[Amps] LF oscillation caused by defective plate choke?

[Jim Thomson]

-----Original Message----- 
From: Carl
Sent: Wednesday, October 09, 2013 8:17 AM
To: Jim Thomson ; amps at contesting.com
Subject: Re: [Amps] LF oscillation caused by defective plate choke?


----- 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

#####  I buy the .0047  from both henry radio + mouser. I bought the 
Ceramite
brand, which come in both 10 and 15 kv.  I think you can also get em in 6 
kv.
Dunno how short you mean by short leads.  Super thin .005 inch cu strap can 
be
soldered to the wire leads, to decrease XL... in cases where short leads are 
not
always possible.   I have bins full of these 10 + 15 kv types..and also 
dozens of
200 + 500 pf  HEC TH-57 TX doorknobs.  The commercial broadcast stuff, some 
of it
anyway, will use a parallel combo of a mess of .0047s and also 500 pf 
dooknobs.
To be on the safe side, since I have loads of em, I installed several 500 pf 
caps in there.
They would serve more use in the RF deck + HV supply, than sitting in bins.

Jim  VE7RF