Bottlehead C4S as a differential pair constant current sink?

[Mikey]

Paul, you wrote:

"To be careful, check the 6SN7 bias voltage"

What voltage am I measuring?
In other words, where do I place the voltmeter probes?

"and compute the dissipation in the small transistor"

Earlier, you showed me how to calculate the dissipation in Q2, but I didn't see
anything in the C4S manual about the dissipation in Q1....how is it calculated?

Mike

[Paul Joppa]

In normal use, the bias on Q2 is +0.7v, leaving about 2.5 drop across Q1 (2*1.6v across the LEDs, minus 0.7v base to emitter on Q2). If the 6SN7 bias is (for example) -8v then the drop across Q1 is 11.2v. Measure ground to the 6SN7 cathode in you case. Then if the current is 15mA, the dissipation in Q1 is (0.015A * 11.2v) or 168mW. The 2N2222 is rated fro 500mW without a heat sink, so I would not run it at more than 250mW without a heat sink. The above calculation says that's OK but the margin is not large.

[Mikey]

"Measure ground to the 6SN7 cathode in your case"

PJ, did you mean to write:

"Measure B- to the 6SN7 cathode in your case"

If I measure from ground to the cathodes, I get some funky numbers.
Assuming you meant B-, I measure 10.0v between B- and the 6SN7 cathode.
Plugging into your equation, the drop across Q1 is 13.2v.  At the moment, this
C4S is set for 8mA, so the 2N2222 is dissipating 105mW the way things sit today.

Of course, that assumes that I measured the 6SN7 bias correctly.....

Can't I measure the voltage drop across Q1 directly?  Sure, it may be somewhat
cumbersome because the 2N2222's three terminals are out of reach, but I can
follow the circuit board traces to points that are more accessible.  It would be
nice to compare the results both ways to see if they match!

Mike

[Paul Joppa]

The 2N2222 emitter is connected to B- (yes, that's what I meant) through R1 which drops about 0.855 volts. Its collector is connected to the 6SN7 cathode. So B- to cathode is a slightly conservative estimate of the voltage drop, and much easier to measure.