| Posted by Dave | September 01, 2014 - 09:36 am |
| Hi Stephen -- In the basic EFB installation, there is a 15K resistor from one end of the bias control resistor, to ground. When individual bias controls are added for each channel, the 15K resistor should be changed to a 13K resistor, but it is no longer connected to ground on one end. Rather, it is connected to the two extra 2K pots added to the circuit. The resistive elements of these two extra pots (outside terminals) will be connected together (i.e., wired in parallel), with the 13K resistor connected to one set of terminals on these pots, and the other set of terminals grounded. R35/R36 in each channel would have their ground connection lifted, and these two resistors then connected to the wiper (center) terminal of the control designated for each channel. And that's it. The original bias control remains, but now becomes a "Bias Range" control, with the two new controls providing a fine adjustment for each channel. You always want to use matched tubes not only in a push-pull amplifier, but between the channels of a stereo push-pull amplifier. Even though individual bias adjustments might be provided for each channel, using a match quad of tubes will always deliver the best performance, versus using unmatched tubes -- even though they might be adjusted for balanced current flow. To this point then, the new individual channel bias controls are intentionally designed to produce a limited range of adjustment -- that required to bring a matched quad into perfect balance. This approach is then most in keeping with the spirit of the original Dynaco design, that being of achieving maximum performance from maximum simplicity. I hope this helps! Dave |
| Posted by Stephen Zaneteas | August 31, 2014 - 10:52 pm |
| hI dave….just finished a big mod on an sca-35 which i posted here. Would love to do your mod and have all parts. one question. If i was to make individual bias for both channels as you have indicated, how do u wire the two-2k pots to r35 and r36 of the output tubes? Physically how does it look? connected. i mean?? Im hazy as to the proper location. Also, do you still need matched quads with individual left right bias?? |
| Posted by Dave | April 21, 2013 - 12:08 am |
| Hi Pete -- I obviously missed your post here, so you have my deepest apologizes! Ah, yes. Fisher's alternate DC heater supply! EFB(tm) is entirely appropriate for the output stage of the X-100, but the problem then is, how to power the small signal tube heaters. Basically, an alternate source of power would have to be provided since those tubes would not be powered by the output stage anymore. While that would complicate things somewhat, the effort would still be very worthwhile, as installing EFB would make a huge difference in this amplifier with regards to (at least) output tube life, since in stock form, the output stage has to conduct enough current to power the heater string, as you well know. By comparison, since the power amplifier section of the X-100 is similar in design to that used in the 400 (minus the heater string issue), you might check out my recent power amplifier rebuild of that unit over on the AK Fisher forum to see just how beneficial it was to add EFB to that unit. I have two new universal EFB boards that should be available hopefully around July, one of which would work well with your amplifier. That would greatly simplify the effort to install EFB, but the small signal tube heaters will still need an alternate source of power. However, I believe that Radio Shack carries all the parts you need to make a small power supply to handle that job. Dave |
| Posted by Pete M. | April 01, 2013 - 12:35 pm |
| Hi Dave, Great article but can this be done on a Fisher X100? It uses the cathode currents to run the preamp tube heaters. Perhaps an alternate plan could be devised? Thanks, Pete |
| Posted by joe w. | March 30, 2013 - 12:25 am |
| hi guys 7247 tubes are new from russia with love lol, god love the russians. |
| Posted by Dave | February 25, 2013 - 11:20 pm |
| Hi Ken -- The process is underway even now. I am awaiting delivery of the first board -- that being the SCA EFB power supply board. They are slated to arrive around the end of the month. I am currently in the process of setting up the retail site to facilitate their sale, but see no reason why at least that board should not be available some time next month. Going forward, I would like to try and add an additional board every month until all the SCA and ST boards are available. As for modifying the boards, of course, the SCA-35 power amplifier boards are already properly engineered to use 6GH8 tubes in the driver position. As for the ST-35 boards however, I know of no readily available tube that could be substituted, while still delivering the performance of the 7247/12DW7 used in that amplifier. Multiple tubes could be used to replicate the original tube, but space on the boards is just not permitting of this approach. Ultimately, the boards basically have to be taken for what they were intended to represent, which is a very high quality modern replacement board for the original product, with common sense improvements (i.e. the conversion to use a 6GH8) made where practical. Thanks for your interest. I'll post here as soon as the first board has arrived, and a definite date for sale/price can be determined. Dave |