needed if its got a rising or falling response with different transformers.
 
I've heard these quite a bit; they sound great. Not that you asked ;-) but, interestingly, the SP-10 sounds perhaps even better. The Hot Club of New York has used both of them (mono) with an LC-1 and has switched permanently to the SP-10. (It might also be the synergy of using period RCA components together, however...)
 
SP10 looks like basically the same amp with 2 output tubes vs 4. I wonder if thats just an issue of the parallel output tubes. they really need to be well matched in order for that to behave correctly.
 
gadget73 said:
SP10 looks like basically the same amp with 2 output tubes vs 4. I wonder if thats just an issue of the parallel output tubes. they really need to be well matched in order for that to behave correctly.
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Yes, good point; it could just be the reduced power required to drive the LC-1 that makes it sound better...

Does the SP-20 also have double rectifiers? I forget, now...

Edit: Nope.
 
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Ken Boyd said:
Would something like these work?
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Isn't the SP20 a push-pull amplifier? Those transformers are SE.
Ken Boyd said:
They use a 12Au7 input phase splitter, 2 6Au6 voltage, 2 6V6 output and 1 5U4G for the rectifier.
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The schematic shows four 6V6 output tubes. It's parallel push-pull. Primary impedance between 4K and 5K is appropriate.

Jack
 
BassKulcha said:
Yes, good point; it could just be the reduced power required to drive the LC-1 that makes it sound better...
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wouldn't really be less drive power, though the load from the grid isn't zero. Its not very much though. If the tubes don't perfectly match on each "side" of the transformer you get some extra stuff from the two tubes flowing slightly different amounts of current at any given moment.

possible it would do better with a 6L6 but maybe not. I would also wonder about drive capability of the phase inverter, the voltage amp stage seems biased in a strange way for a 12au7 operating as a voltage amp, and the splitter also seems a bit oddly biased. A 6L6 would need more drive voltage so that may need altering.

at some point it turns into a "ship of Theseus" deal, if you change everything do you still have what it was?
 
I built an SP10 clone last year. I had some PC boards for the bulk of the passive components which was designed by an AK'er. I used a pair of Heyboer 8k PP outputs. The original OPT's for the SP10 were specified as 10k. There are many 5k choices available which should work great for the SP20. There are good threads about these amps on DIYaudio.

I had one issue with mine that I didn't expect since there is no global feedback in this amp. That was a HF oscillation which I quelled with a shelf network or zobel across the 6AU6 plate loads. That settled it but probably caused a bit of HF loss. I also substituted 6SN7 for the 12AU7. Actually the power transformer I used only had a 12.6V filament winding so I used all 12V tubes.

Here is a good AK thread
elpulpo88

Thread 'RCA SP-10 Restoration'

So recently I’ve been falling further down the rabbit hole of vintage hifi. What a tragedy! I’ve been hooked on the sound of simple, high efficiency speakers, idler drive turntables, and tube based electronics. My main amp for a while has been a Primaluna Dialogue, but I’ve been hunting for some vintage monoblocks.

the other day I came across a listing for a really clean pair of RCA SP-10 amplifiers. What little I could find on these amps got me very interested: designed by Harry Olson’s team in Camden, overbuilt transformers, 1/2% tolerance parts in the signal path. These were originally...
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And a good thread on HiFiHaven
hifihaven.org

RCA SP Series amps clones?

Hello Hifi Haven! I am a newbie here, but some of you may know me from AK, Instagram (Retrohifiguy), or the Facebook groups. My name is Jacob W. and I am a tube enthusiast/repair tech in the North GA. area. I have been a longtime lurker, but finally joined at the suggestion of my good friend...
hifihaven.org hifihaven.org
 
TriodeLuvr said:
Isn't the SP20 a push-pull amplifier? Those transformers are SE.

The schematic shows four 6V6 output tubes. It's parallel push-pull. Primary impedance between 4K and 5K is appropriate.

Jack
Click to expand...
Since they don't use any global feedback wouldn't a SE tranformer be OK? The primary impedance on them is 5K if you use the 5 K tap.

I can look for a pair that are for a PP amp.
 
SP-20 is ppparallel, with R-C feedback from the output tubes' plates back to the phase inverter tubes. Superb RCA amps...
 
These look like they might work, but I will keep looking for others that might have better specs.
 
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TriodeLuvr said:
Isn't the SP20 a push-pull amplifier? Those transformers are SE.

The schematic shows four 6V6 output tubes. It's parallel push-pull. Primary impedance between 4K and 5K is appropriate.

Jack
Click to expand...
Oops, yup 4 output, I corrected that.
 
Ken Boyd said:
Since they don't use any global feedback wouldn't a SE tranformer be OK? The primary impedance on them is 5K if you use the 5 K tap.
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No, it doesn't have anything to do with feedback. A single ended transformer simply can't be used for push-pull. The basic configuration is different.
gadget73 said:
possible it would do better with a 6L6
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If we're discussing substitutions, a pair of EL34 could replace the four 6V6 outputs (assuming sufficient heater current is available). I did something similar when I built my guitar amplifier a few years ago. The vintage Peavey output transformer I bought was originally used with four EL84 in parallel push pull. The primary impedance is about 4K, and that worked great with a pair of EL34. The EL34s are also easier to drive due to their higher gain. Have to say, I wouldn't even consider building a stereo PPP amp that requires two matched quads of power tubes. It just doesn't make economic sense, and there's no sonic benefit compared to using a pair of larger tubes in each channel. The 6L6s you mentioned could also be used of course with the appropriate primary impedance.
nerdorama said:
I had one issue with mine that I didn't expect since there is no global feedback in this amp.
Click to expand...
The feedback in this design - and many others that don't connect to the OPT secondary directly - is more "global" than it might appear. The primary and secondary of the transformer are tightly coupled, and this means all the issues we normally deal with at the output, including transformer phase shift and resonance, also appear at the primary. I have to assume a 10kHz square wave test would confirm this.

Jack
 
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