----- Original Message -----
From: "Bill, W6WRT" <dezrat1242@yahoo.com>
To: <amps@contesting.com>
Sent: Sunday, November 08, 2009 11:37 AM
Subject: Re: [Amps] B- questions
> 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. 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.
Most quality USA made meters, and especially the old military ones, will
safely withstand up to 2500V or so which is a common ham amp voltage.
Mounting the meter behind glass is a sure fire way of protection and was
used commercially by Collins and others. A meter in the plate lead measures
plate current. A meter in the cathode measures all currents.
>
> Regarding the "dinky" diode between B- and chassis, conventional silicon
> diodes
> which are damaged by a massive arc always fail shorted, never open. 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's the safest way.
It adds another failure point. A well designed amp should have only the
voltages safely removed during an event and once the fault is cleared a
reset will have it back on line with no other damage. Hams seem to be more
interested in dinky designs and shortcuts. Look at any of the larger
Ameritron amps. The dinky diode shorts after the tube short takes out
everything in its path....some protection!
Carl
KM1H
>
> 73, Bill W6WRT
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