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It seems that my calculations were, in the end, not that spot-on as I would expect.
I need to come back to my biasing transistors lessons from 40 years ago.
But I tend to reach to the same conclusion as you, especially in consideration of the other answers, confirming that this circuit operates rather hot and yours is operating normally.
The idea of adding a top hat to the TO-39 xsistors seems terrific (just be careful to avoid that the heat-sink touches other parts.
On Sun, Apr 29, 2018 at 05:42 pm, Al Holt wrote:
On Fri, Apr 27, 2018 at 08:24 am, Fabio Trevisan wrote:
25V across them. Lift one of their leads and measure their resistance, to be
The 3K9 resistors at the emitter of the top transistors should have about
sure they're still at 3k9. If the resistors are within tolerance, and the
voltage is about 25V, then you know the collector current of the output
transistor pair is correct.
My measurements of the 3k9 resistors:
R1152 -- 3k95 Ohms & 26.83 Volts
R1132 -- 3k97 Ohms & 27.19 Volts
Measure the voltage at the collector of Q1136 and Q1156... they both shouldmeasure about 4V (relative to ground)...
My measurement were:
Q1136 -- 5.75 Volts
Q1156 -- 5.58 Volts
Finally, measure the voltage across the 120ohm resistors at the output... Itwill give you an idea of how much current is being drawn by the load circuitry
downstream. By no means this current can be more than the quiescent bias
current going through the output transistors, which is about 6mA. My wild
guess is that it shouldn't be any greater than half of it, or about 3mA,
because otherwise this would seriously upset the quiescent point of a Class A
My measurements were:
R1157: measured 122.3 Ohms, Voltage Drop: 6.2mV
R1137, measured 123.3 Ohms, Voltage Drop: 1.5mV
All told, I'd say the section is operating normally; the voltage drops across
the 120 Ohm resistors varies a lot from your hunch, but maybe that's a good
thing. For now I'll do a bit more cleaning up with larger 20k resistors and a
top-hat heatsink for Q1156 and try some more checks of the entire instrument.