Blumlein's Garter Bias question

jkeny · 24th August 2008, 10:03 PM

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 · 24th August 2008, 11:53 PM

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, 01:19 AM

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, 01:52 AM

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

jkeny · 25th August 2008, 01:55 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, 02:11 AM

I may well try Shoog's variation on this biasing seen here: http://www.diyaudio.com/forums/attac...amp=1190309883

kenpeter · 25th August 2008, 06:17 AM

R14 1M keep that series pair of 1000uF (electolytics I would
assume) bridged across the cathodes with a safe DC bias?

24th August 2008, 10:03 PM	#1
jkeny Banned Join Date: Feb 2007 Location: Dublin	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?

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24th August 2008, 11:53 PM	#2
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	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.

25th August 2008, 01:52 AM	#4
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	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

25th August 2008, 01:55 AM	#5
jkeny Banned Join Date: Feb 2007 Location: Dublin	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?

25th August 2008, 02:11 AM	#6
jkeny Banned Join Date: Feb 2007 Location: Dublin	I may well try Shoog's variation on this biasing seen here: http://www.diyaudio.com/forums/attac...amp=1190309883

25th August 2008, 06:17 AM	#7
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	R14 1M keep that series pair of 1000uF (electolytics I would assume) bridged across the cathodes with a safe DC bias?