[Amps] Replacing bias string diodes in Harbach rectifier board

Bill VanAlstyne W5WVO w5wvo at cybermesa.net
Wed Aug 11 15:01:52 PDT 2010

Carl, the rectifier diodes that fused short were 2.5A (251Ds).

I have more questions:

(1) One of the other destroyed components in the bias path was a small SPST 
relay that I used to bypass the 50Kohm cutoff self-bias resistor during 
transmit and QSK keying. Since it was in SSB Tx mode when the arc occurred, 
the contacts were closed, and they fused. I'm replacing this relay (which 
was kinda noisy anyway) with a reed relay that will be silent as well as 
even faster (< 2 ms). But I'd like to protect it from the same fate. I'm 
thinking that if I want to protect the relay contacts, I will need a fuse in 
the grid bias line. I'm thinking a 500 mA fast-blow fuse. Any reason this is 
a bad idea? Or unnecessary?

(2) In looking more closely at the converted 6m SB-220's RF output area and 
comparing this with the original HF schematic, I see that there used to be a 
pi-network filter in the B+ line consisting of an 8.5 µH choke flanked on 
either side by 1000 pF 6KV bypass capacitors, one stud-mounted doorknob type 
(closer to the tubes) and one radial-lead disc type (closer to the power 
supply). The original choke was apparently removed during the 6-meter 
conversion and replaced with a piece of wire; the capacitors are still 
there. I doubt the wire (4") has enough inductive reactance at 50 MHz to 
look like anything but a wire, so the two 1000 pF bypass caps are basically 
just paralleled and become 2000 pF of B+ bypass capacitance. Between these 
two capacitors, where the 4" wire now is, looks like an ideal spot to put 
the glitch resistor, which will be four paralleled 82-ohm 10W Ohmite 
vitreous enameled wire-wound resistors. I can also put the glitch resistor 
between the rectifier PCB and the first bypass cap, which would get it a 
little further away from the RF components. Would one position be better 
than the other? Replacing the 4" wire with the resistors would be the most 
convenient, as well as being better for getting them in the cooling air 

(3) Regarding the tubes (3-500ZGs) -- I pulled them and tested them for 
shorts using just a digital multimeter, and infinite resistance is shown 
between cathode-grid and grid-anode. Is this a good indication that the 
tubes escaped damage, or is this unpowered test relatively meaningless?

Bill W5WVO
New Mexico

From: "Carl" <km1h at jeremy.mv.com>
Sent: Wednesday, August 11, 2010 19:37
To: <dezrat1242 at yahoo.com>; <amps at contesting.com>
Subject: Re: [Amps] 50 watt stud mount zener diodes

> ----- Original Message ----- 
> From: "Bill, W6WRT" <dezrat1242 at yahoo.com>
> To: <amps at contesting.com>
> Sent: Wednesday, August 11, 2010 1:14 PM
> Subject: Re: [Amps] 50 watt stud mount zener diodes
>> On Wed, 11 Aug 2010 10:45:42 -0400, "Carl" <km1h at jeremy.mv.com> wrote:
>>>A tube arc will take out a
>>>50W zener as fast as a 10W version.
>> More nonsense from our favorite supplier.
>> A truly massive arc would take out either one, but an arc just big
>> enough to damage a 10 watt zener would do no harm to a 50 watt.
>> 73, Bill W6WRT
> Its a shame that Bill is so insecure that he has to start off a reply with
> an insult. Perhaps if enough readers complain to our administrator he will
> take steps to remove him from this forum and we can get back to our normal
> discussions and even arguments with civility.
> If anyone needs that email address send me a direct request.
> I didnt realize that there were baby and big arcs once a plasma is 
> created.
> IMO they range from big to bigger depending upon the PS capacity to 
> destroy
> things before something opens up the AC or HV line.
> Since the event that started this thread destroyed 1A diodes (or were they
> 2.5A) and would also slam grid choke pi's together in a SB-220 then its
> pretty much a given that either size zener would also be history.
> The customer LK-550 I have on the bench had your basic 3-500 arc and the 
> 50W
> zener was instant history.
> Carl
> KM1H
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