Peter Chadwick wrote:
>
>Rich says;
>
>>Adding 3A diodes across meter movements is probably a good idea
>
>Unless the meters are 500mV drop (rather than the usual 100mV), I'd argue that
>there's no 'probably' involved - it is a VERY good idea - in any amplifier.
Even if the meter has a 500mV drop at full scale, a 1N540x in parallel
will make <1% difference in the reading. Go much above 500mV, and you do
see a difference because the diode starts to conduct.
On the other hand, if the full-scale voltage drop is significantly less
that 500mV, the diode offers much less protection. For example, if the
normal voltage drop is 100mV full-scale, the diode won't even start to
conduct until the meter is more than 5x overloaded.
500mV is a kind of "magic number" that offers the best balance between
accuracy and protection. IMO it's very worthwhile to *make* the full-
scale voltage drop exactly 500mV if it's presently lower than that.
You can do this by increasing the shunt resistor and inserting a trimpot
in series with the meter. Adjust the trimpot by pushing the correct
full-scale current through the meter circuit from an external DC supply,
with the amp totally unplugged. The meter scale calibration stays the
same.
The only disadvantage is that the effective cathode bias is increased a
little more than usual at higher grid currents.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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