Re: a single output transformerstereo tube amp ??

On Fri, 27 Oct 2006 20:30:24 -0400, "Boborann" <trottier@xxxxxxxxxxx>
wrote:

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


Here's a higher resolution scan of that schematic.

http://www.personal.psu.edu/users/a/j/ajc177/stereo1.gif

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

acuffe@xxxxxxxxx
.

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