Comments for page: A New Look

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Posted by Dak January 05, 2016 - 04:44 pm
Thank you for the recommendation Dave. So, as far as the cathode bias arrangement, I would keep it as designed? Right now it is using a capacitor bypassed resistor per each output tube. cheers, Dak

Posted by Dave January 05, 2016 - 04:02 pm
Hi Dak -- Christmas was great this year -- thanks for asking!

For EL34s, you would want to use an EFB(tm) control grid regulator, and (if necessary) an EFB screen grid regulator. This is because the typical bias that EL34s require is right at the limits that can be placed across a typical 3-Terminal regulator. There are solutions for this, but by the time they are implemented, it would be far easier to simply use an EFB Control Grid regulator in connection with a conventional negative bias supply. The 3-Terminal regulator approach works well with low bias requirement tubes like the 6BQ5 family, 7591 family, and even the 8417, but for higher bias requirement tubes a control grid regulator is the way to go. If you are not familiar with this concept, the AK Fisher forum has some typical examples of how this is implemented.

Good luck with your project!


Posted by Dak January 05, 2016 - 12:31 am
Hello Dave, I hope you and yours had a very merry Christmas and New years. I am thinking of implementing your EFB in cathode bias in a stereo pp EL34 amp. Is there any previous documentation of this that you are aware of that I can peruse? tc&br, Dak

Posted by steve November 18, 2015 - 03:25 pm
Thanks Dave.....i never would have thought the pot. its just a simple turn 5k bias pot. Guess I'll need another one.

Posted by Dave November 14, 2015 - 07:49 pm
Hi Stephen -- It could be, but I rather doubt it. Solid state components typically fail in "short" mode, which in this case would cause an extremely high bias voltage that would not be adjustable. You have a bias voltage that is too low and not adjustable, but within the normal range of the control.
Therefore, I would concentrate on a defective bias pot. If you are using a multiple turn type, it may be stuck on the high end of its travel (the end connected to the 360K 2W resistor). Other than that, there is really nothing else that can cause the circuit to become non-adjustable -- and specifically so within the normal adjustment range of the circuit. Failure of any other component would cause the bias to become non-adjustable, but also well outside of the normal range of adjustment.

I hope this helps!


Posted by stephen zaneteas November 14, 2015 - 05:55 pm
Hi again...just wanted your opinion...Suddenly I can't adjust my bias-it shovering around .245..instead of .270...Could the lm337 be bad?

stephen z

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