An Active Load for the Chaperone

(A current source, with a turn-on delay)

MODELED and TESTED
It was about 1/2 as hard as the preamp itself was to build.
I used C1 = 470 uF 35V (Radio Shack) and C2 = 180 uF 63V (Digikey).
With a 100V DC supply, the turn on delay was 8.5 seconds and the current stopped rising after 22 seconds.

I was curious what an active load for the Chaperone output would sound like. I wanted the active load to have a turn-on delay that automatically reset when the high voltage power supply dropped below a certain level. If it did not have an automatic reset circuit, it could take an hour to completely discharge on its own. This is the circuit I am going to try: 

* R1 with C1 and D1 sets the turn on delay.
* R1 and C1 + C2 set the rise time from no current to full current.
* R7 bleeds C2 to ground at power off. R6 sets the output current to 5.6V /R6 - 60 uA.
* R4 protects Q1 when spike occur on the output. It also buffers the output capacitance of Q1 so that the tube does not see a capacitive load at radio frequencies.
* When -V_RHT is near zero, Q3 is off which allows Q2 to turn on. When Q2 is on, C1 and D1 are pulled low.
* When the voltage on -V_RHT is high enough, Q3 turns on and turns Q2 off which allows C1 to charge. High enough is approximately 0.5V * (R1 + R5) /R5 = 50V
* R3 should give approximately 0.5 mA to 1 mA into D2 after you subtract the leakage currents of C1 and C2.
* R1 = R2 = 4 to 10 times R3. If R2 is made too low in value (supplying more than 1 mA), Q2 can be damaged when it discharges C1.
* Buy C1 and C2 first on life expectancy at 105C and second on ESR at greater than 10 kHz. C1 and C2 should be greater than 0.4" in diameter. That seems to be where the life expectancy levels off on several brands of aluminum electrolytics.


This figure above shows the power supply rejection Pspice thinks the Chaperone Current Source will have.
Note: My 6.2V zeners are running at 5.8V because of the low current they are biased with.


This is the part stuffing guidelines. The resistors and diodes are actually one hole smaller than shown. The top 220 uF is C1:

This is the X-ray view of the wiring diagram from the top. The wires are all on the back side of the perf board. The wires are not shown as dashed lines (hidden lines) because the dashes were hard to see on a web page. "G" goes to ground. "Out" goes to the cathode of the tube. "Vneg" goes to the -100V.

I had only hoped for an reasonable improvement of the turn-on thump. What I got was a huge improvement in the turn-on thump and a very noticeable improvement in the sound. The sound was much silkier now and lost none of it's detail and image. .

First issue 3-Sep-00 Last Rev 15-Sep-00. 
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