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Old 22nd October 2007, 07:55 PM   #1
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Default Cathode Feedback How-To?

I link to this image in a recent thread. I'd like to try and implement cathode feeback in a modest pair of Hammond output transformers I purchased from eBay.

Click the image to open in full size.

The thing is, I don't have the faintest clue how to work these extra windings. Is there a certain web page or book that might help? I just have Morgan Jones' Valve Amplifiers and will give it a look pronto. Should I just go with the DCR's of each winding and add any extra resistance I need in order to bias the tubes? Should I wire the CFB windings criss-crossed between tubes? I'm not even sure I can figure out which way is which with the wires, anyway...

Thanks for any help.
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Old 22nd October 2007, 11:44 PM   #2
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So, you finally noticed the tertiary winding. Good!

Your diagram tells you how to connect things up. Use A1 on the plate of 1 tube and K1 on the cathode of that tube. Wire the 2nd tube to A2 and K2. Combination bias is (IMO) the way to go. Use a 100 Ohm resistor in parallel with 330 muF. between the CT (black) of the tertiary winding and ground. A negative supply with 1 idle current set pot./channel allows experimentation with the operating point.

The connection of the tertiary winding to the cathodes provides local NFB. It's an excellent way of improving both linearity and damping factor in pentode mode "finals". Of course, TANSTAAFL applies and a higher drive voltage is necessary. A few dB. of loop NFB will polish things off nicely.

6V6 family "finals" with the screen grid B+ provided by a VR150 seem OK. For starters, set the idle current at 35 mA./tube and the plate to cathode differential at 300 V. You must protect the O/P trafo cores from saturation by rolling infrasonic garbage off at the amp's I/Ps. That 15 W. "iron" is going to be pushed to its limits.
Eli D.
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Old 23rd October 2007, 12:54 AM   #3
KSTR is offline KSTR  Germany
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Old 23rd October 2007, 02:25 AM   #4
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Default Re: Cathode Feedback How-To?

Originally posted by sorenj07
The thing is, I don't have the faintest clue how to work these extra windings.
January 20, 2009: Bush's last day
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Old 23rd October 2007, 05:04 AM   #5
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thanks for all the info
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