Anyone who has lurked around the AK Fisher Forum for very long has probably come across threads by Fisher Folks who either bought a KX-100 or X-100B with one or both OPTs bad, or had an otherwise good one die on them while in service. This usually prompts calls for help either in fitting or trying to locate a suitable aftermarket replace piece, or in obtaining a good original piece from a parts donor unit somebody has on their shelf. It's a story that's repeated itself seemingly a number of times here on AK. And so, my own brush with this issue has now come to life.
Some years ago, I had restored a KX-100 for a client, where in that process, I had also modified the unit with many of the modifications/recommendations I've posted on this forum for Fisher equipment. For the purposes of this thread, the only modification that is relevant from that effort was the installation of a separate DC Heater/Bias supply that allowed the unit to have its output stage converted to fixed bias operation, with the end result that output tube quiescent current could then be reduced to sane levels that would benefit the life of the tubes and all three transformers, as well as providing significant performance improvements as well. Just recently then, he has sent this unit back to me - still in perfect working order -- asking only for me to install the EFB™ modification into it as well.
That process begins with running a series of tests, to determine the optimum parameters to set the EFB Screen Grid and Control Grid Regulators at for optimum performance of the unit. One of those tests involves driving both channels to the onset of clipping at the same time, to gauge power supply performance under that condition. And so the test began, where above about 15 watts of power, the left channel scope display started to scatter and display added trash to the sine wave that was starting to partially break up as well. I backed the drive down, checked all connections, turned up the drive again, where upon it did the same thing. More testing showed that the drive signal to the output tubes was clean on both sides of the push-pull connection, so the next hope was that it was the tubes or their sockets causing the issue.
I swapped the power tubes between the channels, and gave it another whirl. This time, the left channel rose right up to the onset of clipping without incident, which was a relief. The trouble is, so did the right channel as well. That seemed to eliminate the tubes, leaving (hopefully) just the sockets. After a good cleaning, the tubes were reinstalled in their original locations, and the test run again. This time, the left channel again rose cleanly right up to the onset of clipping, so at that point, the previous scattered display was (cautiously) written off to a dirty tube socket, which I informed my client of as the only thing the unit apparently suffered from over the time since I last had the unit on my bench.
The rest of the testing was then finished uneventfully, parameters determined, and the EFB circuits installed. At that point, more testing then began to determine if the new circuits were performing as intended, and the expected increase in performance was achieved. Unfortunately, the same hash/trash appeared at the same power level, in the same channel, once again. So as before, the power tubes were swapped. But unlike before, the same hash/trash still appeared at the same power level with now cleaned (and well tensioned) sockets, in the same channel, using different tubes. So now the drive, the tubes, and the sockets were all positively eliminated. Uh oh.......
What I was witnessing was an original KX-100 OPT going through the process of passing from this world to the next. Below 15 watts of power, there were no apparent problems. Clearly, my client had never pushed the amplifier beyond this power level, as he had no complaints as to its operation at all -- in fact, he loved it. He just wanted EFB installed. Whenever the hash/trash appeared, I would always back off on the drive, not wanting to hasten the transformer's demise, but at this point, there was no doubt -- it was only a matter of time before the transformer would die completely. Of course, when that happens, it often does damage to the tubes involved, as well as potentially damage to other components as well, so I made the recommendation that the transformer(s) get replaced. Hardly what my client was expecting from a request to simply install EFB into a perfectly operating amplifier, and yet, unfortunately, far too common an event for owners of X-100B and KX-100 amplifiers. I have to believe that had the biasing system still been stock, with the output tubes running at much higher quiescent current levels, at or beyond their dissipation limits, and everything getting cooked in the amplifier at much higher temperatures, that this transformer would have been toast much sooner.
In the discussions that followed with my client, I expressed my concern with using the common go-to transformer manufacturers/re-winders, as near direct experience with the go-to re-winder when rewinding OPTs yielded basically incompetent results, and otherwise only scant claims of great work performed (but with no published performance/evidence), or, with with the go-to new manufacturer, direct experience (and observed performance published by others) has shown that performance is well off the mark of the high quality high fidelity pieces of yesteryear -- including those of the KX-100/X-100B as well. Alternately, finding a good original replacement is always possible given enough time, but of course then the same fate is likely inevitable to occur at some point.
With the recent apparent shuttering of Transcendar, I was always disappointed that I never got a chance to try out some of their products. They certainly look to be quality devices (but then so do other's products), and have built up their followers who praise them (as have others) -- but again, I am not aware of any published performance data as a result of using their products in vintage designs. However, based on their description as to how their transformers are built, it offered hope. And, there have been sputtering signs of life at Transcendar lately, if only to (possibly) sell off some of their remaining inventory. With that thought, I decided to check on line where at auction, as luck would have it, Transcendar was offering for sale a few direct replacement OPTs specifically for the KX-100/X-100B (isn't that telling). I immediately informed my client, where upon the decision was made to purchase two of them. In spite of an indicated March delivery, the two transformers arrived in one short week's worth of time.
With that, new tests were then run to determine just how good the new transformers were, how well they mimicked the original pieces, and how well they could perform in the KX-100/X-100B design.
I'll get into all of that next time.
Dave
Some years ago, I had restored a KX-100 for a client, where in that process, I had also modified the unit with many of the modifications/recommendations I've posted on this forum for Fisher equipment. For the purposes of this thread, the only modification that is relevant from that effort was the installation of a separate DC Heater/Bias supply that allowed the unit to have its output stage converted to fixed bias operation, with the end result that output tube quiescent current could then be reduced to sane levels that would benefit the life of the tubes and all three transformers, as well as providing significant performance improvements as well. Just recently then, he has sent this unit back to me - still in perfect working order -- asking only for me to install the EFB™ modification into it as well.
That process begins with running a series of tests, to determine the optimum parameters to set the EFB Screen Grid and Control Grid Regulators at for optimum performance of the unit. One of those tests involves driving both channels to the onset of clipping at the same time, to gauge power supply performance under that condition. And so the test began, where above about 15 watts of power, the left channel scope display started to scatter and display added trash to the sine wave that was starting to partially break up as well. I backed the drive down, checked all connections, turned up the drive again, where upon it did the same thing. More testing showed that the drive signal to the output tubes was clean on both sides of the push-pull connection, so the next hope was that it was the tubes or their sockets causing the issue.
I swapped the power tubes between the channels, and gave it another whirl. This time, the left channel rose right up to the onset of clipping without incident, which was a relief. The trouble is, so did the right channel as well. That seemed to eliminate the tubes, leaving (hopefully) just the sockets. After a good cleaning, the tubes were reinstalled in their original locations, and the test run again. This time, the left channel again rose cleanly right up to the onset of clipping, so at that point, the previous scattered display was (cautiously) written off to a dirty tube socket, which I informed my client of as the only thing the unit apparently suffered from over the time since I last had the unit on my bench.
The rest of the testing was then finished uneventfully, parameters determined, and the EFB circuits installed. At that point, more testing then began to determine if the new circuits were performing as intended, and the expected increase in performance was achieved. Unfortunately, the same hash/trash appeared at the same power level, in the same channel, once again. So as before, the power tubes were swapped. But unlike before, the same hash/trash still appeared at the same power level with now cleaned (and well tensioned) sockets, in the same channel, using different tubes. So now the drive, the tubes, and the sockets were all positively eliminated. Uh oh.......
What I was witnessing was an original KX-100 OPT going through the process of passing from this world to the next. Below 15 watts of power, there were no apparent problems. Clearly, my client had never pushed the amplifier beyond this power level, as he had no complaints as to its operation at all -- in fact, he loved it. He just wanted EFB installed. Whenever the hash/trash appeared, I would always back off on the drive, not wanting to hasten the transformer's demise, but at this point, there was no doubt -- it was only a matter of time before the transformer would die completely. Of course, when that happens, it often does damage to the tubes involved, as well as potentially damage to other components as well, so I made the recommendation that the transformer(s) get replaced. Hardly what my client was expecting from a request to simply install EFB into a perfectly operating amplifier, and yet, unfortunately, far too common an event for owners of X-100B and KX-100 amplifiers. I have to believe that had the biasing system still been stock, with the output tubes running at much higher quiescent current levels, at or beyond their dissipation limits, and everything getting cooked in the amplifier at much higher temperatures, that this transformer would have been toast much sooner.
In the discussions that followed with my client, I expressed my concern with using the common go-to transformer manufacturers/re-winders, as near direct experience with the go-to re-winder when rewinding OPTs yielded basically incompetent results, and otherwise only scant claims of great work performed (but with no published performance/evidence), or, with with the go-to new manufacturer, direct experience (and observed performance published by others) has shown that performance is well off the mark of the high quality high fidelity pieces of yesteryear -- including those of the KX-100/X-100B as well. Alternately, finding a good original replacement is always possible given enough time, but of course then the same fate is likely inevitable to occur at some point.
With the recent apparent shuttering of Transcendar, I was always disappointed that I never got a chance to try out some of their products. They certainly look to be quality devices (but then so do other's products), and have built up their followers who praise them (as have others) -- but again, I am not aware of any published performance data as a result of using their products in vintage designs. However, based on their description as to how their transformers are built, it offered hope. And, there have been sputtering signs of life at Transcendar lately, if only to (possibly) sell off some of their remaining inventory. With that thought, I decided to check on line where at auction, as luck would have it, Transcendar was offering for sale a few direct replacement OPTs specifically for the KX-100/X-100B (isn't that telling). I immediately informed my client, where upon the decision was made to purchase two of them. In spite of an indicated March delivery, the two transformers arrived in one short week's worth of time.
With that, new tests were then run to determine just how good the new transformers were, how well they mimicked the original pieces, and how well they could perform in the KX-100/X-100B design.
I'll get into all of that next time.
Dave