[Amps] Replacing bias string diodes in Harbach rectifier board

[Bill, W6WRT]

ORIGINAL MESSAGE:

On Wed, 11 Aug 2010 03:52:24 -0000, "Bill VanAlstyne W5WVO"
<w5wvo at cybermesa.net> wrote:

>For the bias string, I'm going to go with the 1N5408 diodes I already have, 
>which have a max forward surge current of 200A. If I put in a glitch 
>resistor of 20 ohms, that will limit the short current to 150A, which should 
>be good enough to protect the 1N5408s. With a plate current of 1A (more than 
>it would really run at), that would be 20 watts, which I would put up to 30W 
>for margin, since I run data modes on 6m. I'm assuming since this resistor 
>is below the RF choke, it is OK to use wirewound. (If not, what would be 
>better? I'm probably going to end up ordering it from Mouser anyway.)
>
>The bypass cap in the SB-220 is 1,000 pF (0.001 µF). If I want this R-C 
>circuit to prevent spikes that could blow things up, is this enough 
>capacitance? I don't want to go much more than 20 ohms on the glitch for 
>regulation reasons, as well as being a waste of power. The 1,000 pF bypass 
>capacitor is a 5 KV stud-mounted doorknob type. I should be able to replace 
>it with something bigger if that would help. Advice, please.

REPLY:

Wirewound is fine as they have a very large tolerance for surges. If
you can physically fit in a resistor even larger than 30 watts I would
suggest you do. The cost is not much more and you'll sleep better.
Some designers intentionally use a low wattage resistor there,
thinking it will blow like a fuse. The only thing I trust to act like
a fuse is a fuse. In my own homebrew amp I do not have any HV fuse but
I have very fast acting fuses in the AC line. 

As for the bypass cap, I notice you mentioned six meters. Apparently
you have converted your amp. 1000 pF at 50 MHz has a reactance of
about three ohms. Myself, I would use something bigger to help keep RF
out of the power supply. There are lots of Russian doorknob caps
listed on eBay. I'll go out on a limb and say the 1000 pF *probably*
is ok, but it makes me a little nervous. RF and electrolytic caps do
not mix well. One possible option would be to add a small choke and
another 1000 pF to form a low pass filter right at the base of the
plate choke. 

73, Bill W6WRT