[Heathkit] SB-230 grid current meter is wrong

[Kurt Fitzner]

Hello All,

I have just refit my SB-230 to use a GI-7BT tube - since the Russian 
tube operates with a higher grid current, my intention was to alter the 
grid current meter resistors to scale the meter differently.  However, 
as I was going over the circuit, for the life of me there seems to be an 
error in it.

The grid current on the SB-230 is supposed to read 0 - 80mA.  The meter 
is a 1ma meter with an internal resistance of 120 ohms.  The grid 
current shunt resistor (R25) is 1.5 ohms, and simply math indicates 80mA 
* 1.5 ohms = .120 volts, which is correctly full scale 1ma with the 120 
ohm internal resistance.  The problem is, the circuit then goes to route 
the grid current with a 47 ohm resistor (R33) in series with the meter.  
This renders the meter a 0-111mA meter and throws everything off.

When searching for an answer, I found this old thread 
(http://www.eham.net/ehamforum/smf/index.php/topic,96457.0.html) over at 
eHam discussing this very thing.  No one there understood why Heathkit 
had done this either, and I'm wondering if anyone here knows why it's 
this way?  Is there something I'm missing?

Thanks,

      Kurt (VA1DER)