Comments for page: A New Look

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Posted by Joseph July 22, 2012 - 01:16 am

What parts value changes are needed to build the EFB (TM) to control just one channel on an ST-35. My plan is to make my ST-35 into a (more-or-less) dual mono amp - so I'd like to have one EFB (TM) per channel. I presume that this approach could be used for the other Dynaco mono amps, the Marks 2,3, and 4.

Posted by Dave June 16, 2012 - 09:59 am
Hi Denny --
The best way to add a choke to either of the SCA or ST-35 amplifiers, is to use the choke that Dynaco originally installed in the ST-70 and MK III amplifiers, and connect it so as to replace the 50 ohm 5 watt power supply resistor in the SCA/ST-35 amplifiers. These chokes are 1.5 Hy units of about 60 ohms resistance, will handle the full power needs of the smaller amplifiers, and provide superior filtering to that of the simple resistor originally provided.

If you are adding this choke to a unit that has not had a replacement power supply board (of any type) installed, then the 50 ohm resistor is located close to (and connected to) the terminals of the original power supply can cap.

If you are adding the choke to to a unit that has had (or will have) the Audio Regenesis EFB board installed (or most other aftermarket power supply boards for that matter), then the resistor will be located on the new board.

In any event, the goal would be to remove the resistor, and connect the choke in its place. Like the resistor, it makes no difference which way the leads from the choke are connected.

Good luck with your project, and I hope this helps!


Posted by denny June 15, 2012 - 10:28 pm
Using that efb cap board would it be of any improvement to add a choke?? I've read that it can improve the sound. Sca-35..are,efb power supply board. Were would one wire the choke in thanks

Posted by Dave May 30, 2012 - 11:04 pm
Hi David --

To install EFB in your Fisher will first require you to provide some alternate way of powering the heaters of the four 12AX7 tubes in the preamp/line amp section of the amplifier. As you are likely aware, the design of the X101C uses the output tube cathode current to power the heaters of these tubes, which not only provides an ability to bias the output stage, but also provide DC power for the preamp tube heaters. Therefore, to convert this amplifier to operate with EFB will first require establishing how this basic issue will be dealt with. If you leave the heater string in the circuit as is, it will still be leaving the basic cathode biasing element in place, so that any benefit from the inclusion of the EFB circuit will effectively be nullified.

If you would like to discuss this more, send me a PM through AK, or an email through the Yahoo websites and I can help you decide how best to proceed. In any event, once the heater issue is dealt with, the basic EFB circuit could likely be installed as is, with the exception that the B+ source for the divider network would be the screen B+ source in the X101C, rather than the main output stage B+ as used in the Dynaco SCA/ST35 amplifiers.

I hope this helps!


Posted by TPStatic May 30, 2012 - 12:50 pm
Hi: Dave
Hi Dave

I'm pumped about installing this modification in my cathode based Fisher 101C with 7591 tubes. Can you suggest the proper starting values to pllace on the input side of the LM337? The schematic calls for 40V for the bias settings.

Thanks large
David T Hobday

Posted by Dave May 07, 2012 - 11:44 pm
Hi Ricco --

Regarding EFB(tm) circuit action, it is true that any traditional fixed bias output stage certainly acts as a shunt regulator to its power supply as the B+ level might rise or fall. I mention this point very prominently in the article introducing the EFB concept. However, EFB acts much more like a cathode biased stage with regards to this issue. That is, as the B+ is elevated, the tendency towards increased current draw in the output stage is largely compensated for with EFB, resulting in minimal change in quiescent current flow. When the B+ is reduced, compensation again takes place to largely maintain the original current flow. In this way, the tubes are constantly being adjusted for changes in their operating environment, in much the same way a traditional cathode biased stage is automatically adjusted.

However, much unlike cathode bias, the bias voltage of the output stage is quite stable with respect to changing current flow conditions due to dynamic excitation of the output stage. As a result, the stage is now operating exactly as any traditionally fixed bias output stage would. This is much unlike the action of a resistively cathode biased output stage, where increased power output demands raise the bias voltage of the output stage.

Therefore, from a regulation standpoint, the voltage at the cathodes is absolutely regulated against changes produced from changing current draw demands from the tubes themselves, but automatically adjusted for changes in operating environment.

It is the "Enhanced" portion of the design, where the regulator's adjust terminal reference voltage is developed from the B+ supply, that allows EFB circuit action to provide the best of both cathode and fixed bias operation. Developing the adjust terminal reference voltage from the output of the regulator would destroy this feature, causing the regulator to operate in a "dumb" fashion at a fixed voltage set point only, regardless of changes to the B+ supply. The dangers of this type of operation (regulated bias with unregulated B+) are also well covered in the article. It is (in part) because the output tube operating point is continually adjusted for the prevailing B+ conditions that such a significant reduction of distortion was achieved in spite of both channels being driven to full power output in the EFB modified amplifier.

Finally, no doubt a negative voltage bias supply could be developed from one side of the HV winding as you suggest, and has been done in many a guitar amplifier design. And while that approach would have some compensating effects as the B+ conditions varied similar to the way the EFB circuit operates, it would not operate with the same level of accuracy or response as the EFB circuit does.

The performance enhancements that the EFB(tm) modification has produced in the Dynaco SCA/ST-35 amplifiers is now well established, and has been verified by numerous people worldwide. I hope this helps to explain the circuit's action more clearly for you. Thanks for your comments!


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