| Posted by Dave | July 22, 2012 - 07:20 pm |
| Hi Joseph -- To use the EFB(tm) modification as presented with a single channel of the ST-35 would required no component changes at all. In fact, to illustrate that point, in a standard SCA/ST-35 that has been modified with EFB, you could simply remove the output tubes from one channel at will, and then operate the unit as a single channel mono unit with no other changes what so ever. The B+ would rise somewhat due to the reduced load on the power supply, but the EFB circuit would automatically adjust the bias on the remaining two tubes to compensate for their removal -- just as it compensates for any changes in B+ due to changes in line voltage or power output conditions when all four tubes are installed. This action is the very essence of how the "Enhanced" portion of EFB operates, and is one of the reasons that the performance improvements it provides in these amplifiers is so effective. You certainly couldn't just remove two output tubes at will with the original single cathode resistor design. The remaining two tubes would be seriously under biased, and red plate rather immediately. For your other questions, I would encourage you to reference the "Housekeeping" section at the end of my article, as there I discuss alternate ways to implement EFB to allow the control of individual tubes or channels, while still using a single 337 regulator device. Of course, you could certainly use two separate EFB circuits for each channel if you're using two separate power supplies, but with a common supply, I just wanted to make sure you are aware of the other options available to you. Also in that section, I make mention of using EFB in other applications with other tubes. While the concept can certainly be applied to other designs, in the form presented here, it is best applied to designs that require a relatively low bias voltage, so as to minimize the voltage drop across the regulator, and therefore limit its maximum voltage, dissipation, and heat sinking requirements. Thanks for your interest in EFB(tm), and good luck with your project! Dave |
| Posted by Joseph | July 22, 2012 - 02:41 am |
| Dave; What part value changes are needed in the EFB(TM) to have it control a single channel on the ST-35? I plan to make my ST-35 into (more-or-less) a dual mono unit, so one EFB(TM) unit per channel seems in order. May I presume that the EFB(TM) is applicable to the other Dynaco mono amps - Marks 2,3, and 4 - with appropriate parts value changes? Thanks for your research and your willingness to share it with the rest of the DIY community. |
| Posted by Joseph | July 22, 2012 - 01:16 am |
| Dave, 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! Dave |
| 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! Dave |