I got the amplifier chassis out of that RCA console that I picked up a couple of months ago. Having been having serial surgeries I haven't had what it takes to get started until now.
Speakers are 4 ohm 8 inch mid/woofs and cone tweeters. The terminal strip is broken off of one woofer but it looks like the whiskers may be intact. It appears that the PO was messing around with the speaker wiring as the caps on the tweets were disconnected and the wiring on the tweeters is a rat's nest. I haven't traced the wires out but I am guessing that they were having some trouble and disabled the tweets. If I have any bad drivers I do have some options laying around.
Tube compliment is 5Y3 Rect, 2-6BQ5 (EL-84), and a 6EU7 preamp tube.
Chassis doesn't look too bad. A few caps and resistors I plan to replace on principle but they look pretty easy to access. There is only one component pack... hope it is OK.
When I first got it I tried to find schemo but no luck as of yet.
Speakers are 4 ohm 8 inch mid/woofs and cone tweeters. The terminal strip is broken off of one woofer but it looks like the whiskers may be intact. It appears that the PO was messing around with the speaker wiring as the caps on the tweets were disconnected and the wiring on the tweeters is a rat's nest. I haven't traced the wires out but I am guessing that they were having some trouble and disabled the tweets. If I have any bad drivers I do have some options laying around.
Tube compliment is 5Y3 Rect, 2-6BQ5 (EL-84), and a 6EU7 preamp tube.
Chassis doesn't look too bad. A few caps and resistors I plan to replace on principle but they look pretty easy to access. There is only one component pack... hope it is OK.
When I first got it I tried to find schemo but no luck as of yet.
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IMO, you should not spend money on a schematic. Just reconstruct to simple triode driving pentode "final". Save some money and switch to a 12AX7, by rewiring the driver socket. The triodes in a 6EU7 and a 12AX7 are "identical".
Don't forget the proper, 3 wire, safety grounded, power cable. BTW, Jim McShane has F&T twin section 32 muF. 'lytics that will be perfect for rebuilding the PSU.
Don't forget the proper, 3 wire, safety grounded, power cable. BTW, Jim McShane has F&T twin section 32 muF. 'lytics that will be perfect for rebuilding the PSU.
Thank you Eli. I am in the process of tracing the wiring right now. I think I will rewire for 12AX7 and use the 6EU7 (if it is good) in a piggy back phono preamp for it. I have a 70s vintage TT with MM cart that will sit in the cabinet just peachy.
I will do some updating on the wiring while I am at it. Nothing is twisted. Not heaters or signal either. No shielded cable either. I would guess that twisting the signal wires should be just as good as shielded cable for runs this short yes?
I can rewire the jack that was used to send 120 AC to the phono to provide B+ for the piggyback phono pre. I was considering using a polarized two wire since there is no exposed metal on the console but I suppose that the new TT might allow connection through the cart somehow so probably best to convert. I will need to make sure the signal ground is separate from the chassis and laid out properly for best hum performance.
I will do some updating on the wiring while I am at it. Nothing is twisted. Not heaters or signal either. No shielded cable either. I would guess that twisting the signal wires should be just as good as shielded cable for runs this short yes?
I can rewire the jack that was used to send 120 AC to the phono to provide B+ for the piggyback phono pre. I was considering using a polarized two wire since there is no exposed metal on the console but I suppose that the new TT might allow connection through the cart somehow so probably best to convert. I will need to make sure the signal ground is separate from the chassis and laid out properly for best hum performance.
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Around 61-63 is my best guess. In some ways it is too simple.
I have traced it out for the most part. The color coding on the PT was a bit strange but I figured it out. But there are some other quirks too.
1. It uses an RCA trick I have seen before. The OPT has a screen tap but it is not UL. It is laid out so that the B+ is in between the screen tap and the plate supply. It is apparently a tricky way to reduce noise so they can cheap out on the PS filtering. A portion of the noise being injected into the screen kind of Akido like. The rectifier output is sent to 35u filter cap and then to the OPT B+ lead. The normal plate lead of course goes to the 6BQ5 plate. The screen tap of the OPTs go through individual resistors to be summed at the second section 80u filter cap and then to the screen and to the VAS without any further filtering.
2. Pentode mode no FB. As near as I can tell there is no FB either local or global. Amazing. I am tempted to start by recapping and replacing a few key resistors to see how it sounds. The next oddity may make adding FB a challenge.
3. The signal from the input selector goes to a rat's nest of a tone control circuit and then to the VAS. The output of the VAS then goes to... the volume control and balance control which then feeds the grids of the 6BQ5s. This puts the volume and balance right smack dab in the middle of where you would be adding FB.
The left and right signals go to their respective volume pots and the output from those pots goes to each end of a single pot with the wiper grounded and then to the respective 6BQ5 grids. On the other side of the volume pots are two additional variable resistances that vary between 113k and 134k IIRC and these are connected both to the component pack and via a resistor to their respective volume pot connections. My guess is some sort of loudness contour.
So a normal global FB loop or even plate to plate would include the volume and loudness circuit in the loop. I don't think that will work. I am not sure whether it might be possible to RC couple the output tube's plate directly back to its grid. What say the experts?
Of course a complete tear out and redesign is a possibility. Another wacky thought I had is that I have a couple of 6GV8 (NOS I think). It would be a little less power but I could put those in place of the 6BQ5s and wire them as an RH style SE pentode mode with plate to plate and the use the 6EU7 socket for the last gain stage (12AX7) of the piggy back MM phono pre discussed in my other thread and either add another socket on the chassis or put the first phono stage on a separate small chassis.
It is a small little thing but would be a great fit for our piano/organ room.
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Really good tone controls require complex circuitry. I suggest you eliminate tone controls, when you rebuild the amp.
No NFB whatsoever is bad. A global loop is likely to cause OEM O/P trafo core saturation.
So, a simple 'X7 section voltage amplifier feeding a full pentode O/P stage, along with Schade (AKA "plate to plate") short loop feedback, is my recommendation.
BTW, investing in new O/P "iron" is (IMO) not a bad idea. Edcor GXSE15-5K trafos would allow "conventional" GNFB circuitry to be used and allow the use of UL mode "finals" too.
No NFB whatsoever is bad. A global loop is likely to cause OEM O/P trafo core saturation.
So, a simple 'X7 section voltage amplifier feeding a full pentode O/P stage, along with Schade (AKA "plate to plate") short loop feedback, is my recommendation.BTW, investing in new O/P "iron" is (IMO) not a bad idea. Edcor GXSE15-5K trafos would allow "conventional" GNFB circuitry to be used and allow the use of UL mode "finals" too.
Really good tone controls require complex circuitry. I suggest you eliminate tone controls, when you rebuild the amp.
No NFB whatsoever is bad. A global loop is likely to cause OEM O/P trafo core saturation.
BTW, investing in new O/P "iron" is (IMO) not a bad idea. Edcor GXSE15-5K trafos would allow "conventional" GNFB circuitry to be used and allow the use of UL mode "finals" too.
A couple of questions. Do you thing that a plate to grid feedback would be possible with the original layout...
x----6BQ5-----x----balance----Volume---12AX7----tone-----<<
|----Rf---Cf----|
Also, is the problem with the passive tone circuits primarily poor response curves or do they degrade other parameters? Not sure I want to mess with adding Bax.
Loop NFB of any kind requires adequate open loop gain. IMO, a single 12AX7 section can't deliver sufficient gain to feed a NFB loop and make up for the tone circuitry losses. If you eliminate all signal shaping elements, coming out ahead is highly likely. Do you want gadgets or do you want music?
You bet! That's where they belong. There is a "gotcha" with 'X7 CMiller and a large total grid to ground resistance. It's HF roll off.
If you find that a ceramic phono cart. (frequently a record eater 
) and/or the tuner need a very light load, cascoded pairs of DN2540N-3G MOSFETs should handle the necessary buffering chore. Cascoding takes reverse transfer capacitance worries out of play.The 6GV8 does not look promising. Figure on a 50% O/P power loss and the triode's gain may be insufficient, unless CCS loading is employed. Remember, the SE 6BQ5s are providing only approx. 5 WPC.
Use speaker level I/Ps on a SS S/W "plate" amp. Don't "daisy chain". Just hang the cabling for the SW in parallel with the main speakers. You will pick up some of the tubed circuitry's sonic signature.
Instead fussing, I suggest you buy Jim Hagerman's Bugle2 as the means by which you switch to a magnetic phono cartridge. FWIW, I suspect that old record changer needs to go too.
Use speaker level I/Ps on a SS S/W "plate" amp. Don't "daisy chain". Just hang the cabling for the SW in parallel with the main speakers. You will pick up some of the tubed circuitry's sonic signature.
Instead fussing, I suggest you buy Jim Hagerman's Bugle2 as the means by which you switch to a magnetic phono cartridge. FWIW, I suspect that old record changer needs to go too.
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