Comments for page: A New Look

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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!

Dave

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

Posted by Dave July 11, 2015 - 12:16 pm
Hi Dak -- Great questions! In order then:

1. The data sheets for the LM337 (page-4) will help you with the terminal designations for the regulator chip:

http://pdf.datasheetcatalog.com/datasheet/nationalsemiconductor/DS009067.PDF

2. You don't need to do anything to compensate for your amplifier not having a hum adjustment pot. No current flows through the connection: The output of the regulator is elevated (about 15) volts above ground, and serves to positively bias the heater circuit, just as the original connection in the stock SCA-35 design did. It is merely a convenient place to get the positive low voltage needed to provide the bias. The connection serves no purpose for the EFB(tm) regulator action.

3. You don't need to adjust the EFB regulator for your application, as it will automatically do that for you -- that's precisely what EFB action is: As the B+ is reduced, it automatically reduces the bias voltage to the output tubes to maintain the operating point of the stage. Therefore, it acts to make any fluctuations in the output of the power supply invisible to the output stage.

4. You will in fact need to heatsink the devices, but simply using a (relatively) cool, convenient place on the chassis will suffice. In this application, the devices typically dissipate about 1.6-1.7 watts of heat, which is why the chassis does the job just fine. I recommend that you use thermal compound and insulate the mounting tab from the chassis -- not because it needs to be due to electrical differences in potential, but to eliminate the possibility of ground loops. The mounting tab of the 337 is electrically the same as the INput lead, which itself is grounded in this application.

I hope this helps!

Dave

Posted by Dak July 10, 2015 - 11:53 pm
Hi Dave, I got my amp up and running good so I can replace the cathode bias with your EFB. I have a couple of questions in order to make the EFB applicable in my amp. First I need to verify which legs of TO220 LM337 package corresponds to in, out, and adj? If you were looking at it with the legs in the downside from the front? 2. in your amp you have the "out" going to a hum adjustment pot. I have no hum pot in my amp what do you suggest to install to replace it? Also on the other channel do I need the same thing? I also found that my B+ is 320vdc, do I need to adjust the 360k 2w resistor value to compensate? And finally do I need some heatsinking on the lm337 device to assist in heat dissipation? Best regards, Dak


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