For those following my project 400 receiver, the path started with modifying it for EFB operation for extended tube life and lower distortion, in a thread that can be found here:
http://www.audiokarma.org/forums/showthread.php?t=509090
That effort had specific goals which were met, but also brought to light other areas where significant improvement can be made in this receiver as well. Additionally, there are also some rather simple, but logical changes that can also be made, to render the unit more useful in today's audio environment. In this post however, I'll start by laying out the problems with the phase inverter stage.
The previous work of installing EFB revealed that Fisher took precautions in this stage to protect the output tubes in these receivers. If you will access the circuit of either version from the data base, it will be helpful in following along.
Specifically, the design includes a 150K resistor connected between the B+ point supplying the phase inverter (PI) stage, and the cathode of this stage. This resistor is effectively an electrical noose around the PI stage, as will be shown.
From the schematic, the relevant information is:
1. The B+ level supplying this stage is 320 vdc.
2. The plate voltage of the PI stage is 292 vdc, which is the result of a 28 volt drop produced across the 47K plate load resistor for this stage.
3. The cathode voltage of the PI stage is 136 vdc.
From this information, since we know that the aforementioned 150K resistor is connected between two points representing 320 volts, and 136 volts, it has a 184 volt drop across it, which by Ohms law represents a current of 1.23 ma flowing through this resistor.
We also know that a 28 volt drop produced across the 47K plate load resistor represents a current flow of 0.6 ma through this resistor. Since the resistor is in series with the PI tube, it means that the PI tube itself is only passing .6 ma as well. This is a pitiful operating point for any output stage driver tube to operate at. With the 150K resistor flowing twice the current flow "around" the tube, versus that flowing through the tube, it represents a very big noose indeed. What is the net effect of this design then?
First, it reduces the effective supply voltage for the stage from 320 vdc, to 184 vdc (the voltage across the 150K resistor). With the majority of current flowing through the cathode resistor of the PI stage coming from the 150K resistor then (rather than the tube), it draws the cathode voltage up to a much higher level than would be produced by the current flow of the PI tube itself -- effectively lowering the available voltage for the tube to operate from, hence, the noose.
Secondly, with only a 28 volt drop produced across the plate load resistor, it means that even if the 12AX7 tube used in this stage is driven clear through the non-linear portion of its curves to cut-off, the stage only produced an output of 28 peak volts -- and a very distorted 28 volts at that since the non-linear portion of its curves would be used to develop this output level. Staying out of the non-linear portion of the operating curves for this tube means that the stage is really only capable of developing about 14 volts peak at each output. With the output tubes receiving a bias voltage of some -17 volts, it ultimately means that the PI stage runs out of gas before the output stage does. At higher frequencies, this becomes worse, since capacitance in the output tubes requires greater drive capability from the PI stage to develop full power. And if the 330K grid resistors of the output stage were reduced to 220K? It only ups the driving requirements of the PI stage even further, adding insult to injury.
Again, this was clearly all done to protect the screen grids of the output tubes, based on how they are operated in the stock design. But since EFB can properly deal with the output tube screen grid concerns, it means that the noose can be lifted from the PI stage, moving it to a much more linear operating point for the tube, allowing for a significantly improved drive signal to the output stage.
But the PI stage is directly coupled to the previous AF amplifier stage (the other half of the 12AX7 PI tube), meaning that correcting issues in the PI stage starts with a properly operating AF amplifier stage.
For starters however, the plate voltage of the AF amplifier stage rests at some 135 volts, which helps (in part, as discussed earlier) to set the cathode voltage of the PI stage at 136 volts. This is already a red flag, since the heater circuit for these tubes references ground in the design, meaning that a 136 volt potential at the cathode then exceeds the average 100 volt maximum heater/cathode voltage rating for these tubes. That places significant stress on the heater/cathode insulation in the tubes used in this position.
In looking to correct that issue however, that required a review of the feedback/stability circuits so that any changes made to the AF amplifier stage would not significantly alter the performance of these circuits -- but what a can of worms that opened! I'll lay out those concerns in the next installment, but stay tuned, because there is excellent resolve for all of these issues.
Dave
http://www.audiokarma.org/forums/showthread.php?t=509090
That effort had specific goals which were met, but also brought to light other areas where significant improvement can be made in this receiver as well. Additionally, there are also some rather simple, but logical changes that can also be made, to render the unit more useful in today's audio environment. In this post however, I'll start by laying out the problems with the phase inverter stage.
The previous work of installing EFB revealed that Fisher took precautions in this stage to protect the output tubes in these receivers. If you will access the circuit of either version from the data base, it will be helpful in following along.
Specifically, the design includes a 150K resistor connected between the B+ point supplying the phase inverter (PI) stage, and the cathode of this stage. This resistor is effectively an electrical noose around the PI stage, as will be shown.
From the schematic, the relevant information is:
1. The B+ level supplying this stage is 320 vdc.
2. The plate voltage of the PI stage is 292 vdc, which is the result of a 28 volt drop produced across the 47K plate load resistor for this stage.
3. The cathode voltage of the PI stage is 136 vdc.
From this information, since we know that the aforementioned 150K resistor is connected between two points representing 320 volts, and 136 volts, it has a 184 volt drop across it, which by Ohms law represents a current of 1.23 ma flowing through this resistor.
We also know that a 28 volt drop produced across the 47K plate load resistor represents a current flow of 0.6 ma through this resistor. Since the resistor is in series with the PI tube, it means that the PI tube itself is only passing .6 ma as well. This is a pitiful operating point for any output stage driver tube to operate at. With the 150K resistor flowing twice the current flow "around" the tube, versus that flowing through the tube, it represents a very big noose indeed. What is the net effect of this design then?
First, it reduces the effective supply voltage for the stage from 320 vdc, to 184 vdc (the voltage across the 150K resistor). With the majority of current flowing through the cathode resistor of the PI stage coming from the 150K resistor then (rather than the tube), it draws the cathode voltage up to a much higher level than would be produced by the current flow of the PI tube itself -- effectively lowering the available voltage for the tube to operate from, hence, the noose.
Secondly, with only a 28 volt drop produced across the plate load resistor, it means that even if the 12AX7 tube used in this stage is driven clear through the non-linear portion of its curves to cut-off, the stage only produced an output of 28 peak volts -- and a very distorted 28 volts at that since the non-linear portion of its curves would be used to develop this output level. Staying out of the non-linear portion of the operating curves for this tube means that the stage is really only capable of developing about 14 volts peak at each output. With the output tubes receiving a bias voltage of some -17 volts, it ultimately means that the PI stage runs out of gas before the output stage does. At higher frequencies, this becomes worse, since capacitance in the output tubes requires greater drive capability from the PI stage to develop full power. And if the 330K grid resistors of the output stage were reduced to 220K? It only ups the driving requirements of the PI stage even further, adding insult to injury.
Again, this was clearly all done to protect the screen grids of the output tubes, based on how they are operated in the stock design. But since EFB can properly deal with the output tube screen grid concerns, it means that the noose can be lifted from the PI stage, moving it to a much more linear operating point for the tube, allowing for a significantly improved drive signal to the output stage.
But the PI stage is directly coupled to the previous AF amplifier stage (the other half of the 12AX7 PI tube), meaning that correcting issues in the PI stage starts with a properly operating AF amplifier stage.
For starters however, the plate voltage of the AF amplifier stage rests at some 135 volts, which helps (in part, as discussed earlier) to set the cathode voltage of the PI stage at 136 volts. This is already a red flag, since the heater circuit for these tubes references ground in the design, meaning that a 136 volt potential at the cathode then exceeds the average 100 volt maximum heater/cathode voltage rating for these tubes. That places significant stress on the heater/cathode insulation in the tubes used in this position.
In looking to correct that issue however, that required a review of the feedback/stability circuits so that any changes made to the AF amplifier stage would not significantly alter the performance of these circuits -- but what a can of worms that opened! I'll lay out those concerns in the next installment, but stay tuned, because there is excellent resolve for all of these issues.
Dave