Comments for page: A New Look

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Posted by dak January 06, 2016 - 01:37 pm
Hi Dave, thank you for the tip. The amp that I wish to modify is a PP EL34 with Dynaco A420 OPTs. The front end is a Williamson type with a 6sn7 to 2 6sn7 phase splitter/drivers. The power supply is LCLC type with a B+ of just over 400vdc. The amp sounds good but I am not happy with the cathode bias situation and would like to try something different other than fixed bias. My feelings are that your EFB design would be an elegant solution to the cumbersome 4 bias resistors and bypass capacitors.
If I decide to use the TL 783, will following your schematic circuit and installation be the correct way to approach this? Or do you think I need to adjust the various component values? Should I just build it and see what kind of voltages I will get? If this works out I will be happy to post my results on AK and here. Thank you so much for your time and consideration, best regards, Dak

Posted by Dave January 06, 2016 - 01:04 pm
Hi Dak -- I have just be been (thankfully) gently "reminded" by Steve that higher voltage 3 Terminal regulator packages do in fact exist, and in the required negative configuration as well. One such example he provided is here:

http://www.irf.com/part/Hi-Rel-Adjustable-Voltage-3-Terminal-Negative-Regulator/_/A~OM7612ST

With a 50 volt maximum differential, it should be safe for use with EL34s, but that's about as far as I'd go with it. Use of larger tubes like the 6550 would stretch the regulator to its limits.

Understand however that using the cathode regulator approach with higher bias tubes (versus using a control grid regulator) means requiring larger amounts of B+ (as would be required if normal cathode bias were used), but it also means greater heat dissipation from the regulator as well. Therefore, proper heatsinking becomes even more important. In any event, it will let you use the EL34s with an EFB cathode regulator. With the proper precautions taken then, it should work just fine, but understand that I personally have not used this particular device, nor any similar device in the application you are proposing.

Good luck with your project!

Dave

Posted by Dak January 06, 2016 - 01:12 am
Ok, so , I need to create a negative bias of appropriate voltage. I was hoping to avoid something like that as my amp's power trans has no bias supply voltage. Is the reason I can not use your EFB design because the SS device is not capable to support the extra current demands of the EL34?

Posted by Dave January 05, 2016 - 06:13 pm
No -- The cathodes would get grounded though traditional 10 ohm resistors for current measurement, and an EFB Control Grid Regulator would be used in conjunction with a traditional negative bias supply to provide EFB action.

Dave

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!

Dave


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