The Millett Hybrid MOSFET-MAXed - Tweaks
Millett MOSFET-MAX Tube CCS (Constant Current Source)
The Millett MOSFET-MAX implements the same CCS as used in the original MAX for the first time: a dual-transistor, dual-resistor
constant current sink, a common transistor circuit. In the case
of the MOSFET-MAX, however, it is operating as constant current source. The operation of
the circuit is simple. If the voltage across the emitter resistor of QA1 attempts to exceed
0.65V (the base turn-on voltage for a silicon transistor), then QA2 will turn on, and short
out all base current to QA1 except for exactly that amount required to maintain the specified
current. If the collector current of QA2 falls, then the voltage across the emitter resistor
also falls. This turns off QA1 until the current is again stable at the preset value.
The preset value is determined by: I = Vbe/RA8 , where Vbe
is the base turn-on voltage for a silicon transistor, 0.65A. Traditionally, the CCS current for
the Millett has been set at 0.56A. Therefore, to get 0.56A, we need a resistor value for RA8 that
results in 0.56A for Vbe:
R = 0.65/0.56 = 1.16K ohm.
RA9 needs to be at least 10 times RA8. So, RA9 = 11.6K ohm, or greater.
Single or small number purchase quantity is not possible for Vishay-Dale resistors at that size from
the preferred outlet. So, a value of 1.13K ohm was selected for RA8, and 11.3K ohm for RA9. This
results in a CCS current of: I = 0.65 / 1.16K = 0.575ma .
Referring to the tube operation curves below, one can see that this value falls in the midrange for the
typical Millett supply voltage. Other resistor combinations may be tried, but this one is closest to
traditional Millett parameters. Divide 0.65 by the desired current to obtain the resistor value. Then
size RA9 for at least 10 times the value of RA8.
This type of CCS was chosen for several reasons:
The combination of two of the same type transistors used elsewhere on the board is economical,
likewise, the cost of the two resistors is trivial, and
unlike traditional CCS diodes or depletion mosfets, availability is good worldwide and is not
exclusive to a single supplier.
Finally, since the current is set by adjusting the values of the two resistors, the CCS is adjustable
without replacing the semiconductor. In fact, for those enterprising builders, socketing the RA8 and RA9 resistors might be a good thing to do. Most of the CCS discussion above is based on the 12AE6. However, the 12AE6's CCS current is not necessarily optimum for the 12FK6 or 12FM6. Varying the currents may result in better performance from either of those two tubes. Refer to the data sheets for the respective tubes in the Millett Tubes section for more details.