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Modified
Broskie Autobias Circuit

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Topic: Modified Broskie Autobias Circuit  (Read 2761 times)

Geek

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Modified
Broskie Autobias Circuit

« on: November 18,
2005, 14:50:31 »

I'm so dang proud of the Muntzing and
tweak I came up with tonight, if I were vain I'd call it
the "Broskie-van der Sluys Autobias"

As a continuation of this thread, I pulled a little 4558
off a board laying around the shop and went to
work.

So, what does it do? It's actually three
circuits in one - DC servo, Broskie bias compensator and
foldover circuit (think of "runaway" brakes, like on a
train).

DC servo. The 1/2 of the 4558 that
feeds the base of the A1380 shunt-regulator is the heart
of the entire kaboodle. By itself, it's a DC servo. The
"-" input (because of the transistor, the inputs are
inverted) is fed with reference, through a 220K
resistor. The "+" is fed with sense voltage, developed
across the 10 ohm cathode resistor. The opamp tries to
keep its differential inputs equalized and as a
byproduct of that, we get a constant cathode
current.

Broskie bias compensator. As John
Broskie pointed out in his TubeCad articles here and here, the failure of the typical DC
servo is when the tube input signal drives the bias
toward zero volts and is assymetrical, the tube starts
coming out of class-A, due to the audio appearing across
the cathode resistor that confuses the servo, making it
push the bias more negative. With a seriously
assymetrical waveform (what music is a sinewave
anyway?), the servo can push the tube into bad
non-linear conduction. Result, even before you are
*near* and not *at* 0V on g1, you have
terrible sounding music.

What Broskie figued was
that a DC servo worked quite excellent as long as the
grid didn't conduct and the tube stayed in class-A. But
as the tube approached the borderline, the circuit
should up the bias to keep the tube class-A. It was a
very simple task to accomplish - toss a diode in there
so it turned on as the point was reached. That's what
the 1N4148 in series with the 220K resistor does. As the
tube starts developing odd audio-related votages across
the cathode resistor, the diode contucts and "assists"
the reference voltage, effectively increasing the bias,
keeping the tube in the class-A area with large input
signals.

This brings us to those two 0.1uF
polycaps... the crossover/recovery time must be as
balanced as possible from DC servo to bias-compensator,
unless you want to introduce some distortion (it's a
subsonic bass pulse, your woofer will hate you if you
get it wrong ). Make the values too large and the circuit
will be too slow on transients. And on recovery, the
tube current shoots up for too long. Too small of values
and audio will leak into the bias circuit. Since when
this part of the circuit is active, the two 220K
resistors will be acting in parallel, hence keeping the
time constant equal with the 100K/0.1uF on the sense
line.

The Broskie circuit does a wonderful job
and can actually increase the output power of an
amplifier. Its flaw though, is it doesn't know when to
quit! If you keep pouring more signal on the input,
it'll keep raising the bias through the tube. On my
CV4060/S11E12 with mu-follower driver, as I turned the
function genny to max, the current through the tube was
an insane 130mA with 465V on the anode - twice its 60mA
bias and well over Pd.

Foldover
protection. So, we need a third circuit in here,
which tells the device that "enough is enough" and cut
back on the current. The other section of the 4558 does
just that. There is a divider on the input that sets
this "trip point" as to when to hit the brakes. The
values shown (in my CV4060 circuit) will cut the current
very abruptly at 72mA, where any higher input causes the
current through the tube to fall lower and lower, until
it hits the DC servo mode again, despite the waveform.
Use the variable alternative to tweak the trip point.
This is all simply accomplished as the foldover diode
turns on, it places voltage on the sense input of the
control opamp, making the chip realize there's too much
current flowing and because its voltage will be stronger
because the impedance on that diode is lower (thanks to
the opamp driving it), the Broskie compensation is
overridden.

Caveats. As with anything,
nothing is free. The higher the trip point, the better
the sound at full power and the more power you get (I
get a solid 10.5W sinewave with the 72mA trip point).
The disadvantage is shortened tube life and a power
supply that may moan and whine.

Also, in this
"Muntzed" version, there is some component interaction.
If you change the 220K resistor on the Broskie
compensation diode, your trip point will go all to heck.
But this interaction is one-way... the trip point
resistor doesn't change the Broskie section at
all..

So, I got this all down with one dual
opamp and some peripheral parts. Thanks to the PSRR of
the opamp, a regulated supply isn't needed. I also don't
see why most any dual opamp can't be
used.

Because this is based on the Broskie
circuit, Copyleft dictates it's available for use under
the same conditions as the original - any and all use,
including commercial, for attribution.

Cheers!

Modified-Broskie.png (4.59
KB, 667x443 - viewed 2230 times.)

«
Last Edit: November 18, 2005, 14:52:21 by
Geek »

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Re:
Modified Broskie Autobias Circuit

« Reply #1 on:
November 20, 2005, 09:37:27 »

No comments?

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Thomas H. Drage

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Re:
Modified Broskie Autobias Circuit

« Reply #2 on:
November 20, 2005, 10:01:05 »

I don't even know what I'm looking at,
so thats my excuse

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Jason

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Re:
Modified Broskie Autobias Circuit

« Reply #3 on:
November 20, 2005, 11:07:15 »

I didn''t know what to say. Don't
really understand what's going on anymore

Did you email John Broskie? He might
be interested to see the variation on his
circuit.

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Re:
Modified Broskie Autobias Circuit

« Reply #4 on:
November 20, 2005, 11:18:36 »

Quote
from: Jason on November 20, 2005, 11:07:15

Did you email John Broskie? He might
be interested to see the variation on his
circuit.

Yup!

I dunno if he'll reply
the email He does say in his "contact" guidelines he
doesn't do forums, but he need not post.

Quote

I didn''t know what to say. Don't
really understand what's going on anymore

It's a simplified version of his
articles:
http://www.tubecad.com/2005/May/blog0045.htm
http://www.tubecad.com/2005/May/blog0046.htm

...with
the addition of a foldback to keep the bias from going
so high, the circuit destroys the tube.

Bottom
line - drop in an output tube and forget about it - no
bias adjustments.

The only thing is the circut
works with a "normal" voltage developed across the
cathode resistor of 0.6V. So, with a 10 ohm resistor
works good for 60mA of normal current. For other
currents, just use a different cathode resistor so under
normal conditions, 0.6V is developed across it

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Re:
Modified Broskie Autobias Circuit

« Reply #5 on:
November 20, 2005, 12:06:33 »

Ahhh.... I think I get it now

Very cool. Umm... I mean, hot.

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Re:
Modified Broskie Autobias Circuit

« Reply #6 on:
November 20, 2005, 12:13:05 »

I'll write an article for the main
website, once I put it through its paces, to make sure I
ain't made a lemon

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Re:
Modified Broskie Autobias Circuit

« Reply #7 on:
November 20, 2005, 14:41:38 »

Greg,
When you have
perfected it maybe you could approach Fairchild or
somebody
to encapsulate it into a little 4 leg chip
or, better still, a glass tube thing.
Mustn't frighten the phools with heresy.

? paul

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Yves

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Re:
Modified Broskie Autobias Circuit

« Reply #8 on:
November 20, 2005, 15:30:53 »

Heuuuuuuu?
Yes..., sure..., but..., ahem..., what do
you fear about?

Tube thermal runaway, lack of bias
supply, shorted link cap??
Just a fast fuse should be sufficient and
less prone to failure that all this thermal sensitive
sand?
KISS
? ?Yves.

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Geek

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Re:
Modified Broskie Autobias Circuit

« Reply #9 on:
November 20, 2005, 22:31:11 »

Quote
from: 99 on November 20, 2005, 14:41:38

Greg,
When you have perfected it
maybe you could approach Fairchild or somebody
to
encapsulate it into a little 4 leg chip or, better
still, a glass tube thing.
Mustn't frighten the phools with heresy.

? paul

Yeah, a little glass
drop, like some thermistors

Quote
from: Yves on November 20, 2005, 15:30:53

Heuuuuuuu?
Yes..., sure..., but..., ahem..., what do
you fear about?

Tube thermal runaway, lack of bias
supply, shorted link cap??
Just a fast fuse should be sufficient and
less prone to failure that all this thermal sensitive
sand?
KISS
? ?Yves.

Honestly, I was
looking for just a fool-proof way to drop in an output
tube and go - no bias adjustments

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