Carl wrote:
>
>Would not diodes across the resistors change meter calibration?
>
A normal silicon rectifier diode has no detectable effect on meter
calibration at 500mV (I checked with a DMM) but it starts to conduct
significantly at around 600mV. That's why it's nice to design the
metering resistors for a full-scale drop of just 500mV, so that
protection starts almost straight away on an overload.
In the case of R3 in the SB-220, the full-scale voltage drop is 350mA x
0.82R = 287mV so there is no problem with meter calibration. Surge
protection will start at about 600mV across R3, which corresponds to
about -700mA. Note the minus sign - a current surge coming up from
chassis will drive the meter NEGATIVE. The current through the grid
meter itself will be limited to about -300 to -400uA, which it should
handle OK.
For the anode metering resistor R1, the full-scale voltage drop is 1A x
1R = 1V, so it needs two diodes in series. Protection starts at about
1.2A, and the maximum current through the anode meter would be less than
2x full-scale.
>That metering circuit is begging for a complete redesign. The meters BTW
>are 200ua at 1400 Ohms.
>
It doesn't look too bad as it is, if the protection diodes are added.
The critical factor is that the protection diodes MUST hang in there
until the surge is over. Considering the cost of the meters, big fat
diodes like the GI750 look like a good investment.
Rich - this is basically your protection circuit with the diodes strung
in series. I hesitate to make specific recommendations for an amp that I
haven't seen in years, so please can the SB-220 experts confirm whether
the following make sense?
* One big diode with anode connected to point K (chassis) on the
rectifier board, cathode to F.
* While you're there, add a 1N400x from F to K (the opposite way) to
protect the grid meter against forward grid-current overloads.
* Two big diodes pre-wired in series, with cathode to F, anode to B or
C.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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