I've removed some degeneration on the emitter, but I actually have more DC feedback from the collector. How does this affect bias stability? It actually doesn't that much! Probably the coolest characteristic of the original is there pretty much intact: bias is almost impervious to supply voltage changes or beta variations, which is why the original can run at 27V too. In simulation, the collector voltages were pretty much multiplied by the same amount as the supply is, with Q2 being 19.5V at 27V supply (6,5*3=19.5!
). In practice this means that you can try any voltage up to 27V and any combination of transistors and you can expect the circuit to behave as intended with no changes.Q2 is pretty much the same as the original, with the obvious exception of being NPN. I'm not preventing anyone to try this with PNP transistors, so for that there's the version below. The Q1 has been rebiased to have about 6,5V on Q2's collector (why not 4,5V? the original biases at about 2.5V, but being NPN instead, the same bias should be taken from the positive rail, so that gives 6.5 the off-center bias is an important part of the sound so I kept to it), and simulation shows the headroom is the same as the complementary version, so I guess I've done things right.
Some things are gone, some things are back. Like the power supply filter resistor, or the gain stopper that prevents the output from cutting off, or C11. Oh how glad I am to get C11 back. I never had luck and always got oscillation with it, no matter if I was breadboarding the narrow, the broadcast, with silicon or germanium, with or without transformer. In the end some general bandwidth limiting/dominant pole compensation did the trick with C5, which doesn't otherwise affect response much at all. I guess you really need poor bandwidth to begin with otherwise, and even my germs were too good...
Did I say transformer? That one is on the bench still. I even got a proper "10K" transformer and made comparison, and I didn't hear anything so I keep my stance. And since it makes the circuit more affordable and easy to build, I'm all for it. A 25K or 50K volume pot gives a similar load as the original, accounting for all the series and parallel output resistors. Taper is log.
As it's usually the case for me, values have been standardized to the E6 series where this doesn't mean a compromise, with gain and AC response (in this case I've also done transient analysis) as close as the original as possible. Here's a sweep of gain and bass controls comparing narrowcast and broadcast (minimum and maximum bass, four gain settings) in low gain mode:
