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Re: [Amps] Amps Digest, Vol 93, Issue 11

To: rezycle.bin@gmail.com, amps@contesting.com
Subject: Re: [Amps] Amps Digest, Vol 93, Issue 11
From: TexasRF@aol.com
Date: Fri, 10 Sep 2010 09:54:37 EDT
List-post: <amps@contesting.com">mailto:amps@contesting.com>
In my view, a diode pair reverse connected from B minus to ground is the  
preferred combination. Back to back would look like an open circuit until one 
or  both fail shorted.
 

The inclusion of a series R in the B plus lead, usually in the 25 to 50  
ohm range, will limit current enough to prevent a diode from going open 
circuit  during a glitch. That leaves the failure mode as a short from B minus 
to 
ground  and protection is in place. 
 
The grid current metering will no longer work but should be easily  
repairable.
 
73,
Gerald K5GW
 
 
 
In a message dated 9/10/2010 7:08:34 A.M. Central Daylight Time,  
rezycle.bin@gmail.com writes:

Good  idea to use diodes instead of drop resistor Bill. Thank you for
helping  out! I would think that the diodes should be able to withstand
very large  surges, as you mention in case of a glitch. If the back to
back diode  combination should become open circuit (in case of a very
large glitch) and  someone (not me) then removes Bminus before Bplus,
there is no return  current via the chassis path to Bminus. This would
place the RF deck at  Bpluss. This also would apply to a defective grid
drop resistor of course.  However I am not sure if a resistor is more
likely to survive a glitch than  a 6A10. I would tend to trust the
resistor, but thats just  me.

>From all advice here in the last mails and a small FMEA  analysis I
have carried out, here are my list of safety measures   (please review,
comment and expand on it):

-Use a connector that  cannot disconnect Bminus before Bplus

-Isolate B- from chassis ground  if a SHV connector is used

-Use HV rated cable (where do I get hold of  proper specced HV cable
that will match the SHV connectors. I dont trust  RG8 coax, or should
I?)

-Make sure there is an extra PSU to RF deck  chassis ground strip

-Use either a large wattage low resistance  resistor or a set of back
to back 6A10s from Bminus to chassis ground in  the HV psu chassis

-Never run meters on the "hot" side of the tube due  to risk of arcing
from the meter to the operator

-Run a separate  spun metal "sock" outside the HV cable and ground that
metal shield in the  HV PSU chassis so that in case the cable
insulation breaks down and Bplus  arcs over, the path of lowest
restance is via the sock back to the HV psu  and not via any other path
(for example the operator).

-Design in a  safety interlock so that the PSU cannot be engaged w/o
the controller in  the RF deck commands it to engage. Run an extra
chassis ground in this  interlock cable for safety.

-Design in a RF deck safety interlock so  that in case the RF deck
cover is opened, the PSU is cut immediately (and  possibly the HV is
cut via a vacuum relay to avoid waiting for the cap bank  to
discharge). Any vacuum relay must not disconnect the  bleeding
resistors over the cap bank.

-Make sure no holes in the PSU  chassis or RF deck may expose any
humans to HV in case something is  inserted accidentically in the holes

-Use MOVs over the primary of the  HV trafo to prevent spikes arriving
from the mains causing overvoltage and  exceeding the cap banks max
voltage, damaging the tube or causing  arcs.

-Use MOVs from L and N side of the primary to chassis ground  to
prevent any dangerous voltages to be present on the L and N lines  in
case both the diode recifier bank fails (short) and the HV  trafo
insulation between secondary and primary fails at the same time.  If
this happens B+ may be present on the L or N lines and the  insulation
in the mains wiring in your house may have insulation enough to  tackle
that for some time. This may cause dangerous overvoltages to  be
present on the mains system damaging sensitive equipment (e.g.  your
spouses TV) eventually causing an arc to the ground  connector
somewhere on the mains system in your house when the insulation  breaks
down. Note: In norway the Neutral (N) wire is not connected to  earth
on the supply side of the main breaker at the house (as in the  USA)
since we run an isolated terra system. Both L and N are routed to  the
main distribution transformer here and the center tap of the  main
distribution transformer is connected to earth via a disneuter  (spark
gap). Earth current return happens via capacitive  coupling.

73
Marius
LB3HC





>
>  REPLY:
>
> You are correct to leave the B- lead isolated from  chassis, but you
> don't need the high wattage resistor you mentioned.  Instead, you
> should connect a pair of diodes back-to-back from the B-  lead to
> chassis ground inside the PS chassis and another pair across  the grid
> meter inside the amp chassis as described  below.
>
> Use high surge current rated diodes such as the 6A10 or  equivalent.
> With the diodes present, you do not need the high wattage  resistor you
> mentioned above. For GS-35b I would use a 500 mA meter.  You should
> protect the meter with a low value resistor in series with  the meter.
> The value of the resistor should be chosen so the diodes  begin to
> conduct when the drop across the meter + resistor equals  about .7
> volts.  This combination of series resistor and the  back-to-back
> diodes mentioned above will protect the grid meter  against even
> massive arcs. The negative side of the grid current meter  is connected
> to ground and the positive side is connected to the  positive side of
> the cathode current meter, below.
>
> The  cathode current meter is placed in the B- lead and should also
> have a  series resistor on it's negative side and protective diodes
>  back-to-back across the combination of meter and series resistor. The
>  value of the series resistor is chosen by the same means as above for
>  the grid current meter.
>
> If any of this is not clear, please  ask questions. Good luck and keep
> us posted on your  project.
>
> Bill,  W6WRT
>
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