[Amps] Possible issue with B- "floating down"

[David H Craig]

    Foating B- above chassis ground is necessary & sound practice.  That 
said, you don't need to fear metering or otherwise called "float" resistors 
between B- and chassis ground, so long as they are accompanied by a SINGLE 
(or pair) of diodes facing ground (from B-)- Lunar link, e.g., uses a 1N5408 
(plus resistor & cap) to ground  in the PS section (& and also the same in 
the RF deck ro ground -remote PS)- you just have to make the parallel 
resistors = to the necessary value in parallel for the meters.  1N15408s and 
single resistor from B- to ground on each side of the amp (PS & RF deck) 
plus a HV cap work great.  If a resistor(s) blow open, no problem, the 
diodes blow the breaker or fuses.  I use this configuration on all my HB 
amps and 0 issues.  Ommiting the diodes, however, will create an issue if 
the float resistor opens- then you'll be hunting shorts like a a madman 
until you realize the float resistor is open (been there, done that, in 3rd 
world countries).  The potential from B- to chassis ground will be so great 
if the resitor(s) blow you'll get immediate shorts (or arcs anyway) from all 
over until you find the float / shunt resistor is blown.  Use the diodes in 
parallel, keep the shunt, and you'll be fine.  Just be prepared to replace 
the shunt R & (maybe) the shunted diodes in case of major mishap.

    73 Dave N3DB


----- Original Message ----- 
From: "Carl Clawson" <carlclawson at verizon.net>
To: <amps at contesting.com>
Sent: Tuesday, September 14, 2010 6:18 PM
Subject: Re: [Amps] Possible issue with B- "floating down"


> Gary,
>
> The scenario begins with B+ being shorted to ground by some kind of fault.
> This tries to drive B- negative because the power supply is attempting to
> maintain a voltage between B- and B+. If B- is forced to stay near ground 
> in
> this circumstance, the effect is to short the supply which will then
> presumably blow a fuse. But if B- is not held somewhere near ground (i.e.
> left "floating") it will be driven to a negative potential equal to the 
> full
> value of the power supply.
>
> Perhaps "float" was an inapt choice of word here, but the idea is to
> indicate the lack of anything holding it at a low impedance to ground.
>
> 73, Carl WS7L
>
>> -----Original Message-----
>> From: amps-bounces at contesting.com
>> [mailto:amps-bounces at contesting.com] On Behalf Of Gary Patterson
>> Sent: Tuesday, September 14, 2010 1:32 PM
>> To: amps at contesting.com
>> Subject: Re: [Amps] Possible issue with B- "floating down"
>>
>>
>> Could someone comment on the "floating down"  of the b-.
>> What is actually causing this?
>>
>> > From: carlclawson at verizon.net
>> > To: amps at contesting.com
>> > Date: Tue, 14 Sep 2010 10:28:40 -0700
>> > Subject: Re: [Amps] Possible issue with B- run in wire with
>> less than HV rating? (Separate HV PSU and RF decks)
>> >
>> > Hi Marius,
>> >
>> > Your scenario can happen. I recently saw B- arcing to the chassis
>> > through the insulation on low voltage wiring in an LK500-ZB
>> amplifier.
>> > It had suffered an HV arc during which the grid meter shunt
>> resistor blew open.
>> > This resistor was the only thing holding B- near ground, so
>> it could
>> > now float downwards to a dangerous value. An unwary person
>> trying to
>> > probe the low voltage end of things in this situation could
>> get a nasty surprise.
> ...
>
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