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Old 9th August 2008, 11:33 AM   #1
GordonW is offline GordonW  United States
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Default Positive feedback, improperly drawn screen, or WTF?

Ok. This was pointed out to me- the SAMS schematic for the Magnavox 196 SE EL84 integrated amp has a very strange depiction of the output transformer wiring.

In this drawing, it appears that the screen is wired to a primary tap... but in OPPOSITE polarity to a normal ultralinear arrangement.

Now, by my use of basic math, that would make it POSITIVE feedback on the screen, as opposed to the degree of effective negative feedback that a normal ultralinear tap induces (compared to straight pentode mode) on the plate of the tube?

Someone please tell me if I'm wrong... I've been claimed to be so, on the MagnavoxFriends Yahoo group, but I'm not at all convinced. Not when I can readily generate math that shows that my conclusion seems to be consistent with basic principles...

I've attached a pic, showing an excerpt from the Magnavox 196 schematic, compared to an excerpt from a more normal ultralinear wiring scheme, for comparison.

I'm convinced that the Sams schematic for the Magnavox 196 is just drawn wrong (which would be far from the first time THAT happened, FWIW)... someone please tell me if I'm off-base on this!

Thanks!

Regards,
Gordon.
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Old 9th August 2008, 12:34 PM   #2
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Oh yes, the old hum-canceling connection. A small bit of the power supply ripple at the plate is fed to the screen in reverse phase, canceling it out. It was cheaper for a volume manufacturer to add a tap on the transformer than to add another electrolytic.
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Old 9th August 2008, 07:32 PM   #3
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Quote:
Originally posted by Tom Bavis
Oh yes, the old hum-canceling connection. A small bit of the power supply ripple at the plate is fed to the screen in reverse phase, canceling it out. It was cheaper for a volume manufacturer to add a tap on the transformer than to add another electrolytic.
That is a pretty neat cost reduction...

What ratio is it? so to say, how close is the B+ connection to being a CT? If it is 10-15% I would be quite tempted to swap it and see how low ratio U-L works( or for that matter regrdless of the ratio ).
cheers,
Douglas
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Old 9th August 2008, 07:35 PM   #4
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Quote:
Originally posted by Bandersnatch

What ratio is it? so to say, how close is the B+ connection to being a CT? If it is 10-15% I would be quite tempted to swap it and see how low ratio U-L works( or for that matter regrdless of the ratio ).

The 7 ohm DCR of that half of the coil suggests it's a pretty small %, given the 333 ohm DCR of the rest of the primary.
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