a single output transformerstereo tube amp ??

"Boborann" wrote in news:BI6dnRDDT_paWt7YnZ2dnUVZ snipped-for-privacy@comcast.com:

Your analysis is correct, as far as it goes. It's the *second* transformer in the output circuit, connected to the output transformer's primary and secondary, that makes stereo come out of the speakers.

Jim Land spake thus:

Ah, there it is; I missed that the first time I looked at that (crappy) picture of the schematic.

So now I'm wondering: what, if anything, would be the advantage of such a circuit? Again, it looks like they're trying to save a nickel or two on transformers, but there are two anyway: why not just use two standard output transformers? Must have been someone's brainstorm.

The difference signal is small so the extra transformer is much smaller. Also, instead of two full tube push pull amps you have just one. Same design philosophy as Detroit.

Sorry, no, this is not stereo, this is not even a stereo circuit.

This is a standard two 6L6's in push-pull mode circuit, one pushes the up-swing of the sine wave, the other pushes the down-swing of the sine wave, each half of the wave is fed into the opposite side of the transformer.

Regardless of what output tubes they use, nearly every push-pull circuit I have seen has been designed this way.

That little transformer that is hooked to the primary side's center tap does NOT make it stereo, if you follow the circuit backwards from the transformer it leads back to B+ voltage coming out of the rectifier tube. For whatever reason that they decided to do it this way, the center tap of the secondary of the output transformer self modulates the B+ voltage going into that transformer in phase which the signal that is going through the output transformer.

My best guess on this is that it's a poor mans' volume booster to get more volume going to the speakers than the amplifier design would do on it's own.

Secondly, the speaker's are not on separate circuits (this was mentioned in another posting), the two tweeters and woofer are arranged in a series-parallel circuit with a capacitor stuck in there to act as a poor man's cross over circuit to separate the high and low frequencies going to the speakers.

The way that these are arranged is MONO but instead of using SIGNAL and GROUND to wire the speaker(s) to they used the PUSH-PULL of the up and down swings to drive the speakers with.

Looking at the INPUT of this entire circuit there IS something on there that says STEREO, it is probably a phonograph input. The rotary switch that switches between RADIO, STEREO, and MONO, in Radio mode the lower half of the 12AXT tube is grounded out and it only runs in MONO mode, when switched to MONO mode the same thing happens.

But in STEREO mode both halves of the 12AXT are turned on and it also acts as a PUSH-PULL circuit and feels both halves of the signal to the rest of the circuit, which from here on out is in push-pull mode.

Hope this clears up the confusion.

-Landon

Nope.

Does too.

Nope.

That is one of the two speakers.

Go look again. This amp has STEREO input, STEREO volume control, STEREO tone controls and two sets of speakers, left and right.

Maybe its time the OP posted the cct somewhere we can all see, eg tinypic. Not everyone has binaries groups access.

NT

Didn't he do that? He also attached it here (naughty, naughty).

You can analyse it by assuming the two inputs are tied together; and then using one only while tying the other one to ground. First one transformer works, then the other.

I can get binaries, but I don't see a link.

I really would like to see the print for this contraption.

JURB

snipped-for-privacy@aol.com spake thus:

What's up with this? There's no link; look at the OP's 3rd post here. It has a JPG of the schematic attached.

-- Just as McDonald's is where you go when you're hungry but don't really care about the quality of your food, Wikipedia is where you go when you're curious but don't really care about the quality of your knowledge.

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Many ISPs (including mine) strip attachments off text only newsgroup posts. Otherwise their servers would be jammed with HTML files...

not that I saw anywhere.

Unfortunately analysing a circuit without knowing the circuit is one of those skills I never did learn.

NT

Hi,

Yes I will agree with you that it has two speakers, it could have four, eight, or sixteen speakers and still be a mono circuit depending on how the speakers are wired together.

Yes it has stereo inputs, they stay stereo until they get to the 6L6's when it becomes a MONO push-pull circuit.

Take another look at the schematic, the two 6L6's are creating a Push-Pull circuit, one produces the up-swing of the sine wave while the other produces the down swing of the sine wave, then they mix together in the output transfer to form an alternating current (complete) audio sine wave.

Now, take a look at the output of the output transformer, follow the wires going to the speakers (notice that there are only two), one wire goes to the left side of one of the speakers, the other goes to the right hand side of the other speaker.

Here is the important part, the two speakers are tied together right in the middle (series), then the woofer is wired to the left side of the left speaker and the right side of the right speaker.

This is a series-parallel MONO circuit (using Ohms Law this probably comes to 4 or 8 ohms, maybe 16), to make this a true stereo circuit you would need to have at least three wires (two signal wires and a ground).

For a final test, play something through it that you know is STEREO, The Doors and old Pink Floyd are well known for doing weird freaky things with stereo.

If the guitar is on one speaker and the keyboard/organ is on the other on a song that you know plays this way on a stereo they yes this amp is stereo. If the same sound is blended and comes out the same on both speakers it is mono.

-Landon

Try

Don't whine about the server.

Nope. In mono it is a parallel system. Only the stereo signal activates the split transformer which is probably much smaller.

That would require one side to have an inverted signal. Where does that happen?

cant get anything from it. Is it up at specific times only?

NT

Okay, I understand how you could be deceived, but you're WRONG, this is a stereo circuit.

The way it works is:

(1) The SUM of the left and right channels is amplified the usual way, with the output tubes being driven in push-pull, but CLASS A, so the average B+ current does not vary. That is important, as we'll see later on..

(2) The DIFFERENCE is applied to the output tubes IN PHASE. This signal cancels out in the main output transformer, so it doesnt bleed into the main output signal, the SUM.

(3) The difference signal, applied to the outut tubes in phase, shows up as current variations in the output transformer primary. That signal is picked up by the smaller transformer that is in series with the center tap.

So we have in effect TWO separate class A amplifiers. The transformers do the magic of separating the in-phase and out of phase signals.

(4) Then at the secondary of the output transformers, more magic. The secondaries are hooked up so that the difference signal gets added to one half, mathematically (L+R) + (L-R ) gives 2L, then in the other half, (L+R)-(L-R) gives 2R. Voila! Stereo!

No. You have to step through its annoying process but it's only 30 seconds.

Where does the inversion of one channel take place?

I wish all you gurus would figure this thing out i am intrigued and on the edge of my SEAT !

when does the movie come out ?

BTW isn't this just a version of a Williamson amplifier adding inverse feedback using the transformers for phase inversion to control the distortion

( sorry for blurt , the intrigue forced me to break out my Buchsbaum and Douglas-Young books )