EPILOGUE
So the question that remains boils down to this: Why did the approach that worked consistently well in the 400 receiver, not work so consistently well in the 500C/800C units? After all, the power amplifier section of all three units are very very similar. The technical reasons have been uncovered as to why this is so. But in doing so, the study also revealed another important observation that ultimately will need further discussion in another thread. For this thread however, the technical reasons include:
1. B+ VOLTAGE: In the 400 receiver, B+ voltage to the phase inverter stage is given as 320 vdc, while that to the tone control stages is given as 290 vdc. Stabilized real world measurements actually indicate (about) 335 vdc and 295 vdc at these supply points in typical restored units, which is an important element of the issue. By moving the B+ supply for the AF Amplifier section of the driver stage down to the next lower B+ source (schematic indicated 290 vdc), this allowed the plate voltage of the AF Amplifier stage to be lowered sufficiently to then allow the phase inverter section to develop a workable bias voltage with a wide variety of tubes, without the need for the noose, while operating with (about) a 47K cathode load.
In the 500C/800C, these same two B+ sources are given as 350 vdc and 295 vdc. However, stabilized real world measurements in these units actually indicate (about) 370 vdc and 325 vdc at these supply points in typical restored units. As a result, even if the B+ supply to the AF Amplifier stage is moved down to the next lower B+ level (schematic indicated 295 vdc), the actual voltage appearing at that point is not sufficiently low enough to lower the plate voltage of the AF Amplifier stage to a level that will allow the phase inverter section to develop a workable bias voltage with a wide variety of tubes, without the noose, while operating with a 47K cathode load.
NOTE: Current draw by the individual circuits of the small signal audio tubes is nearly identical between all three receivers. But the particular output tubes used and the total quiescent (no signal) current they draw will have an impact on the actual voltage the small signal tubes operate from, as output tube manufacturing tolerances and their bias setting has a notable impact on the voltage drop produced across the screen grid B+ dropping resistor. Since current flow for all the small signal stages also pass through this resistor, any change in voltage drop across it affects them as well. Lower output stage quiescent current then will raise general operating voltages due not only to the rise in main B+ voltage this causes, but also due to reduced voltage drop developed across the screen grid dropping resistor as well. For the actual voltage levels mentioned above, all units observed had CL-80 current limiting devices installed, operated from a line voltage of 121-122 vac, with output tube quiescent currents averaging 35 mA per tube at the time of measurement.
2. AF AMPLIFIER STAGE BIAS: Here, the difference in design between the 400 and 500C/800C receivers is significant. In the 400 design, a combination of cathode bias and fixed bias is used to bias this stage, while in the larger receivers, the stage operates with pure cathode bias. While both approaches have benefits and drawbacks, one of the benefits of cathode bias is that it acts to stabilize a tube's operating parameters in the face of varying conditions around it. In this case then, lowering the B+ supply point for this stage did not produce the needed effect of lowering the plate voltage of the stage sufficiently to allow the inverter stage to operate properly without the noose installed.
3. SAVING GRACE: Since the combination of higher overall B+ voltages and different AF Amplifier Stage biasing approach prevented the needed plate voltage reduction for the AF Amplifier stage in the large receivers when moving that stage's B+ supply to the next lower voltage source, it left the phase inverter section under biased when the noose was removed. However, those that implemented this approach also invariably found that once the noose was removed, they could then no longer make the appropriate Phase Inverter adjustment as well, and so invariably increased the value of the 33K inverter stage plate resistor to (likely) about 56K so the proper adjustment could once again be made. This move was a saving grace that lowered the voltage to the inverter stage enough that it at least allowed some tubes to operate properly (just like the original design), and some not. In other words, the improvement sought by installing the modification used in 400 receivers into the large receivers produced no real improvement at all, ultimately producing the same hit or miss operation versus tubes used that the original design produced. But there's more.
4. OBSERVATION: To make the study as complete as possible regarding the phase inverter noose and all of Fisher's MPX receivers, the study also included taking a box stock 400 receiver, and installing the usual phase inverter modification into it. Before doing so, a tube was targeted that was found to performed poorly in the stock design (early clipping, reduced power output). With the modification installed, tests were again run with the target tube installed. Improvement was readily apparent, as the tube was now able to drive the output stage to full power output with low distortion. However, the improvement was not of my level of expectation, because the tube could just barely do so. This was same new German made Amperex tube that performed so poorly in the original design of the 500C receiver of this thread, and also showed such a dramatic turn around when the modification designed for that receiver was installed into it. In the 400 setting, there was a turn around with this tube when the usual modification was installed, but not nearly as dramatically so, which of course prompted further investigation. That will be dealt with in a sister thread I will start soon, employing the same title as this one, but targeting the 400 receivers. And, because of the particular 400 receiver used for that testing, the answer to an unrelated, but long standing question about the production of the 400 receiver has finally been uncovered. But that will all be discussed in the new thread.
For this thread, the phase inverter modification offered herein for the 500C/800C receivers represents a significant improvement over that of the stock design, enhancing both the performance and dependability of the tubes used in the driver stages of these receivers, while at the same time, allowing for a very wide range of tubes to be used, without forever wondering if they are operating at optimum performance. By converting the inverter stage to operate with pure cathode bias and standing it on an R/C tail, the intentions of the original Fisher design are maintained, but optimum performance is assured with virtually all tubes, allowing them to easily perform the necessary functions of the driver stage under worst case conditions, and you to receive maximum return on your tube dollar spent.
Happy listening!
Dave
