----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: "[Amps]" <amps@contesting.com>
Sent: Wednesday, August 11, 2010 1:01 AM
Subject: Re: [Amps] Replacing bias string diodes in Harbach rectifier board
ORIGINAL MESSAGE:
On Wed, 11 Aug 2010 03:52:24 -0000, "Bill VanAlstyne W5WVO"
<w5wvo@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
The idea of the glitch resistor is to do its job and survive. The fuse or
breaker is supposed to open but that is up to the whim of the gods and the
amp owner choices.
Anyway, only the wirewound vitreous enameled resistor has what it takes for
this.
A 1000 pf choke bypass is more than sufficient at 6M as the series resonance
with the typical 1/2" lead length as used in a Heath SB series amp is right
around 50 MHz. This data goes back into the 60's and Ed Tiltons testing. In
any case Ive yet to hear of any HF or 6M amp having filter cap problems with
a 1000pf. OTOH, a .0047 or even a .01 is suggested for 160 which is
considered a MF band. Dentron was known for PS cap problems as they used
500pf in a few of their models.
A wirewound resistor has low Q inductance and does an excellent job of
suppressing any RF that gets around the plate choke, place it right at the
point that the HV line leaves the RF enclosure. However a seperate Hi Q VHF
choke can cause problems as mentioned before; that is 60's design and is not
used in modern construction of the last decade or more. A 20-25W size will
survive an arc that even destroys the grid chokes in the SB-220 and fits the
area OK (size accordingly when running serious QRO or ridiculous amounts of
filter C), Ive deliberately used a defective 3-500 to test this as well as
the survivability of 1N5408's in the PS and as bias diodes. Use another
1000pf bypass at its far end; this cap is already in the SB-220 series.
Carl
KM1H
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