[HBR] Need assistance: external meter shunting

[Walt Hutchens]

> I have a 50 uA meter (full scale deflection). <SNIP> The parallel
> rheostat R5 is a 200 ohm unit on the schematic. I suspect that
> because of my meter's lower current requirments, R5 should decrease
> as well. Since the proportion of decrease of the meter current is 20
> times, I suspect it stands to reason that the shunting rheostat
> should also be reduced 20 times to 10 ohms. Is this thinking correct
> or am I off my rocker?

Not off your rocker but this is a bit tricker than it looks.

The circuit in question is at:

http://www.qsl.net/k5bcq/HBR/HBR13Cp2.jpg

This is a bridge-type circuit.  R6, the cathode resistor of V8A, is
adjusted so that when there is no AGC voltage (no signal) the bridge
is balanced, that is, the current through the tube is the same as the
current flowing in the 56K resistor.  That makes the voltage at the
two ends of the meter the same so no current flows through it and it
indicates zero.

When the AGC voltage increases (more negative) the current in V8A
decreases and the voltage on the plate of the tube and the + end of
the meter rises, causing it to deflect upscale. I haven't read the
directions for this design, but typically the 200 ohm variable
shunting the meter would be adjusted for something like full scale
deflection with V8 pulled out of its socket -- i.e., with no current
flowing in the tube, corresponding to complete cut-off by AGC voltage.

Basically the current flowing through the upper 470 ohm resistor and
the internal resistance of the meter as shunted by the 200 ohm
variable is produced by the voltage drop across the lower 470 ohm
resistor in series with the 56K resistor.

Now here's the tricky part: the internal resistance of your 50 uA
meter will be WAY higher than the 0-1 MA unit in the design.  I just
measured a few meters: Two 1930's GE 0-1 MA meters were 44 and 71 ohms
respectively; a more modern GE meter (60's?) was 100 ohms.

However two 1960's 100 uA meters were 1115 and 1230 ohms respectively.
I couldn't immediately find a 50 uA unit but I'd guess it might be
2000 ohms or more.

The good news about this is that very roughly speaking the full scale
VOLTAGE across the meters will be the same -- ballpark 0.1 volts for
full scale. And the same (probably) would be true for an 0-50 uA
meter.

Thus I would bet that an 0-50 uA meter would be 'plug-and-play' in
this circuit.

However when you look more deeply at the circuit, you can see that
W6TC did better than just provide an S-meter that works: The circuit
is so designed that only a complete scrambling of parts could put
enough current through the meter to damage it. If you get the right
parts in the right places then any single part failure, rosin joint,
or other OOPS! will leave that meter undamaged. (When I first drew a
pay check in 1961, new meters cost about three days pay ...) With the
50 uA unit that won't be quite so true: The 56k resistor will pass
about 5 MA; enough to likely be fatal to the meter movement.

I'd consider putting parallel (opposing polarity) silicon power diodes
across the meter before turning the new set on for the first time.
That would limit the voltage to something like 0.6 V which the meter
should survive. (There would be no effect on readings.) In addition,
I'd wire the circuit but leave the connections to one meter terminal
tied together but not hooked to the meter, then use a voltmeter to
confirm (when the set is powered on) that the voltage between the
points that will be connected to the meter can be adjusted to zero
with R6 and that with V8 pulled, the 200 ohm variable can be used to
set 0.1 volt with the proper polarity.

This circuit does have the meter at roughly plate supply voltage.
Shorting the negative meter terminal to the chassis (in the W6TC
circuit) with the power on or a charge in the filter caps is likely to
destroy the movement. Additionally, putting high voltage on such
prominent above-chassis points is a safety hazard. Although I would
never, ever, make either of these mistakes (oh no, never), I prefer a
circuit that puts the meter in the cathode circuit, where voltages are
much lower.

Walt Hutchens
KJ4KV