daylite
Active Member
A bias voltage design for TA-500, K-100, 500 Mono
that's suitable for vintage & modern output tubes.
There have been several discussions of the design of the Fisher “bias-voltage from heater-string” method that Fisher used to generate the negative potential needed to bias output tubes. Dave (dcgillespie) just posted an update to his simple and elegant use of Zener diodes to regulate the negative voltage. I got interested in the Zener regulation approach earlier this year. Here’s a project I planned in June (the Mouser order went out July 12) that started with Dave’s original Zener ideas. It led to a place less simple and less elegant than Dave’s plan (!), but it kept me happily occupied in the second summer of the pandemic. I’m new to the Fisher world and the forum – please forgive and correct errors I may make in this post. Oh – and I will be repeating some of the things Dave said in his recent post. That’s partly to explain what I was thinking in June, and mostly to test/reveal my understanding/misunderstanding of things.
As Dave explained, in the Fisher design when the receiver uses more current the negative voltage made available for bias will change, because it is developed by the voltage drop (to below ground) as current flows from ground through the heater filaments back to the center tap. With more current through the heaters the voltage drops to greater negative potential. In many cases, and in the Fisher K-100 chassis, Fisher used a 400R 5 W resistor to shunt some of the current past the heaters, in order to adjust the voltage drop (and thus the current) to allow the heaters to operate as wanted. (EDIT: Link added to K-100 schematic.) In the K-100 there are three 12AX7s to be heated. Fisher liked to run them at 10.5-11 volts each, so the target voltage drop over the heaters would be -31V to -33V, and that’s also just about what was wanted for bias with the original tubes and design. (The K-100 called for -31.5 V with EL37 OTs.) I have only new-issue 6L6GC tubes so I needed to get the bias potential many volts more negative.
Dave’s Zener approach is excellent. As a challenge to myself I decided to try to eliminate using 5 W Zener diodes and the high currents through them and target a supply that could give a widely variable range of bias voltages. Adapting a common approach used with Zeners suggested a circuit like this:
Current from the receiver flows through ground (at the top) to the center tap (at bottom) and is split between the heaters and the transistor TIP122 (the stock 400 ohm resistor R81 is removed). Three ¼ to ½ W Zeners set the base ('B') on the TIP122 to -33V relative to ground. I think of the TIP122 and the 220R then working as a voltage divider, with the TIP122 letting through variable amounts of current – whatever is needed to keep the emitter (at 'A') from falling below -33V (ignoring the base-emitter voltage). The 220R resistance was selected to give -33V for the heaters when plate current is at the low end (more precisely, when the receiver return-current to the center tap is at the lowest expected level). Under those conditions, with the chosen 220R resistance, the TIP122 is idle and not passing current. When the receiver uses more current the voltage at ‘A’ may begin to drop due to the extra current. (I’m calling it ‘extra’ because it’s more than is wanted for the heaters. From the bigger picture it’s not extra, it’s what the receiver needs to use to do what it’s being asked to do.) When extra current pushes ‘A’ more than a little below the -33V base voltage TIP122 starts conducting and the extra current bypasses the heaters, preventing any more voltage drop. The extra current is dumped through the transistor to the 10W resistor and onward to the center tap. All Zeners should see <10 mA. Of course, the C.T. voltage varies a lot as the receiver current changes. That’s the reason for an additional two low-power Zeners in the final circuit below. They give a more stable low-voltage end for the bias pot range, setting ‘D’ 24 volts below ‘A’.
My goal was for the supply to provide a steadied bias voltage adjustable from -26V to -44V (the range was just a guess - I didn’t have the experience to know what would be needed). In the end the complete voltage supply circuit looks as shown below on the right:
The 10W/220R resistor is moved off-board, connected to two of the seven terminals. Two other terminals provide for the end of the heater string and ground. Another two terminals provide outputs for about -25V and -53V for the two ends (CW and CCW) of the bias pots. The seventh terminal is for the center tap. (For reference, the mono IBAM, coupling caps, and grid resistors circuit is shown on the left.) The voltage divider at 'E' provides a nominal -23V for one end of the bias range. Two 12V Zeners D4/D5 give a low-current voltage source of nominally -57V, 24V below the -33V at point ‘A’, for the other end of the bias range.
I applied this to the Fisher K-100 chassis, a mono receiver. The installation came out like this:
Near the center of the photo is the green bias voltage supply board with the Zeners, TIP122/heatsink, and terminal block. In a stock chassis that’s where the center tap meets the heater string and where R81 would be. The 10W/220R resistor is behind the supply circuit board in contact with a plate that’s flush to the chassis – that plate also anchors the standoffs for the circuit board. At the top of the photo is a bracket that holds a small board with the bias pots and test jacks for cathode voltages. My DVM probe has slipped often enough measuring high voltages that a safer test point seemed a good idea. That’s what you see to the right of the voltage supply board. A resistor limits the potential hazard from the 390V anode voltage. No holes were made in the chassis.
So that’s the project report. The way I use the receiver there’s no great heat from the TIP122 or the 220R resistor (output is typically running at 60%-70% max plate dissipation). The temperature of the TIP122 rises, but not alarmingly, as output goes higher. The heater voltage does vary a little with operating conditions but stays in the -31V to -34V range throughout the practical range of bias voltage adjustment settings. The design provides more bias voltage range than needed using Shuguang, Sovtek, Svetlana, or some nice Tung-Sols Jim McShane kindly sent. I don’t have any vintage tubes to try. BTW, with the Tung-Sols passing 42.4 mA and 42.3 mA (@390 V), the bias needed is -38.6V and -38.8V (that’s a measure of the quality of Jim McShane’s tube matching). I hope you enjoyed reading. It’s not a project for everybody, but I learned a lot – and I had fun doing it.
that's suitable for vintage & modern output tubes.
There have been several discussions of the design of the Fisher “bias-voltage from heater-string” method that Fisher used to generate the negative potential needed to bias output tubes. Dave (dcgillespie) just posted an update to his simple and elegant use of Zener diodes to regulate the negative voltage. I got interested in the Zener regulation approach earlier this year. Here’s a project I planned in June (the Mouser order went out July 12) that started with Dave’s original Zener ideas. It led to a place less simple and less elegant than Dave’s plan (!), but it kept me happily occupied in the second summer of the pandemic. I’m new to the Fisher world and the forum – please forgive and correct errors I may make in this post. Oh – and I will be repeating some of the things Dave said in his recent post. That’s partly to explain what I was thinking in June, and mostly to test/reveal my understanding/misunderstanding of things.
As Dave explained, in the Fisher design when the receiver uses more current the negative voltage made available for bias will change, because it is developed by the voltage drop (to below ground) as current flows from ground through the heater filaments back to the center tap. With more current through the heaters the voltage drops to greater negative potential. In many cases, and in the Fisher K-100 chassis, Fisher used a 400R 5 W resistor to shunt some of the current past the heaters, in order to adjust the voltage drop (and thus the current) to allow the heaters to operate as wanted. (EDIT: Link added to K-100 schematic.) In the K-100 there are three 12AX7s to be heated. Fisher liked to run them at 10.5-11 volts each, so the target voltage drop over the heaters would be -31V to -33V, and that’s also just about what was wanted for bias with the original tubes and design. (The K-100 called for -31.5 V with EL37 OTs.) I have only new-issue 6L6GC tubes so I needed to get the bias potential many volts more negative.
Dave’s Zener approach is excellent. As a challenge to myself I decided to try to eliminate using 5 W Zener diodes and the high currents through them and target a supply that could give a widely variable range of bias voltages. Adapting a common approach used with Zeners suggested a circuit like this:
Current from the receiver flows through ground (at the top) to the center tap (at bottom) and is split between the heaters and the transistor TIP122 (the stock 400 ohm resistor R81 is removed). Three ¼ to ½ W Zeners set the base ('B') on the TIP122 to -33V relative to ground. I think of the TIP122 and the 220R then working as a voltage divider, with the TIP122 letting through variable amounts of current – whatever is needed to keep the emitter (at 'A') from falling below -33V (ignoring the base-emitter voltage). The 220R resistance was selected to give -33V for the heaters when plate current is at the low end (more precisely, when the receiver return-current to the center tap is at the lowest expected level). Under those conditions, with the chosen 220R resistance, the TIP122 is idle and not passing current. When the receiver uses more current the voltage at ‘A’ may begin to drop due to the extra current. (I’m calling it ‘extra’ because it’s more than is wanted for the heaters. From the bigger picture it’s not extra, it’s what the receiver needs to use to do what it’s being asked to do.) When extra current pushes ‘A’ more than a little below the -33V base voltage TIP122 starts conducting and the extra current bypasses the heaters, preventing any more voltage drop. The extra current is dumped through the transistor to the 10W resistor and onward to the center tap. All Zeners should see <10 mA. Of course, the C.T. voltage varies a lot as the receiver current changes. That’s the reason for an additional two low-power Zeners in the final circuit below. They give a more stable low-voltage end for the bias pot range, setting ‘D’ 24 volts below ‘A’.
My goal was for the supply to provide a steadied bias voltage adjustable from -26V to -44V (the range was just a guess - I didn’t have the experience to know what would be needed). In the end the complete voltage supply circuit looks as shown below on the right:
The 10W/220R resistor is moved off-board, connected to two of the seven terminals. Two other terminals provide for the end of the heater string and ground. Another two terminals provide outputs for about -25V and -53V for the two ends (CW and CCW) of the bias pots. The seventh terminal is for the center tap. (For reference, the mono IBAM, coupling caps, and grid resistors circuit is shown on the left.) The voltage divider at 'E' provides a nominal -23V for one end of the bias range. Two 12V Zeners D4/D5 give a low-current voltage source of nominally -57V, 24V below the -33V at point ‘A’, for the other end of the bias range.
I applied this to the Fisher K-100 chassis, a mono receiver. The installation came out like this:
Near the center of the photo is the green bias voltage supply board with the Zeners, TIP122/heatsink, and terminal block. In a stock chassis that’s where the center tap meets the heater string and where R81 would be. The 10W/220R resistor is behind the supply circuit board in contact with a plate that’s flush to the chassis – that plate also anchors the standoffs for the circuit board. At the top of the photo is a bracket that holds a small board with the bias pots and test jacks for cathode voltages. My DVM probe has slipped often enough measuring high voltages that a safer test point seemed a good idea. That’s what you see to the right of the voltage supply board. A resistor limits the potential hazard from the 390V anode voltage. No holes were made in the chassis.
So that’s the project report. The way I use the receiver there’s no great heat from the TIP122 or the 220R resistor (output is typically running at 60%-70% max plate dissipation). The temperature of the TIP122 rises, but not alarmingly, as output goes higher. The heater voltage does vary a little with operating conditions but stays in the -31V to -34V range throughout the practical range of bias voltage adjustment settings. The design provides more bias voltage range than needed using Shuguang, Sovtek, Svetlana, or some nice Tung-Sols Jim McShane kindly sent. I don’t have any vintage tubes to try. BTW, with the Tung-Sols passing 42.4 mA and 42.3 mA (@390 V), the bias needed is -38.6V and -38.8V (that’s a measure of the quality of Jim McShane’s tube matching). I hope you enjoyed reading. It’s not a project for everybody, but I learned a lot – and I had fun doing it.
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