[Amps] LF oscillation caused by defective plate choke?

[peter chadwick]

The 50 microhenry will effectively have around 2000pF across it - the capacitors to ground at either end - and so will be resonated around 450kHz. At that frequency, the plate choke is about 700 ohms. The second choke is best mounted outside the PA compartment to avoid any coupling, and it doesn't need too high a Q either, as the RF current in it is pretty low.

Where this technique is really useful is when wanting good RF bypassing while plate modulating. I suspect the problem of bypassing the HV was one reason for using push-pull back in the days of triode PAs, although push-pull made neutralising easier. Showing my age again!

73

Peter G3RZP