Thanks for the ideas, Bill. OK, so let's say I want to clean this amp up
once and for all. No more accidents.
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.
Bill W5WVO
--------------------------------------------------
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
Sent: Wednesday, August 11, 2010 00:04
To: "[Amps]" <amps@contesting.com>
Subject: Re: [Amps] Replacing bias string diodes in Harbach rectifier board
> ORIGINAL MESSAGE:
>
> On Tue, 10 Aug 2010 20:43:55 -0000, "Bill VanAlstyne W5WVO"
> <w5wvo@cybermesa.net> wrote:
>
>> Maybe the original Heath zener wasn't such a bad idea after all.
>
> REPLY:
>
> A couple of options:
>
> 1. Install a 50 watt stud-mount type zener. Part numbers 1N3305-1N3350
> and 1N4549-1N4556. Browse the Mouser catalog for the needed voltage.
> About $10 each.
>
> or perhaps even better although more work
>
> 2. Replace the diodes with 6A10 type. They have a 400 amp surge
> rating. Assuming your B+ lead has a glitch resistor which holds the
> dead-short current below that point, they should last forever. For
> example, with 3000 volts and a ten ohm resistor, the max possible
> surge is 300 amps. The leads are a larger diameter and you'd have to
> drill out the PCB or mount them separately. About 40 cents each.
>
> Good luck.
>
> 73, Bill W6WRT
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>
>
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