[patrick-turner]

HpW-Works Aug 9 (20 hours ago)


Other recipients: Patrick,
Noticed an error in the MJE340 current source circuit in the first
schematic. (Correct in the others.)
why do you use only one transistor. Using two would also increase the
output impedance....

----------- V+
| |
R LOAD
| |
| C
--------B Q1
| E
C |
Q2 B--------
E |
| Re
| |
----------- GND

Hp

I have ammended the first schematic on that page Basic Tube 5.

It now shows the emitter resistor 2k2 of MJE340 being taken to -45Vdc rail.

There is no point at all in using 2 transistors in a series arrangement because with just one bjt, the Rout is well over 1Meg ohm. If it was 100Mohms, the operation difference and mathematics involved would not change enough to warrant using the 2 transistor pair. The beauty of having an active solid state CCS means at least it saves having a 1M x 100Watt resistance taken to a -10,000Vdc rail. Its bound to be dangerous. 1.37Vac across 1M = 0.00137mAac, 1.37uA, so no need to worry about Rk being too low. I have used a larger Re with -100V supply rail to increase the finite R value for CCS.

But feel free to use two bjts if you insist.

In old days the makers of scientific gear used a pentode between common cathodes of LTP and say -150Vdc rail, and they included an Rk of say 3k0 under the pentode k so that effective Ra' = Ra + [ ( µ + 1 ) x Rk ] and if the pentode was 6BX6, gm = 7mA/V at 10mA, and Ra = 250k, then µ = 1,750, and calculated Ra' would be 250k + ( 1,751 x 3,000 ) = 5,503k ohms, or about 5.5M. The screen must be well bypassed to cathode, and sit at about 0V, with high value R feed from say a +400Vdc elsewhere in the gear.

But no manufacturer who made large numbers of amps ever put a pentode CCS in any home audio gear because it was never ever thought to improve the sound or lessen the overall THD measurements of the amp.
But Now an MJE340 costs 1/100 of the cost of a 6BX6 if we include the tube and socket, wiring, extra chassis space and labour etc.
Patrick Turner.