[Amps] LF oscillation caused by defective plate choke?

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at contesting.com>
Sent: Wednesday, October 09, 2013 6:11 AM
Subject: [Amps] LF oscillation caused by defective plate choke?


> 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


The 500pf arent necessary as modern .0047 discs still make excellent 
bypasses at low VHF as long as the leads are short.

Carl
KM1H