Flea market find

[gui_tarzan]

I went to a hamfest/flea market yesterday and stumbled across a stereo amp that looks like it came out of a console. It's a Columbia "stereophonic" but it is a circuit I've never seen before. I'm sure some of you have so maybe you can enlighten me. It has two 6BQ5s, a 12AX7 and a 6CA4. Close examination tells me the OT is a dual transformer in one, as each side has three wires like the attached schematic has. I can't find the right schematic but this one is very close to the layout of the one I have.

It looks to me like the separate sides run in a sort of push-pull configuration but in separate output circuits. I say "pseudo" PP, because it only uses the one transformer, but electronically it can be separate signal paths, right?

[Chuck H]

I've read that it's possible to create a stereo signal in a single OT with the CT secondary arrangement. I never gave it much thought because, well, how often would we use this for a guitar amp? But the schematic is pretty clear. Has to be strictly class A with that arrangement I expect. And strictly a low enough power tube grid input that the bias doesn't shift at any point. Pretty useless for a guitar amp as it's wired I think. The PI uses grid leak bias!?! You could probably test the ratios and run the formulas for figuring impedance on the different secondary taps and figure out how to wire the OT for mono use in push/pull. It'll probably handle it. The tube compliment is good. Since you have a 6ca4 (probably on the same filament supply?) you can change to a diode rectifier and add another 12ax7 to facilitate a more typical "guitar amp" topology. The voltage is probably somewhat low now because of the careful, strictly clean class A arrangement so the diode rectifier may not cause a high voltage problem. If not you can always drop the voltage with zeners.

Neat design to see and probably a good find for a conversion

[Jazz P Bass]

I think the 'trick' is the secondary coil going to the CT of the output transformer secondary.
That and the NP caps.

I would sell the amp long before I gutted it.

It is probably a really nice sounding low power amp.

[eschertron]

Back in the days of vinyl recordings, the stereo encoding made it difficult to get a phono needle to track phase-variant low frequencies without the needle flying out of the groove
So if the phase-variant (stereo) information is limited to high frequencies (like most live space reverb is) then I'd wager the electrical limitations of the OT are inconsequential when conservatively-mastered vinyl recordings are played back. All bets are off in the digital age!

[gui_tarzan]

I have no intentions of using this as a guitar amp, but I have been wanting to build a tube stereo amp for a while and this might just fit the bill. I don't listen to music up loud anymore, playing in a live band for years has cured me of that.

[gui_tarzan]

I'm drawing out the schematic (in between tv shows) and this actually has one leg of the heater wires going to ground as do two other leads from the PT. There is no fuse, it's definitely not safe as is. There are also a few capacitors that have five leads and I haven't run into these before. Time for a search.

[Enzo]

Probably the mains fuse was elsewhere in the unit this assembly lived in.

Five-legged things are not caps, strictly speaking, they are caps and resistors in a small circuit. AMpeg used them in a number of models, had the tone stack parts in one thing, just wire it to the pots - like your photo shows - for a complete tone stack. Look at Ampeg V4 or VT40 for examples. Look up Ampeg PEC. I think that stands for "printed electronic circuit".

I don't know your TV schedule, but I watch the Untouchables at 4PM, and the Man From Uncle at 4AM. I have a lot of time between shows...

[Mick Bailey]

Back in the days of vinyl recordings, the stereo encoding made it difficult to get a phono needle to track phase-variant low frequencies without the needle flying out of the groove
So if the phase-variant (stereo) information is limited to high frequencies (like most live space reverb is) then I'd wager the electrical limitations of the OT are inconsequential when conservatively-mastered vinyl recordings are played back. All bets are off in the digital age!

The RIAA curve de-emphasizes bass on recording but a preamp specifically for vinyl (pretty much every phono stage has this) re-emphasizes the bass using an inversion of the same curve. So on playback the bass is present as per the original source. You still need good transformers to reproduce the bass properly.

[gui_tarzan]

Here's what I have done so far, I just need to verify the preamp and connections to the power amp so they're not connected.

[Jazz P Bass]

I would redo the drawing for the B+ to the Output Transformer.
The OT should feed directly off of the rectifier.

Not too sure what is up with the 270/ 5 watt resistor.

[gui_tarzan]

Redo it why? That's how it's wired. I looked this thing over and over and over again because it's not what I'm used to seeing. I even put it down for a couple of days and went back just because I wanted to make sure. I followed the wires forward and backward.

Look at the first picture I posted of the underside. B+ comes off the rectifier through the orange wire to the first cap in the can. Then it goes through the cement resistor (270/5w) to the second cap and no other wire comes off that first cap. Unusual? Yep. Why? I have no idea but if you follow the second cap it goes to the first transformer that then goes to the OT.

[Jazz P Bass]

You got me.

The original posted schematic is correct.

Why would you want to limit the B+ to the output tubes?

[gui_tarzan]

I don't know, it seems very odd to me. But I looked it over carefully and everything is original and hasn't been worked on so your guess is as good as mine. Maybe Enzo has some insight?

That schematic is not the one for this amp, it uses different power tubes but it was the closest I could find. This seems to be a very uncommon stereo amp.

[Enzo]

I'm not the circuit guru around here really. I have no issue with the thing as drawn. It simply has an extra stage of filtration before it starts using it. Less ripple hits things. Might be gilding the lily, since push pull stages tend to cancel B+ ripple. But it costs a few volts to get smoother DC.

Look at Champ 5C1 and 5E1 as examples.

[J M Fahey]

II have no issue with the thing as drawn. It simply has an extra stage of filtration before it starts using it. Less ripple hits things. Might be gilding the lily, since push pull stages tend to cancel B+ ripple.

Just thinking aloud:
"push pull stages tend to cancel B+ ripple" : true, they work in a differential way, so whateverīs present on both plates at the same time, such as ripple, gets cancelled out.
But in this funny amp, I *think* it works as a push pull mono amp combining "signals which are the same left and right", typically the bass guitar, often the bass drum, some vocals, (hey, they are already "mono" by definition) , and/or "low frequency signals were mixed to mono to make needle tracking groove easier"
Notice full power reaches a woofer boc, with an added low pass filter.
I wonder why you crossed it out because itīs *essential* for this system.
Mid/high frequencies, which are easier to handle and provide stereo information are differentiated and sent to satellite speakers , notice 100uF NP caps in series with each, that would give as a 200Hz highpass if running 8 ohms speaker.

That said, they are getting differential signals to recover right/left information.
***Maybe*** that re-differentiation kills the hum-cancelling in "normal" push pull and they had annoying hum present in satellites, so they needed super clean +B
Losing a few Volts would not be a big deal.

In any case, the amp looks more like a "proof of concept" job than anuthing else.
Maybe it was designed by an Engineer experienced in disc cutting Electronics and he applied his specialized knowledge here.

I faintly remember some 60īs designs which did that (decoding stereo/mono information hidden in commercial vinyl records) but in the preamp, using some complex decoding system, never saw somebody doing it in such a direct way.