[Amps] LF oscillation caused by defective plate choke?

[Jim Thomson]

From: peter chadwick <g8on at fsmail.net>
To: N1BUG <paul at n1bug.com>, 'Amps' <Amps at contesting.com>
Subject: Re: [Amps] LF oscillation caused by defective plate choke?

Paul,

1000pF is 45 ohms at 3.5 MHz. If the plate choke is 250 microhenry - typical value - that's 5500 ohms. There's around 2kV RMS of RF on the plate, so that's 360mA of RF in the choke and 16 volts of RF across the 1000pF. I think that one is better off with a larger bypass - 3000pF say - and then another RF Choke (which can be a lot smaller) and another bypass capacitor to keep RF out of the power supply - some electrolytics don't like much RF current.

It gets even worse on 160, though.


73

Peter G3RZP


###   250 uh is fine for the plate choke.     Instead of the 1000 pf bypass cap,  use 2-3 x 4700 pf disc ceramics.     That combo makes for an excellent lower freq bypass cap.  
Then wire 1-3 x  500pf ceramic doorknob caps in parallel with the 4700 pf caps.   The 500 pf caps make for an excellent higher freq bypass cap.  If you have any spare 4700 pf caps left,
and /or 500 pf doorknobs,  wire em between B+  of the lytics to the chassis. 

Then you won’t have any RF  getting into the B+   back to the lytics...... guaranteed.

Later... Jim  VE7RF