[Amps] 1N Diodes

[jim.thom jim.thom@telus.net]

Date: Thu, 1 Jul 2021 17:39:56 -0500
From: Mark - N5OT <r-emails at n5ot.com>
To: amps at contesting.com
Subject: Re: [Amps] 1N Diodes


> Just use the right diode to begin with..and that's not the 1N400X series.


<Not sure how to interpret your comment.? It comes with a 1N4001 from the
<factory.

<Mark N5OT

## The  1N4001 in the mouser online catalog is a whopping  2.8  CENTS
each..bought in bulk, which is what ameritron /  MFJ does.   That
1N4001..along with other items in your amp, are the lowest rated you can
buy.....and done that way to  enhance profits.

##  It gets worse.  Ameritron wires a .1 uf  @  50 vdc rated  disc ceramic
in parallel with the puny  1N4001.   In case the 1N4001  fails open, the
fault path is then across the  50 V rated  .1 uf cap. With the diode open,
the full B+ is across the  50 V rated cap..and of course it will arc.   So
now you have 2 components, with the cap taking the job of the puny diode,
after the diode fails open.   I have never seen a safety diode fail open.
They always fail shorted, every time.   Full B+  is more than ample to
turn on any diode.   Anything > .5 vdc  will bias the diode on.

##  You should be able to  cro-bar the  B+  to the chassis repeatedly..and
the diode should not fail.   Either parallel a few  1N5408's... or parallel
a few  6A10's..or  PM-600's.  I have a hb  B+  supply, and it's  5700 vdc /
7700  vdc.   Same deal, cro-bar the B+  to the chassis  (via a hd spst
ceramic vac relay, for safe testing purposes) ..and nothing happens, except
an open  HV fuse...event over.   The safety diodes  remain intact.

##  I installed a 50 ohm  glitch resistor in series with the  BUSS  HV
fuse, (HV fuse 1st, 50 ohm resistor last).   With any  B+ to chassis  arc,
or  B+  to grid arc, inside the tube, the resistor will  limit the fault
current to a safe value.   2000 vdc /  50 ohms =  40 amps of fault
current.   40 amps of fault current will open off a 750ma or  1000 ma rated
HV fuse in  < 2 msecs  every time....event over.   And safety diodes remain
intact.  Any fault current has to flow through those safety diodes.

##  without the glitch resistor, the fault current is limited only by the
ESR of each lytic in the string of lytics.  200uf  to  330 uf lytics are
typ .75 ohm of ESR per cap.   With  6-8 caps used in series,  total ESR is
only  4.5  to  6 ohms.  Then maybe another 1-2 ohms in the plate choke.

##  IF  u have to ever replace the diodes in the B+  supply, the 1N5408 is
a good choice.  The bigger  6A10 / PM-600/ 10A10 / 10AO8  will drop right
into place where a 1N5408 once stood.  Wire leads on a 6A10 are the same 16
gauge / .052"  diam as the wire leads on a 1N5408. The length of the 6A10
is the same as a 1N5408.  However the diameter is double that of a 1N5408.
The length and also the diameter of the 6A10 are the same.  In the case of
the B+  supply,  I installed a 2nd HV fuse, between  plate xfmr and input
to diode board...one leg only.   Cro-bar the B+  right at the HV caps..and
the 2nd fuse opens up..event over.  Power supply diodes survive.

##  with the above setup, you can cro-bar anything, anywhere..and nothing
happens..except for open  HV fuses.

Jim   VE7RF