[Amps] LF oscillation caused by defective plate choke?

[Carl]

The Ameritron choke is 225uH and most others are a lot less going as low as 
50uH in the SB-200/220 amps.
In those cases the choke becomes part of the output network.

I always suggest a .0047 at the base of the choke and another after the 
glitch resistor which is usually not included on manufactured ham amps. Keep 
that resistor close to the choke, the wire wound resistor inductance 
performs double duty as a low Q choke at VHF.

Add another .0047 across the full capacitor string right at the PS to help 
keep out line spikes as well as any residual RF for long wire runs.

When 160 is included another .0047 in parallel is cheap insurance.

Carl
KM1H


----- Original Message ----- 
From: "peter chadwick" <g8on at fsmail.net>
To: "N1BUG" <paul at n1bug.com>; "'Amps'" <Amps at contesting.com>
Sent: Wednesday, October 09, 2013 4:48 AM
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
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