Bill, W6WRT wrote:
> ORIGINAL MESSAGE:
>
> On Sun, 8 Nov 2009 09:05:10 -0600, Rob Atkinson <ranchorobbo@gmail.com> wrote:
>
>
>> Bill thanks for your answer. I appreciate the lack of gobbldygook
>> But I fail to understand why all this metering of B- is somehow safe,
>> but doing the same thing with B+ is not.
At the bottom end of the chain with a resistor and meter to ground there
are only a few volts to ground. Only if the meter "and shunt" open is
the true B- at that point. When measuring at the B+ line you have the
full B+ inside the meter case. OTOH "IF" the meter is across the
bottom resistor in the PS bleeder chain AND it's protected by a good
hefty diode (I really dislike the term anti parallel...which logically
means series, but is used to mean "parallel diodes of opposing polarity"
and requires a expanded definition)
Rephrasing what others have said, the diodes are purely a safety device
with the protection of the operator and meter (if lucky). They play no
part in protecting the tube or amplifier circuitry.
As I believe Carl has said, they are a short cut. Amplifiers need fast
acting circuitry to shut down all power in case of a fault be they
triodes, tetrodes, or pentodes. In solid state amps this type of
protection is a given due to the nature and price of semiconductors.
Although they add complexity and are seldom activated, they would be
well worth including , but then the manufacturers would be competing on
quality and not price.
73
Roger (K8RI)
>> If B- is around minus 3000
>> v. it seems to me that having only a dinky resistor and/or a diode and
>> meter between that and the chassis is dangerous. There must be
>> something counterintuitive about this I am missing.
>>
>
> REPLY:
>
> The issue is safety for the operator, not safety for the meter.
>
> The problem with B+ metering is it puts the entire B+ on the meter, which is
> normally mounted where someone could touch it. Personally, I don't trust 1/16
> inch of plastic to protect me from many kV. In fact, with many old meters, the
> adjusting screw is metal, which places the B+ right out in the open, ready to
> kill. With the meter in the B- circuit and protected by diodes, no more than
> one
> volt or so appears on the meter.
>
> Regarding the "dinky" diode between B- and chassis, conventional silicon
> diodes
> which are damaged by a massive arc always fail shorted, never open.
That depends on the size of the diode and it's ability to handle
current. A truly "dinky" diode", compared to the capability of the PS
is likely to fail catestropically and open. IOW, nothing left but a pair
of leads, while a heftier diode will indeed fail shorted.
I've seen many diodes blown apart, but those were some pretty
substantial power supplies too. I agree that in the amateur level power
supplies the likelihood of the diode failing open is vanishingly small,
but I don't like to use the word, "never". Of course in the case of a
major short the meter also fails in a rather spectacular (and smelly)
manner.
> That is what
> you want for protection. The only thing you would notice with a shorted diode
> is
> the meter(s) no longer indicate any current.
That depends on the base meter movement. If it's in the microamp range
you'll probably get some reading even with the shorted diode.
> That's the safest way.
>
> 73, Bill W6WRT
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