Thought this might intrigue...

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From: Boborann
Subject: a single output transformerstereo tube amp ??
Date: Sat, 28 Oct 2006 01:30
Newsgroups: sci.electronics.repair

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

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--
*A bicycle can't stand alone because it's two tyred.*

Dave Plowman London SW
To e-mail, change noise into sound.

"Dave Plowman (News)" ****

From: Boborann

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..


** Never seen one and must be rare as hen's teeth. But the idea is *dead
simple* and is used in all 3 phase transformers - independent windings on
separate limbs of an iron core have only limited magnetic interaction.

BTW: There is NO centre tapped winding - that is a total Phurphy created
by reading the schem literally.

There will be TWO independent primary windings each with overwound
secondaries sharing a singe core ( maybe an E- core ) but on separate limbs.

With two single ended tube stages, the DC idle currents can be arranged to
cancel each other so the core needs no air gap to prevent saturation -
how neat !!

Channel separation will be limited at low audio frequencies where the iron
core will magnetically couple the windings - but should be OK above about
100Hz or so for a good stereo effect.

SET freaks go eat your hearts out.




........ Phil

"Dave Plowman (News)" wrote in message
...
Thought this might intrigue...

Hi Dave.

Fascinating idea:-)

I seem to recall having seen this schematic somewhere. It
may have been in an early issue of Audio Engineering Magazine
(post 1947) by CG McProud, or perhaps in one of the
"When Audio was Young reprints"

Surprising the Chinese haven't come across it:-)

Iain

On Sat, 28 Oct 2006 09:25:31 +0100, "Dave Plowman (News)"
wrote:

Thought this might intrigue...

*********************************

From: Boborann
Subject: a single output transformerstereo tube amp ??
Date: Sat, 28 Oct 2006 01:30
Newsgroups: sci.electronics.repair

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

***********************************

Obviously designed by a telephone engineer, because that is exactly
how the hybrid transformer in a phone works, allowing a separate
signal each way on the wire.

I shudder to think what the sound is like though, because you can only
maintain the necessary balance over a very limited frequency range. I
wonder what exactly it was designed for...

d

--
Pearce Consulting
http://www.pearce.uk.com