Comments for page: A New Look

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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:

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!


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

Posted by dak July 05, 2015 - 02:03 am
Thank you for the tips. I finished wiring the input/driver section to the original values so I guess I am going to stick with the 12ax7 driver tubes. I will install the EFB in the next couple of days and hopefully I will be able to fire it up soon after. Thank you for your support. cheers, Dak

Posted by Dave July 04, 2015 - 06:31 pm
Hi Dak -- EFB(tm) can certainly be applied to this design, although to know how much improvement it would provide over the original cathode bias arrangement would depend on the loading conditions offered by the OPT -- and to a certain extent, the power supply's capability as well. In any event, I'd use two separate EFB regulators -- one for each channel, although one regulator could be used for both if a device of greater power rating is used. Either way, you would implement EFB with the same circuit as used in the Dynaco amplifier here.

As for using 12BZ7 tubes, they are basically equal to two 12AX7 tubes in parallel, so the Gm is doubled and plate resistance is halved. This will require altering the value of the plate and cathode resistors accordingly (half the original values) to maintain the same OLG. This in turn will alter the HF characteristics of the design, almost certainly requiring an adjustment to the HF stability components -- although using an alternate OPT from the original design will likely mandate that anyway.

Good luck with your project!


Posted by Dak July 03, 2015 - 06:09 pm
Hi Dave, hope all is well with you and yours. I am writing to see if it is possible to use your EFB design on a Croft Series 5 "clone". here is a link to the schematic
I have done most of the wiring and layout of the amp which uses Hammond transformers and a dual rectifier. I have not dialed in the B+ yet but I am shooting for 380vdc. I also wanted to use a 12bz7 for the driver tubes and wondered if there was any need to change the driver / splitter components to use that tube. Thank you for help, best regards, Dak

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