a single output transformerstereo tube amp ??

"Homer J Simpson" wrote in news:c2e2h.39876$P7.11279@edtnps89:

Can anyone dig up the name of the engineer who came up with this circuit? We ought to have it available for throwing around, when we come across weird circuits.

Here's a higher resolution scan of that schematic.

The way I see it, the transformer connected to the plates of the 6L6s (T1) is the difference channel and the one connected to the center tap (T2) is the sum channel. The 6L6s are operating in single ended class A. There's no phase splitter, so it can't be a PP amp.

If both plates have the same signal on them (a mono signal), they will cancel out in the primary of T1. Since T2 is in the B+ path, it will have the current from both 6L6s passing through it. This will induce a voltage in the secondary of T2 and pass through the secondary of T1 to both speaker outputs in phase.

Now, if the plates of the 6L6s have a 180 degree out of phase signals, the current through T2 will be constant. DC through T2's primary means no signal on its secondary. Since T1's secondary is grounded at the center tap, you'll get equal, but out of phase signals on the two speaker outputs.

Finally, if only the top 6L6 has a signal, it will result in the same signal on the secondaries of both transformers. T2's secondary will be in phase with the signal on the top half of T1's secondary, but out of phase with the signal on the bottom half. This results in a signal only on the black wire from T1.

It looks ok in theory, but it won't save anything unless it's designed assuming there's limited channel difference. Andy Cuffe

snipped-for-privacy@gmail.com

snipped-for-privacy@care2.com wrote in news:1162489513.751736.135760 @h54g2000cwb.googlegroups.com:

really

in

turned

box

little

browser

pop-up

I'm not talking about a javascript pop-up.......

Lord it's no wonder you were having so much trouble with that site when your response to someone trying to help you is a snotty "ah... my browser isnt popup vulnerable.".

Hi,

I was under the assumption that the "External Speaker" was an option not a permanent part of the circuit, going on that, YES this is stereo. Guess that is what I get for assuming something.

Not a design I particularly like, had it been me I would have used four

6L6's and two output transformers.

-Landon

I bet he designs software these days - probably for Microsloth.

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+ Required crap appended to avoid restrictions imposed by brain   +
+ damaged idiots. 
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+ Server Response: '441 Posting Failed (Rejected by POST filter)', +
+ Port: 119, Secure(SSL): No, Server Error: 441, 
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+ Error Number: 0x800CCCA9 
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[snip]

Here are some comments from amplifier group....

mykeymykey wrote ..

------- sounds like a cool amp, can you post us a link to the schematic? this type of output was class A, about 12 watts per channel and popular among budget hi fi buffs in the 50s-60s. the output tranny was built like this just to save money and space. its not very unusual but its uncommon to find one in good condition that works. But yes tube amps and questions about them are always welcome here.

po excuse 4 me wrote ............

---------- Yeah, it's an old trick to get low $ 'stereo' when 'stereo' only ment 2 speakers, 1 L, 1 r.. I love the way the '3rd' tranz works the OT..Hate to see what it would cost today against $$$ design..hehehehe.

It's almost like old McIntosh tube amps with weird NFB based OT. I got from Ned years ago, a donut OT with a 'screen' tap. NOT a tube screen, but an xtra winding BETWEEN the pri/sec, that can be...controled. Also multi-UL taps, just a cool hunk of tranz.

Ned wrote .......

---------- It's a cheap (but clever) piece of shit, like the rest of the stuff Columbia (apparently) gave away as premiums with record-club memberships.

OK, think about this. Having a single output transformer is cheaper and means no core gap. If one 6L6 is amplifying something the other isn't, the signal appears out of phase on the opposite channel speaker. Feed that back to the input tube of the opposite channel, it cancels itself out, mostly. Not perfect stereo separation, but it's not intended to be, it's intended to give "stereo effect" at the lowest possible cost.

flipper wrote .........

----------- I'm not real familiar with the topology but it looks like an old matrix stereo amp, a technique that was experimented with back in the

1950's with the idea being 'cheap', er 'economical'. You get a 'stereo' amp from not much more than a PP 'mono'. Saves iron in the OPT even though still SE.

I say 'looks like' because the matrix circuits I've seen inverts one channel, and that one doesn't, so maybe it's either a variation or an even cheaper attempt at a similar concept.

In the ones I've seen it's analogous to analog stereo radio multiplexing the L and R channels for transmission and then demuxing in the receiver. In the 'amplifier stereo' case you have R and -L (the inversion on the ones I've seen) amplified through the amp and then summed (subtracted, actually, since on opposite sides of the CT) at the OPT primary for R+L (this, btw, is 'normal' P-P polarity). Since the OPT center tap is a common mode null it is R-L, ah hah!, so if you then feed that R-L back (the purpose of the second transformer on the CTs) into the main OPT you subtract it out (or add it in, take your pick), opposite polarity on each side of the CT. So on one side you have R+L-(R-L) for 2L and, conversely, R+L+(R-L) for 2R on the other (if all your components were balanced, flawless, and without parasitic capacitance, leakage inductance, etc)

It's likely that the missing 'inversion' on the front end, compared to what I've seen, is taken care of by proper phasing of the OPT and CT feedback but I'm not familiar enough with the topology to be completely sure if it's 'done right' or a dodge.

But, what the heck and winging it here, that amp looks like both are in phase, at the input, so it'll be R-L in the OPT primaries. Now, that sounds bad, like there's 'nothing' with a common mode (mono) signal, but the second (matrix) transformer is on the primary side CT and will have the signal (and idle) current from both sides going through it, in phase, like a double tube SE, so it's R+L. Ah hah! Taking again the simple case of a common mode (mono) signal, the OPT secondaries 'on the ends' would, in a normal P-P amp, null each other (R-L) but the matrix transformer couples the primary CT R+L signal to the output CT 'adding/subtracting' (depending on which side of the CT you're on) it in. So, bingo, we have R-L+(R+L) for 2R, again, and R-L-(R+L) for 2L on the other. Dreadfully crude analysis but it illustrates the basic principle.

From what I've read the matrix amps didn't work terribly well, though (components aren't flawless), with poor separation but did have the interesting characteristic of sounding louder than normal for the same power rating.

The cct diag of this bizarre amp is on this horrid server:

Unfortunately the image quality is so poor that some relevant details are not legible, eg switch & input markings etc. A first look at the speaker wiring suggests this is one of those units where one speaker was in the amp case, and the other was in its own separate box. So at least one can rule out the speakers themselves being wired in mono - unless I've misread something.

I'm now wondering why 2 tweeters per cabinet.

OK, I've found a phase splitter. Where the input is, it goes to a double valve, and below that valve on the diag is a switch. In its upper position, ie not as shown on diag, the switch decouples the cathodes, making the double valve a conventional pair of amplifiers. But in the position the switch is shown, there is no cathode decoupling, and the double valve acts as a long tailed pair. Note also in this posn the grid feed to one is shorted out. So in this position we get traditional push pull operation on the output, ie a mono high power class B amp using the bigger transformer. I havent looked at the smaller output tf yet.

6L6 = KT66, a fairly high power high quality high price valve. So this doesnt look much like a cut price unit. And it still has 2 output transformers, which is no cheaper than having 2 as usual.

OK I see the smaller output tf sees the total B+ current of both output valves, and unbalances the speakers by an amount proportional to this. What I dont know is what the tf ratios are, so dont know if theyre the same or out by a ratio of 10:1.

Lets guess theyre the same ratio, as its the most likely and it makes analysis relatively easy. Where B1, B2 are the anode current in valves 1 and 2:

top speaker output is then B1-B2 + B1+B2 = 2x B1 btm speaker output is then B2-B1 - (B1+B2) = -2x B1

but driving both speakers off one valve's anode current, thus having to run it class A, makes as much sense as stuffing mushrooms. As does running the speakers in antiphase!

And the above equation holds regardless of what the early stages do. I must have made a mistake, as it doesnt make any sense to run an amp that way - but I'm not seeing where.

NT

I'll admit to not knowing why it was produced. Understandable to design such things to try to get an economic advantage, but surely 4x EL84s would have been cheaper than 2x KT66 (6L6), and there would be no crosstalk issue. And it would be simple enough to switch the speakers in series or parallel for mono max power use.

NT

"robb" wrote in news:dEx2h.1528$0r.1398 @newsread1.news.pas.earthlink.net:

The "amplifier group" sounds interesting. Where are they located?

hello there are two that i know of they are guitar amp news groups but they like any amplifier related stuff they can be searched/seen on google-groups if your ISP news server does not list them

alt.guitar.amplifiers alt.guitar.amps

2 @ 4 ohm = 1 @ 8 ohm.

There's a demented logic in it but it would have made more sense if the mono was push pull and the stereo was single ended.

Probably be in the next Borat movie?

obviously, but that isnt a reason to use 2x 4 ohm tweeters instead of

1x 8 ohm.

NT

You've never worked in industry? Generally the 'reason' comes down to $$$$.