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Old 9th December 2005, 10:36 AM   #1
beamnet is offline beamnet  Netherlands
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Default Automated bias

Just a wild idea i have never seen and am wondering if possible:

I have a BIg pushpull amp. With fixed negative bias.

That means once in a while i have to measure and adjust the bias.

Wel, i am lazy. Simple feats should be done by computers or electronics.



Could you set u a bias measuring curcuit that automatically adjusts bias?

Offcoarse it should have a clock of about half an hour to let the amp run warm. Than it should measure the bias.

I know you can't measure when playing, the curent differs with th music. But if you'd filter it well, you can measure the "mean" value

Just for starters

A 1 ohm resistor for measuring bias current. Buffer with opamp as to not interfere with the music.

Filter

Something elecronically

digital resistor in negative bias supply


thoughts?
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Old 9th December 2005, 10:42 AM   #2
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Iwas scared to turn my automatic amp into fixed bias, so i tried this:

http://www.machmat.com/sales/kits/index.htm

it is automatic fixed bias......
not everybody likes it (it seems....) but it did improve my amp a great deal!

greets
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Old 9th December 2005, 10:48 AM   #3
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A buddy was just playing around with some auto-bias stuff..
Have a look at:
http://geek.scorpiorising.ca/GeeK_Zo...p?topic=1728.0

Hardcore Hollow-Head's beware,there's SS in there!
(But it's okay-the silicon is slaving to keep our tubes in order!)
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Old 9th December 2005, 10:48 AM   #4
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You could give a look at www.tubecad.com, where John Broskie has some blogs about automatic bias.

Erik

Hup, er zijn nu al drie Nederlanders op een rij..komen er nog meer bij?
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Old 10th December 2005, 10:36 AM   #5
beamnet is offline beamnet  Netherlands
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I haven't read the whole article, but these solutions look kinda passive. They don't "remember their status, and they could give the tube weird voltages on startup i think...

Bas
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Old 11th December 2005, 12:49 PM   #6
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@beamnet:

ermm... not entirely clear which of the circuits you mean...
but for the AFB...
you have to set it the first time without tubes plugged in...
Then plug 'em in and look at how they adjust, eg. when you set 44ma, at startup they will go to around 49ma for 1/2 second and then they'll settle at 44ma.(measured at voltage reference point)

in my setup the hv starts when the rectifiers start, so the overshoot is when there isn't even any current delivered..
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Old 11th December 2005, 01:50 PM   #7
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Quote:
Originally posted by beamnet
I haven't read the whole article, but these solutions look kinda passive. They don't "remember their status, and they could give the tube weird voltages on startup i think...

Bas
I've tried them and they're fast enough. Something dynamic is what you need. A "remembered" state is counter productive with a device like a tube.
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Old 12th December 2005, 02:24 AM   #8
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Have a look at Norman Koren's The Emperor's New Amplifier, in which he sets the bias automatically in order to keep 4 class AB2 parallel push-pull 6550s in balance. All details are provided.
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