Blumlein's Garter Bias question - diyAudio
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Old 24th August 2008, 10:03 PM   #1
jkeny is offline jkeny  Ireland
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Default Blumlein's Garter Bias question

I'm going to try this biasing method on a PP ECL86 which currently uses 270R bypassed with 470uF caps on each tube.

Click the image to open in full size.

I know I need to change the 200 Rs in the diagram to 270 but how do I determine the correct value for the 100K R?
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Old 24th August 2008, 11:53 PM   #2
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I am not sure if you have given us sufficient information.
What is your B+, how do you have the screens strapped?
Are you intending bias for Class A, Class AB, Class AB2?

If I knew idle current or bias voltage, I could guess the
other from the 270R's. But I can't calculate what is your
intention from knowing just the cathode resistors.
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Old 25th August 2008, 01:19 AM   #3
jkeny is offline jkeny  Ireland
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OK, I forgot to include this - bias is 27mA in class A on a baby huey circuit attached (390R bias R on diagram is actually 270R)
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File Type: jpg ecl86 baby huey def final amp.jpg (73.3 KB, 279 views)
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Old 25th August 2008, 01:52 AM   #4
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jkeny,
This scheme puts half the bias from the other push pull side onto the grid. Thus rather than (approximately) having the grid at 0V DC and the cathode at +14 DC you want +14 DC on the grid and +28V DC on the cathode. So BOTH cathode resistors need to be the same size as you are now using. So use 2 x 270R for each cathode.

The 100K shown need to follow the max Rg1 values for the tube - For ECL86 take those 100K up to 470K.

Cheers,
Ian
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Old 25th August 2008, 01:55 AM   #5
jkeny is offline jkeny  Ireland
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Edited out some text

Thanks for the info on the 470K.

Have you tried this biasing method?

Will the feedback R need to be changed?
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Old 25th August 2008, 02:11 AM   #6
jkeny is offline jkeny  Ireland
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I may well try Shoog's variation on this biasing seen here: http://www.diyaudio.com/forums/attac...amp=1190309883
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Old 25th August 2008, 06:17 AM   #7
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R14 1M keep that series pair of 1000uF (electolytics I would
assume) bridged across the cathodes with a safe DC bias?
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