ORIGINAL MESSAGE:
On Wed, 8 Dec 2010 10:56:22 EST, K1SG@aol.com wrote:
>In a thread a couple of months back, there were comments about keeping RF
>out of the stack of electrolytics in a power supply. The suggestion was to
>bypass the stack with a .001 MFD HV disc.
>Does it matter if you bypass the stack directly across the electrolytics
>(plus to minus), or from plus to GROUND? The difference is that there's
>usually a 50 ohm 50 watt (or something) resistor between negative and ground.
REPLY:
The source for the RF (if any) is from the HV line to ground. Bypass
that and there's no way RF can get into the caps. To really be thorough
about it, place bypass caps at both ends of the HV lead - at the cold
end of the plate RF choke and again where the HV lead connects to the
electrolytics. Incidentally, .001 uF is a bit small if this is an HF
amplifier. I'd use at least a .01 and maybe as much as .05.
If that 50 ohm resistor is there as a multiplier for the grid current
meter, it should be bypassed with its own capacitor. Normally, the only
connection from the HV negative line to ground is through the grid
current meter and a low value protective resistor and some diodes.
Otherwise, the negative HV line is floating.
In my homebrew 8877 amp I used a pi type lowpass filter right at the
cold end of the plate RF choke. The caps were .01 Russian doorknobs and
the coil was about 100 uH, homemade. That's probably overkill but it
cost practically nothing to add.
73, Bill W6WRT
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