[Amps] HV wires, back-to-back diodes, and B- wiring

[Jim Thomson]

Date: Sat, 11 Sep 2010 08:52:41 -0600
From: "Jim  Garland" <4cx250b at muohio.edu>
Subject: Re: [Amps] HV wires, back-to-back diodes, and B- wiring

I've been reading these posts with interest. Some of my practices are a bit
different from those discussed here.

 

(1) HV wiring: I use ordinary red-jacketed RG-58 cable with Kings HV BNC
connectors to connect power supplies to amplifiers. My 8877 amp runs 4600V
(no load) and the cable has been in place for the past thirty years and
disconnected/connected  many times with no problems.

##  Those are called  MHV connectors..and are just the HV version of
a BNC.    The hot side is not recessed enough imo... and looks dangerous,
IF connector's Un-plugged  and B+ turned on.   [ I would not be plugging/un-plugging
any kind of HV connector with power on, not safe anyway]




 For years the high
energy physics group at Ohio State used the stuff for photomultiplier tube
power supplies, where the voltage runs in excess of 5 KV. That's where I got
my cable and connectors.  The worse case scenario is that it would short to
the braid and blow a fuse.  RG-59 would be a bit better, though unnecessary
IMHO, but my available connectors were sized for RG58. The key point is
always to use shielded cable for HV interconnects, for safety reasons. Also,
aside from safety considerations, uninsulated wire, such as used in the
Drake L4B and L7, will get extremely dirty over time because the electric
field around the wire will attract dust and grime.

## The L4B/L7, only has a few inches  of what appears to be the
dielectric of RG-59.. exposed at back of RF deck.  I have 4 x L4B's
and never seen much dust attracted to the 2600vdc.   Those silly
Red millens are  WAY more dangerous.. ESP when Un-plugged..
and  B+  is present.   [ with 7700 vdc on my HB supply, and using
10 kv rated silicone rubber Hv wire [ belden]... if I get my forearm
any where near the HV cable.. all the hairs stick up on end !   I doubt
 a shielded cable would do this].    I don't trust that red 10 kv millen 
silicone rubber  hv wire.. for anything EXTERNAL.   It would be very easy
to slice through the soft rubber... and not even see the cut.. [ it would
self seal].   The 18 ga super fine stranded belden wire is super flexible,
and ok  for inside a B+ supply or RF deck. 



 

(2) I don't quite understand the rationale for back-to-back diodes between
B- and Ground.  I've always used a single safety diode for protection, with
cathode tied to B-, since a B+ flashover will drive current from chassis to
B-. (Actually, I put one diode in the power supply and another in the RF
deck.)  Back-to-back diodes would likely distort the grid current reading
because one of the diodes will clamp the voltage across the grid current
sampling resistor ( I use 5 or 10 ohms.) An internal tube arc will channel
the current surge directly into the B- lead (if the arc is to the cathode),
or to the chassis (if to the grid), and in either case a single diode will
provide adequate protection.

##  semi agreed.   I use back to back diodes....between  B-  and chassis
simply so the  B-   can never 'wander'  more than  +/-  .7 volt.   Without
the 2nd diode.. it could 'wander' quite a bit.   I use a dedicated grid 
meter.. and V drop is not high enough to frwd bias the 2nd diode. If it was,
simply use  2 or more diodes  in series.   



(3) For B- wiring, I just use one of the conductors in the multi-conductor
control cable to the power supply. The B-voltage is never more than a couple
of volts above chassis ground, even with a flashover, so there's no need for
a separate B- wire.

## IF  a separate B+ pwr supply is in it's own box..and lets say we used the 
old arrl  method, and wired a 10-100 ohm wire wound  between  B-  and chassis,
and that WW  went open.   OK, now we have a  floating 4 kv  power supply.  Now
if something went wrong, and a wire fell down, or B+  got tangled up with the chassis,
we would now have a -4000  supply !  [neg 4 kv supply].  

##  The last thing we need is the B-  going open, on a normally operating HV supply.  I did
this once, by accident... and forgot to hook up the separate B-  cable  between  the HV
supply and the separate  4-1000 RF deck.  With the B+ turned on... all the .01 uf bypass
caps at the cold end  of the bifilar started going off like firecrackers.   With the B-  gone
open... the cathode will indeed, try and assume full B+ potential.  That was  1977..and dunno
what I used between B- and chassis in the HV supply..or if chassis's bonded together.   

## These day's, I treat the B-  like the B+ side...and use a separate B- cable.   I also use 10/8 ga
RW-90  stranded, insulated cu wire.. with heavy duty CRIMP lugs on each end.. and bolt to both
RF deck  and stand alone B+ cable with  1/4-20 SS bolts and SS nylocks.. and crank em down tight. 
Install 2 of em, if ur paranoid. 

## I also run another  similar cable, from RF deck to SPG.. and  ditto  between B+  supply..and  SPG. 
My SPG is just a large AL plate of 6061-T6... 1/4"  thick... that's mounted on basement wall.  It's
the main grnd for everything. [ single point grnd].   The plate is also drilled and tapped.. and the  henry radio
LP filters are bolted to it... along with lightning protector, line isolator, 2 ga CU  from outdoor
cad-welded grnd rods, +  grnd  from main 200A panel, etc.  

##  2-3 kv is bad enough... but with 7-8 kv  and huge filter caps... I'm not abt to take any chances. 
A small rack, with B+ supply in the bottom..and 'slide in RF deck'  at top of rack  would eliminate the
need for HV connectors... most of the time.    If you want to experiment, and operate the  RF deck
on the test bench, with lid off, etc, then you have to cable the b+ / b-  between the RF deck and HV supply. 

##  I gave up on HV connectors yrs ago.. and hardwired both ends.  I use these new style  strain relief 
chassis fittings, that are popular these days... made  in diff sizes..to take all sorts of cable diameters.
You rotate the fitting, and it shrinks in diameter..and pinchs the cable, good and tight.    Then use
your favourire HV cable, shielded, or other wise. 

## BTW... these SHV connector's come in SHV-5,   SHV-10  and also  SHV-20   [ 5/10/20 kv]  
The SHV-5/10  use rg-58/59 cable..and the  SHV-20  uses  RG-213/RG-393 cable.
http://www.insulatorseal.com/searchs/doc/ConnectorIntro.htm

## They just keep getting longer and longer. The 20 kv type is 4.5" long.  Also.. the
Pasternack version of a SHV connector is rated for 5 kv, that's  it.  Pasternack carries
several version's of the male SHV.. but even the ones for  RG213/ RG-393  are still
only good for 5 kv.  

##  I have not found the mating 10/20 kv bulkhead  female SHV connectors.  The
5 kv chassis females all take RG-58/59 cable .  IE:  cable on the back end of the
mating chassis female is attached the same way as the mating male.   I guess the
idea here is the cable coming off the chassis female is also coax cable, with shield
also grnded..so no micky mouse wire used on the inside of the box. 



## The  5 kv rated pasternack SHV's appear to me, to be able to handle a lot more than 
5 kv.  Here is the  female chassis bulkhead connector for RG-213/ RG-393 
 http://www.pasternack.com/product-SHV-Jack-Bulkhead-Clamp-Attachment-For-RG8-RG9-RG213-RG225-RG393-PE44049-70232.html



##  here's the   mating male SHV's..complete list for all cable types.  Note here, that double braided cable like 214/393 uses
 a different male connector  vs  213 u. 
http://www.pasternack.com/searchresult.aspx?categoryid=5&categoryname=connector-wizard&page=1

## I use a HV meter in both the RF deck..and also the remote HV supply.   The HV meter in the HV supply is wired
to the INBOARD side of the HV fuse /glitch R.   If the B+ HV fuse ever blows open... the HV meter in the RF deck 
reads ZERO... meanwhile the HV meter in the HV supply still reads  7 kv!     I have, in effect, a HV meter on both sides
of the HV  B+  fuse. 





later.. Jim   VE7RF   

     





 

73,

Jim W8ZR