Randy
The is no need to switch out the meter's series resistor when peaking the grid current.
Looking at the manual, the Heath engineers didn't attempt to set the grid current of the AT1 at a particular target value. They just ask you to tune the oscillator to peak the grid current.
They could have designed this circuit to actually measure both grid and plate current... and, if you want to do that, here's how:
I would suggest that you pick a meter whose existing scale makes the most sense for reading the plate current. Since the AT1 manual asks you to adjust the plate current to around 60-80mA, a 10 mA meter seems,
at first, like a reasonable choice... because the scale reads 0-10. However, if you pick the series resistance of the meter so that 100 ma of plate current corresponds to full scale ("10") on the meter... then the meter will tend to be "pinned" until the plate current has been "dipped". The required total series resistance is ~500 ohms. [actually 459 ohms, if you do the calculation more accurately]
The above is how the Heath engineers designed the AT-1.
If you pick the series resistance of the 10ma meter to be approximately 750 ohms, then the meter will read approximately 5 (half scale) when the plate current is 75 mA. [The approximate calculation is: 75mA x 51 ohms ~ 3.83 volts across the plate current shunt resistor. 3.83 volts / 750 ohms ~ 5 ma (half scale on a 10 mA meter). The exact calculation yields a required series resistance of 714 ohms instead of 750 ohms]
Another alternative would be a 15 mA meter, like the Shurite 8304Z.
http://www.primetechnology.com/PartsDetails.aspx?PartsId=77&Parentid=28&ParentType=CWith a 15 mA meter and a 459 ohm* total series resistance (total of meter intrinsic resistance + added external series resistance), the meter will read 7.5 (half scale) when the plate current is 75 mA.
[*This is a more exact calculation than the approximate calculation I used above. With the 459 ohm series resistance of the meter in parallel with the 51 ohm shunt resistor... 90% of the plate current will flow through the 51 ohm shunt resistor, and 10% of the plate current will flow through the meter. Thus, when the plate current is 75 mA, 7.5 mA will flow through the meter]
Now, for the grid current measurement.
Looking at the existing AT1 design... when the meter is switched to the "grid" position, the meter (459 ohms of series resistance) will be in parallel with the lower 47k ohm grid resistor... and thus, most of the grid current will flow through the meter. However, when the meter is switched out of the "grid" position, the actual grid current will drop, because the total resistance from grid to ground will nearly double. Therefore, the original AT1 grid current metering design does not give an accurate measurement of the grid current that flows when the meter is switched out of the "grid" position.
What to do?
Alternative 1: Don't worry about it, just peak the grid current with the meter in the "grid" position", and assume that the grid current that is flowing when the meter is switched out of the "grid" position is about half as much as what the meter read when it was switched into the "grid" position.
Alternative 2:
1. Add a 4130 ohm (3900 ohms would be okay) series resistor between the existing, lower 47k grid resistor and ground.
2. Move the wire that currently goes from the meter switch to the top of the existing 47k ohm grid resistor... so that it now goes to the top of the new 4130 ohm resistor.
When the meter is in the "grid" position, 90% of the grid current will go through the meter, and 10% of the grid current will go through the new 4130 ohm series resistor added in steps 1. and 2. above. (If you use a 3900 ohm resistor instead of a 4130 ohm resistor, then 89.5% of the grid current will flow through the meter)
The meter will read 0.9 mA when the actual grid current is 1ma.
When the meter is switched out of the "grid position", the effect on the total resistance from grid to ground will be negligible.
Best regards
Stu