[Amps] Meter issue

[Vic K2VCO]

Recently, someone explained that diodes alone don't protect meters. The 0.6v across the 
diode is enough to seriously damage many meters.

I found out the hard way the other day when I was trying to fix something that wasn't 
broke and accidentally shorted the screen terminal of a tube to ground. The screen meter 
was destroyed, despite the back-to-back diodes.

Here is the original discussion by W6WRT and GM3SEK -- needless to say I'm going to follow 
their advice:

>> Just a word of advice - back to back diodes across the meters adds zero
>> >protection. Diodes don't conduct until the voltage across them reaches
>> >about .6 volts and by then the meters will be toast. You need to add a
>> >resistor in series with the meter of such value that the voltage across
>> >the meter + resistor equals about .6 volts when the current is just
>> >above full scale. Then place the diodes across the combination of meter
>> >+ resistor. Use heavy diodes such as 6A10 and your meters will be nearly
>> >bulletproof.
>> >
> Alternatively, use a low-value resistor in series with the current path,
> and configure the meter as a voltmeter using a 'multiplier' resistor in
> series.
>
> The protection diodes are then connected in parallel with the low-value
> resistor. As Bill says, use large diodes like the 6A10. The aim is that
> NOTHING should burn out in an overload, except a power fuse or breaker.
>
> Aim for 0.5V across the silicon diodes when the meter is at full scale.
> This is -just- below the threshold of conduction, so the meter will read
> correctly all the way up to full scale; but beyond this point, even a
> slight overload will make the diodes conduct. Thus 0.5V gives maximum
> protection to the meter without affecting normal operation.
>
> There is no need for elaborate calculations or precision resistors. Use
> a trimpot for the meter multiplying resistor and then calibrate the
> whole thing by pushing a known value of current through it from a small
> external power supply. Simply adjust the trimpot until the reading on
> the meter agrees with an accurate DMM.
>
> It's much easier to do this than to describe it. There are more details
> in the user manual for the Triode Board (via the link below).
>




On 5/27/2011 6:48 PM, Mitch Cox wrote:
> BTW, those diodes need to be much larger than 1N4004's.
>    ----- Original Message -----
>    From: Paul Decker
>    To: amps at contesting.com
>    Sent: Friday, May 27, 2011 9:42 PM
>    Subject: [Amps] Meter issue
>
>
>
>
>
>
>    I've been trouble shooting a problem in my amp with the Ip and Ig metering. I've got parallel 1n4004 diodes with opposite polarity accross the meter terminals of each Ip and the Ig meter, nothing new, pretty standard.
>
>
>
>    It's easy enough to replace the diodes, but what would cause one of the diodes in both meters to fail in the shorted case? There are no other apparent problems and the amp still works save seeing the meters deflect.
>
>
>
>    Thanks
>
>    Paul
>
>
>
>
>
>
>
>
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-- 
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/