a single output transformerstereo tube amp ??

[David Nebenzahl]

[email protected] spake thus:

I have never seen such a design, I didn't see a link to the pic, so I
am flying by the seat of my pants here.

First of all such a thing might be stereo, or hi fidelity, but not
both. To explain, my definition of stereo is that there are two
seperate, discreet and identical channels. Perhaps is is very high
fidelity when used as a mono amp, but actually was capable of stereo.

Using a large main output transformer for the mono signal, typical push
pull, but the if the primaries are split up and phased just right I can
see how this can be accomplished. Of course then you have the
secondaries of the small transformer in series with the secondaries of
the big transformer. The way I see it, the arraingment of the primaries
is important, otherwise you get alot of second harmonic distortion, IMO
an unacceptable level.

The only way this can work is to feed L+R to the 6L6s in differential
mode, and the L-R in common mode. By splitting the primaries so that
the differential mode current is bucked effectively in the primaries of
the secondary output transformer, and the common mode current is
doubled, acceptable performance could be achieved IMO.

Nope. No way could it work.

What you say is correct, I'm guessing, for what goes on on the *primary*
side of the xfmr. But on the secondary side, as someone else pointed out
here, there's no way the split secondary windings are going to be able
to selectively pick up only that part of the magnetic flux intended for
that channel. In other words, the secondary can only act as one winding
(albeit one split in half), so, as someone also pointed out, what you
end up with is a stereo amp with crosstalk: 100% crosstalk.


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you're curious but don't really care about the quality of your knowledge.

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[Boborann]

The Way I see it is there is a stereop cartridge where each channel goes
through two stages of 12aX7 amplification and the each to its own 6L6 The
output transformer is CT with one speaked on one side of the CT in the box
The second speaker ( remote ) on the second side of the CT
There is even a stereo /Mono switch when when in Mono mode ties both sides
of the cartridge together nd adds a cap across teh cathode biasing resistor
of teh first amplifier stage
I will put a scope on it tomorrow and see how teh outputs are biased as I
assume teh6L6's are running class A
BOb

 

[Boborann]

Schematic posted to alt.binaries.schematics.electronic

BOb

 

[Homer J Simpson]

Nope. No way could it work.

What you say is correct, I'm guessing, for what goes on on the *primary*
side of the xfmr. But on the secondary side, as someone else pointed out
here, there's no way the split secondary windings are going to be able to
selectively pick up only that part of the magnetic flux intended for that
channel. In other words, the secondary can only act as one winding (albeit
one split in half), so, as someone also pointed out, what you end up with
is a stereo amp with crosstalk: 100% crosstalk.

No, I believe it would work OK and the proof is in the pudding.

 

[Jim Land]

The Way I see it is there is a stereop cartridge where each channel
goes through two stages of 12aX7 amplification and the each to its own
6L6 The output transformer is CT with one speaked on one side of the
CT in the box The second speaker ( remote ) on the second side of the
CT There is even a stereo /Mono switch when when in Mono mode ties
both sides of the cartridge together nd adds a cap across teh cathode
biasing resistor of teh first amplifier stage
I will put a scope on it tomorrow and see how teh outputs are biased
as I assume teh6L6's are running class A

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.

 

[David Nebenzahl]

Jim Land spake thus:

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.

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.


--
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.

- Matthew White's WikiWatch (http://users.erols.com/mwhite28/wikiwoo.htm)

 

[Homer J Simpson]

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.

 

[lj_robins]

I bought a Columbia680 stereo today on Craigs list which I assumed was mono
as it had one output transformer and a pair of 6L6's and a mono tuner input
When I tore it down and found the schematic I found that it was in fact
stereo and used a single 6L6 for each channel and they shared an output
transformer .
What looked like a traditional Push Pull design is actually fed with a
channel per 6L6 and in phase .
The output winding is Center tapped and has a speaker attached to each side
of the winding.
Curious whather anyone has ever seen this and could comment on how common it
was
Thanks
Bob

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

 

[Homer J Simpson]

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

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.

That little transformer that is hooked to the primary side's center tap
does NOT make it stereo

Does too.

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.
Nope.

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.

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.

 

[[email protected]]

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

 

[Homer J Simpson]

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

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.

 

[[email protected]]

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

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

JURB

 

[David Nebenzahl]

[email protected] spake thus:

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

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

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.

- Matthew White's WikiWatch (http://users.erols.com/mwhite28/wikiwoo.htm)

 

[Dave Plowman (News)]

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.

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

 

[[email protected]]

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

not that I saw anywhere.

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.

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


NT

 

[Boborann]

Thanks for the heads up I did'nt realize that how it worked and will post to
a binaries group ( non Porno) in the future
BOb

 

[Homer J Simpson]

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

Others won't let you post html or binaries to a non binary group - and won't
let you post html to a binaries group even when it makes sense. Assholes.

 

[James Sweet]

Thanks for the heads up I did'nt realize that how it worked and will post to
a binaries group ( non Porno) in the future

Could someone email me the schematic? I'd like to have a look at it.

 

[Meat Plow]

Could someone email me the schematic? I'd like to have a look at it.

Ok check your mail, don't share my address.

 

[lj_robins]

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.

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