-----Original Message-----
From: Carl
Sent: Wednesday, October 09, 2013 8:17 AM
To: Jim Thomson ; amps@contesting.com
Subject: Re: [Amps] LF oscillation caused by defective plate choke?
----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Wednesday, October 09, 2013 6:11 AM
Subject: [Amps] LF oscillation caused by defective plate choke?
From: peter chadwick <g8on@fsmail.net>
To: N1BUG <paul@n1bug.com>, 'Amps' <Amps@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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|