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push pull bias servo

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8th May 2013, 10:55 PM

#21

centraltexastubeguy

diyAudio Member

Join Date: Mar 2013

Location: Central Texas

Gentle Folks: Just got back from OTC in
Houston (THE trade show for the Oil Patch, that which feeds the family and
these indulgences) and have a little time to read and post.

All
very informative, thanks! It appears to me that the Menno circuit on Post
17 senses current movement about a level set by the reference, which works
great for Class A, and makes use of the diode clamps around the
buffer/integrator to force the sampling to be within Class A excursions
even when operating at AB. Should work though I would expect some
sloppiness is possible as the tube transitions from A to AB until it's
deep into AB. I'd love to hear actual performance reports. If
satisfactory, great, otherwise I have a paper design which uses a
comparator instead of diode clipping for greater precision, independent of
signal level.

But as stated on another post in this threas, I have
no desire to reinvent the wheel, if the current circuit work well, I'll be
the first one to plagiarize and incorporate to my own.

Y'all please
publish away with actual first hand testimonials, we'll all
benefit!

Rene

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9th May 2013, 07:21 AM

#22

M
Gregg

diyAudio Member

Join Date: Jun 2010

Location: UK

Well,

It looks like the Menno circuit
is in production so how different is the production version to the circuit
we have in the thread?

Tube bias control

Regards
M. Gregg

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the sound of one hand clapping?

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12th May 2013, 11:35 PM

#23

gingertube

diyAudio Member

Join Date: Oct 2004

Location: Adelaide South Oz

The usual way to cope with Class AB is to
clip the current monitoring signal at 2 x bias level to balance the fact
that it cuts off for part of the cycle. The Menno scheme works in a
similar fashion but effectively clips the signal in a much narrower window
about the bias point. The commercial offering has 4 of the posted circuit
plus power supplies (including the bias supply).
Cheers,
Ian

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5th August 2013, 07:34 PM

#24

LinuksGuru

diyAudio Member

Join Date: Aug 2008

Quote:

Originally Posted by SpreadSpectrum

Parts are 0805, here is a schematic
and layout. Dimensions are like 1.5" x
2"

Thanks for the schematics. Looks
like its somewhat different from original Broskie circuit. Can you please
explain your mods (apart from replacing FET with high-voltage mosfet)?

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5th August 2013, 07:41 PM

#25

LinuksGuru

diyAudio Member

Join Date: Aug 2008

There is another mod (made by GeeK) published
here:

Modified Broskie Autobias Circuit

Like Broskie
circuit, it is published under Copyleft license.

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6th August 2013, 08:35 PM

#26

Ketje

diyAudio Member

Join Date: Dec 2012

Location: Flanders

To keep it simple,perhaps like this.Just a
suggestion,didn't try it.
Other tube in PP same circuit.Since the Vbe
of the transistor is the reference,best glue the two together for the same
temperature on both sides.For another baiscurrent change the cathodeR for
0.6V at the desired current.
Mona

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