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The Millett Hybrid MOSFET-MAX - Tweaks
Millett MOSFET-MAX Tube Heater Resistor
A tube works by by heating a cathode until it releases electrons. The electrons pass through the control grid to the plate, causing current to flow. Accordingly, tube performance is often directly related to the heater performance.
Two axioms exist regarding the heater in a vacuum tube:
1. The lower the heater voltage, the longer the life of the tube, but performance is reduced
2. The higher the heater voltage (above recommended), the shorter the life of the tube.
In the Millett MOSFET-MAX, as is longstanding with the Millett heritage, the heaters operate in series, dividing the typical 24VDC power supply into 12VDC per tube heater. However, many Millett owners operate the circuit at 27VDC or higher, because higher plate voltages usually translate to higher performance. However, the voltage that the heaters can tolerate is limited. By the same token, voltage too low can hamper performance.
The specific limiting factor is given in several of the common Millett tubes' datasheets from GE:
So, in a nutshell - never let the heater voltage go below 10V, or above 15.9V. Optimum is 12.6V.
To address this issue, while keeping the legacy Millett circuit, Colin Toole added a heater resistor, "R1" to the basic heater circuit. This allows a bit of adjustment with the voltage that the heaters may see, and provides a base loading for the overall circuit. The R1 resistor provides a way of burning off the extra voltage supplied to the heaters for those enterprising DIY-er's wishing to experiment with 30V.
To calculate the heater voltages, currents, and the size needed by R1, we must consider the potential heat, or power, dissipated by the heaters. This is given in the Millett tube datasheets: 150ma @ 12.6V. That's 1.89 Watts. Within reason, this is the power the heater filaments will dissipate, regardless of the specific voltage. We can use this assumption to calculate the resistance needed at that power to provide 12.6V:
P = (I2)*R, so R=(1.89W)/(0.152) = 84 ohms
Since the Millett circuit is in series, this means we have:
84 ohms (Left tube) + 84 ohms (Right tube) + R1
For a given supply voltage, we can apply this total resistance to find the actual current for the entire circuit. For instance, assuming a typical PS voltage of 27VDC and an R1 of 10ohms, we have:
V=I*R, I=V/R. So, I= 27/(84+84+10) = 27/178 = 0.152A
So the voltage drop across R1 (10ohms) in this case is 10 * 0.152A = 1.52V. Therefore, 27VDC - 1.52VDC = 25.48VDC, or 12.74VDC per heater.
Idealy, one should select a round number value for R1 for convenience, but ensure that this value will not exceed the low limit put forth in the tube datasheet warnings - for any of the voltages you might consider using. Here's a chart with some convenient resistor sizes at common PS voltages. Note that you should probably use the resistor no matter what. As mentioned above, it will base load the heater circuit slightly, and help to regulate the voltage somewhat. Pick a convenient size for the power supply voltages you may use and stick with it. This spreadsheet file includes one worksheet that carries the supply voltages out to 36V:
The spreadsheet and graph illustrate that the precision needed for the selection of this resistor is not very critical. For instance, if you are shooting for the lowest possible voltage on the heaters, a 5, 10, 15, or 20ohm resistor is still in the "safe" zone for all PS voltages from 24VDC to 30VDC. A 30ohm resistor is even further down, but only 0.2V from the absolute minimum safe heater voltage at 24V. So, if you ever anticipated running a PS at 24VDC, then that resistor is too low for safety.
A 10ohm resistor works well at 27VDC. The graph shows that resistance is well within safety limits of all the PS voltages shown (the top line of numbers at the bottom of the graph). More important, for a PS voltage of 27VDC, the heater voltage is 12.7V - which is pretty lose to the optimum heater rating of 12.6V.