09-13-2015, 06:03 AM
Yes, Dave Gillespie is the expert engineer for Fisher equipment on the internet. I worked with him to develop a complete schematic of his design incorporated into the X-101-C. I included a bill of materials too to help others locate the needed parts for the modifications. The schematic Rev A can be found at AudioKarma.org, http://akdatabase.com/AKview/thumbnails....=17&page=5 as Fisher X-101-C Mods Rev A although I also did a Rev B which has some corrections to the original released document.
I was amazed at how much cooler the output tubes run after being used to the heat produced by older unmodified Fisher amps and receivers. By comparison to my Dynakit Stereo-70, these run cool as cucumbers!
Two other people who were involved in providing modifications to improve Fisher tube audio equipment were Terry Dewick and Drew Bolce. They developed circuitry to provide Individual Bias Adjustment Modification to Fisher sets that use the 7591 and 7868 output tubes. These days it is difficult to find matched quads of those tubes that are truly well matched. The IBAM changes allow each output tube's current to be adjusted to its ideal range for best power output consistent with lowest current, especially when combined with Dave Gillespie's EFB modifications. Dave also advises adding 100 ohm 1/4 watt carbon film resistors in series with each output tube screen grid and using a 10 ohm 1/4 watt resistor to ground for their cathode circuits. It provides some extra protection against an output tube that might short. Both the screen gird and cathode resistors will blow open and act as fuses to protect the output transformers if a short occurs. The 10 ohm cathode resistors make it easy to measure each output tube's current to make sure each is reading equal to its mate in left and right channels.
Based on testing done by Dave Gillespie of the various brands of new tubes on the market, the best results are obtained with Tung-Sol and Electroharmonix tubes with others coming in with somewhat lower power output. The older American made NOS 7591 and 7868 tubes are getting expensive, but do work well and usually better than the new tubes being made now. Eventually they will no longer be available, so the modifications will insure that whatever replacements are available can be made to work at their best potential.
Joe
I was amazed at how much cooler the output tubes run after being used to the heat produced by older unmodified Fisher amps and receivers. By comparison to my Dynakit Stereo-70, these run cool as cucumbers!
Two other people who were involved in providing modifications to improve Fisher tube audio equipment were Terry Dewick and Drew Bolce. They developed circuitry to provide Individual Bias Adjustment Modification to Fisher sets that use the 7591 and 7868 output tubes. These days it is difficult to find matched quads of those tubes that are truly well matched. The IBAM changes allow each output tube's current to be adjusted to its ideal range for best power output consistent with lowest current, especially when combined with Dave Gillespie's EFB modifications. Dave also advises adding 100 ohm 1/4 watt carbon film resistors in series with each output tube screen grid and using a 10 ohm 1/4 watt resistor to ground for their cathode circuits. It provides some extra protection against an output tube that might short. Both the screen gird and cathode resistors will blow open and act as fuses to protect the output transformers if a short occurs. The 10 ohm cathode resistors make it easy to measure each output tube's current to make sure each is reading equal to its mate in left and right channels.
Based on testing done by Dave Gillespie of the various brands of new tubes on the market, the best results are obtained with Tung-Sol and Electroharmonix tubes with others coming in with somewhat lower power output. The older American made NOS 7591 and 7868 tubes are getting expensive, but do work well and usually better than the new tubes being made now. Eventually they will no longer be available, so the modifications will insure that whatever replacements are available can be made to work at their best potential.
Joe
