Some of you may recall that before I started out on my modification odyssey for my own Fisher 400, that I put out a request for information regarding possible differences in the OPTs used in the two different versions of the 400. Specifically, versions up to serial number 48000 use part number T1020-116-1/2, while those beginning with serial number 48001 and above get part number T1020-116-1/2AX. There were responses to my request, but the end result was that there was no information available regarding any differences between these transformers.
The usual distinction made between the two versions of the 400 is the additional limiter stage added to the FM IF strip section in the latter version. But since I was gathering data at the time in preparation for adding EFB(tm) to my 400, I was scouring both schematics to make sure the modifications I made would be applicable to both versions. The different OPT part numbers came up in that effort, with other changes also noted suggesting that the change was likely due to more than just a new transformer vendor or part number realignment. Besides the transformer part number change, the latter version also used a revised feedback network, had the grid bias voltage to the output tubes elevated, and importantly, would no longer meet the original specifications for power output that Fisher published for the 400. Since the specifications were never revised by Fisher to address any of the changes made, the changes in the audio section went largely unnoticed, as there was little physical evidence of the changes made in that section. Without any definitive information to highlight any differences in the two transformer offerings, I plowed ahead with developing the data from my own unit for the installation of EFB, hoping someday to cross paths with an output transformer from one of the earlier units to determine if in fact any differences did exist between those, and the ones in my own (latter version) unit. Enter Audiodon to the rescue.
For those that followed along in the thread where I modified my unit with EFB, you may remember my comments at the time about how strangely the output stage was engineered, in my unit at least anyway -- to the point of being dangerous for the output tubes at elevated power levels. The combination of transformer load impedance, tube type, and operating conditions employed resulted in elevated distortion levels at the very least, and at worst case, screen grids that easily became miniature flashlights if you weren't careful. As to why Fisher used these questionable operating conditions in at least the latter units is a question still unanswered. But thanks to Don, we now know that it was ONLY in the latter units that these operating conditions existed.
Today's mail included a package from Don that contained a known early version 400 OPT to perform tests on. The needed answers came real quick: Where as the transformers in my version use an OPT with a quite unorthodox (for the application) 10.2K ohm primary impedance, the transformers in the earlier version reflect a very appropriate 6.5K ohm load to the output tubes. For the tubes and the conditions they operate under, this is a huge shift in operating parameters, that without any attending changes to accommodate this shift in loading, places the tubes under significant stress at elevated output levels. There are still more tests to conduct, but this most basic piece of information answers the lion's share of questions, with the exception of why.
To be sure, there was no engineering reason what so ever for Fisher to change to or use the OPTs in the latter versions that they did, so I can only surmise what I originally thought: that the change was basically a get-by move to help reduce inventory levels when Fisher was ultimately brought in for a soft landing.
On the down side of all of this, it means that the EFB parameters I developed for my unit would NOT be fully appropriate for the earlier version of the 400. That's hardly to say that EFB could not be used to good advantage in that version, but simply to say that the parameters for it would need to be adjusted for the operating and loading conditions employed in that version.
The up side however, is that if any version needs EFB, its the latter version for sure. With it, not only does this version then receive the standard benefits of EFB operation (more power, lower distortion, longer tube life, and cooler operation), but in this case, the opportunity was also taken to further adjust the EFB operating parameters to take into account the significantly higher load impedance offered by the OPTs in this version. As a result, the tubes then operate under conditions that are perfectly matched for the higher load impedance offered, preventing the potentially excessive stress they can be subjected to in the stock design of the latter version.
One final point needs to be made then to any and all whoever need to change out an OPT in their 400: Be sure you get one from a version that matches your own, or short of that, get a pair that came from the same unit, or at least match in part number. While the transformers between the two versions have their own unique advantages and disadvantages, if you must seek a pair, then those from the earlier version would be preferable if only because they offer the proper load for the tubes as they are operated in either stock version of the 400, and are therefore safer for the tubes. For the latter version, it would be a simple matter to make the circuit changes associated with the use of the early version transformer to properly accommodate them. Of course, if you install EFB in a latter version, then only the latter version OPTs would be appropriate for replacement since the EFB circuits are specifically tailored for those transformers.
I have Wynonna cranked on my EFB 400 as I type this, and she never sounded better!
Dave
The usual distinction made between the two versions of the 400 is the additional limiter stage added to the FM IF strip section in the latter version. But since I was gathering data at the time in preparation for adding EFB(tm) to my 400, I was scouring both schematics to make sure the modifications I made would be applicable to both versions. The different OPT part numbers came up in that effort, with other changes also noted suggesting that the change was likely due to more than just a new transformer vendor or part number realignment. Besides the transformer part number change, the latter version also used a revised feedback network, had the grid bias voltage to the output tubes elevated, and importantly, would no longer meet the original specifications for power output that Fisher published for the 400. Since the specifications were never revised by Fisher to address any of the changes made, the changes in the audio section went largely unnoticed, as there was little physical evidence of the changes made in that section. Without any definitive information to highlight any differences in the two transformer offerings, I plowed ahead with developing the data from my own unit for the installation of EFB, hoping someday to cross paths with an output transformer from one of the earlier units to determine if in fact any differences did exist between those, and the ones in my own (latter version) unit. Enter Audiodon to the rescue.
For those that followed along in the thread where I modified my unit with EFB, you may remember my comments at the time about how strangely the output stage was engineered, in my unit at least anyway -- to the point of being dangerous for the output tubes at elevated power levels. The combination of transformer load impedance, tube type, and operating conditions employed resulted in elevated distortion levels at the very least, and at worst case, screen grids that easily became miniature flashlights if you weren't careful. As to why Fisher used these questionable operating conditions in at least the latter units is a question still unanswered. But thanks to Don, we now know that it was ONLY in the latter units that these operating conditions existed.
Today's mail included a package from Don that contained a known early version 400 OPT to perform tests on. The needed answers came real quick: Where as the transformers in my version use an OPT with a quite unorthodox (for the application) 10.2K ohm primary impedance, the transformers in the earlier version reflect a very appropriate 6.5K ohm load to the output tubes. For the tubes and the conditions they operate under, this is a huge shift in operating parameters, that without any attending changes to accommodate this shift in loading, places the tubes under significant stress at elevated output levels. There are still more tests to conduct, but this most basic piece of information answers the lion's share of questions, with the exception of why.
To be sure, there was no engineering reason what so ever for Fisher to change to or use the OPTs in the latter versions that they did, so I can only surmise what I originally thought: that the change was basically a get-by move to help reduce inventory levels when Fisher was ultimately brought in for a soft landing.
On the down side of all of this, it means that the EFB parameters I developed for my unit would NOT be fully appropriate for the earlier version of the 400. That's hardly to say that EFB could not be used to good advantage in that version, but simply to say that the parameters for it would need to be adjusted for the operating and loading conditions employed in that version.
The up side however, is that if any version needs EFB, its the latter version for sure. With it, not only does this version then receive the standard benefits of EFB operation (more power, lower distortion, longer tube life, and cooler operation), but in this case, the opportunity was also taken to further adjust the EFB operating parameters to take into account the significantly higher load impedance offered by the OPTs in this version. As a result, the tubes then operate under conditions that are perfectly matched for the higher load impedance offered, preventing the potentially excessive stress they can be subjected to in the stock design of the latter version.
One final point needs to be made then to any and all whoever need to change out an OPT in their 400: Be sure you get one from a version that matches your own, or short of that, get a pair that came from the same unit, or at least match in part number. While the transformers between the two versions have their own unique advantages and disadvantages, if you must seek a pair, then those from the earlier version would be preferable if only because they offer the proper load for the tubes as they are operated in either stock version of the 400, and are therefore safer for the tubes. For the latter version, it would be a simple matter to make the circuit changes associated with the use of the early version transformer to properly accommodate them. Of course, if you install EFB in a latter version, then only the latter version OPTs would be appropriate for replacement since the EFB circuits are specifically tailored for those transformers.
I have Wynonna cranked on my EFB 400 as I type this, and she never sounded better!
Dave
