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I have this amp that isn't working, and the power amp design seems pretty dicey to me. I'm concerned that the design (pretty early solid state for Ampeg) may have worked OK when new but may not work well with anything other than the new, exact parts used originally. So far I replaced one of the 1% resistors for being way out of spec and throwing everything off. Currently have all the bias, predriver, driver, and power transistors out (don't even know if I still have the parts I removed) but at the point where I would start putting them back in. Some parts of this amp look like something we would have designed in first year electronics. Want a voltage point at 40V from your 80V supply? Just put two 20w power resistors to make a voltage divider to do it, no problem! (actually my prof wouldn't have accepted that)
Anyway, the part I don't get is Q18. It has 0.7V base-emitter drop so it should be "on" (at least somewhat) but it also has 40.1V from collector-emitter would imply that is is "off." (note: looking at an old datasheet for this part it looks to me like it has much softer Vbe-current curve than what I am used to) The on-ness of Q18 is set by the PNP Q17, which has Ve=8.5V, Vb=8V, and Vc=0.7V. It seems like Q17 is supposed to leak just enough current to get Q18 biased up to turn it on but not so much as to blow it up. That seems like a pretty heavy burden to put on Q17's parameters and the resistors that bias it up, especially if Q18's Vbe-current curve is steeper than the original.
I know that in general we assume that amp designs are robust unless proven otherwise, but in this case there are almost none of these amps alive anymore (there may not have been many to begin with). I wonder if we should assume that premise to be true in this case. I really don't want to redesign the PA section, if it came down to that I might put a class D module in place of the orginal PA. The preamp is actually working fine and sounds great.
Thoughts? Am I overthinking this?
TIA,
Greg
Anyway, the part I don't get is Q18. It has 0.7V base-emitter drop so it should be "on" (at least somewhat) but it also has 40.1V from collector-emitter would imply that is is "off." (note: looking at an old datasheet for this part it looks to me like it has much softer Vbe-current curve than what I am used to) The on-ness of Q18 is set by the PNP Q17, which has Ve=8.5V, Vb=8V, and Vc=0.7V. It seems like Q17 is supposed to leak just enough current to get Q18 biased up to turn it on but not so much as to blow it up. That seems like a pretty heavy burden to put on Q17's parameters and the resistors that bias it up, especially if Q18's Vbe-current curve is steeper than the original.
I know that in general we assume that amp designs are robust unless proven otherwise, but in this case there are almost none of these amps alive anymore (there may not have been many to begin with). I wonder if we should assume that premise to be true in this case. I really don't want to redesign the PA section, if it came down to that I might put a class D module in place of the orginal PA. The preamp is actually working fine and sounds great.
Thoughts? Am I overthinking this?
TIA,
Greg
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