Comments for page: A New Look

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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....

Posted by Dave September 28, 2014 - 07:29 am
Hi Stephen -- Congrats on your successful installation of EFB! Your perceptions have been shared by virtually everyone who has installed this modification in the small Dynaco amplifiers. The improvements are both readily audible, and easily demonstrated in the lab as well.

The conversion to fixed bias operation changes the fundamental operation of the output stage, while EFB action basically makes it immune to any fluctuations in power supply operating voltages. With the improved match this creates between the output tubes and output transformer, more power can now be developed, and at notably less distortion as well.

In addition, with the lower quiescent current your output tubes now operate at, they will last longer, and the unit will operate cooler as well.

Adding EFB to the small Dynaco amplifiers really is one of those few modifications where the results belie the simplicity of the modification, and the improvements gained are had without compromise. Congrats on your success!

Dave


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