Comments for page: A New Look

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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!

Dave

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 http://www.southampton.ac.uk/~apm3/diyaudio/Croft5_circuit.jpg
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

Posted by Dave March 03, 2015 - 02:00 pm
Hi Joe -- Thanks for the kind words -- and I'm so glad you got your problem resolved!

As for your tone control bypass modification, it can be place where the current tone controls reside (between the volume and balance controls). The series resistor would go between the wiper of the volume control and the "top" of the balance control, while the shunt resistor would go from the top of the balance control, to ground.

Good luck with your project!

Dave

Posted by Joe Janus March 02, 2015 - 08:15 pm
Hi Dave,
I wanted to let you know, and its taken several months, but I was able to solve my "howling" nfb problem. Thanks much for your on target direction in resolving this. Although I can't say I isolated the problem, I did manage to clear it up by undoing the output binding posts and replacing them with the original output terminal connections along with a few other associated connections from (Z-565, PC-10a and grounds..) Over the last few years I have read many of yours posts and responses on various sites and forums, and I must say you are worth your weight(10x fold) in gold plated, Teflon, paper in oil, silver hazed, etc components to guys like me.

All the best

Joe

now if I could figure out where exactly the series and shunt resistors go (or preferred alternate method) when bypassing the original tone controls, then I could ask myself and my ears if this endeavor was all worth it.... and perhaps just relax and just listen to the music.... the final destination....


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