24-04-2012, 09:54 PM
(24-04-2012, 05:52 PM)Joe Wrote: I tried Alan's idea with mixed results. A cap between the collector and emiter of T4 and/or T5 sent it utterly Deolali
Alan said collector-base, not collector-emitter. I'm not surprised it took off - effectively you were asking it drive the caps in parallel, and amplifiers don't like driving capacitive loads (like Quad ESLs).
When you connect between collector and base, you are introducing negative feedback at high frequencies, and effectively slowing down the silicon transistors to act like slow old germaniums... Actually, all transistors have internal capacitance between base and collector, and this is one of the factors that determines the speed of the transistor.
As I said before, emitter followers are especially prone to bursting out in oscillation, and this tendency is strongly dependent on collector current, as you've demonstrated. Putting ~100 ohms in series with the base (can be done above the PCB - pull out the base and fold it up and joint it to a vertical resistor) is much more elegant and effective for an EF circuit. Not surprisingly, these resistors are known as "stoppers" - you'll note that term in the most recent document I've sent you
Give it a go...
