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jkeny 24th August 2008 11:03 PM

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.

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?

kenpeter 25th August 2008 12:53 AM

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.

jkeny 25th August 2008 02:19 AM

2 Attachment(s)
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)

gingertube 25th August 2008 02:52 AM

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.


jkeny 25th August 2008 02:55 AM

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?

jkeny 25th August 2008 03:11 AM

I may well try Shoog's variation on this biasing seen here:

kenpeter 25th August 2008 07:17 AM

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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