SB221/220 LINEAR AMPLIFIER

[KB5MD]

Can anyone tell me the rating of the meter that reads the high voltage in the Heathkit SB221 amp?  Is it a 0-1ma or something else?
The meter on my amp has opened and I have several heath movements that I can change the face out of if
I knew the value of the existing meter.

[AB2EZ]

It's not a 1mA movement.

Looking at the HV divider chain:

3500V / (3 x 4.7 megohms) = 0.25 mA

The meter probably had a 50uA movement, with 22,400 ohms of built-in series resistance.

If so, 1/5 of the current in the HV divider chain would flow through the meter, and 4/5 of the current would flow through the 5600 ohm resistor that is in parallel with the meter when measuring the HV

Stu

[N8ETQ]

 
  200 uA ...

      Back to back 1N4007's would have saved it!

GL  

/Dan

[KB5MD]

Thanks, problem solved.

[AB2EZ]

Agreed!

350mA of grid current corresponds to a full scale reading, when the meter switch is in the grid current position (even though you would never allow the grid current to reach that high a value).

350mA x the 0.82ohm grid current shunt resistor = 0.287 volts across the meter for a full scale reading.

If the meter is a 200uA (full scale) meter, then its internal series resistance is 0.287V / 0.2mA = 1435 ohms.

Note: If you use a replacement meter with a 200uA movement, you need to make sure that it has a terminal-to-terminal resistance of 1435 ohms, or something close to that value. If the terminal-to-terminal resistance of the replacement meter is less than 1435 ohms, then you need to add a resistor in series with the meter to make up for the shortfall.
 

Cross checking against the plate voltage meter switch position:

3500V of plate voltage corresponds to a full scale meter reading, when the meter switch is in the plate voltage position (even though the actual plate voltage will never be that high).

This implies that there will be 0.248 mA flowing down the 3 x 4.7 megohm HV voltage divider chain when the meter is reading full scale. I.e. 3500V / 14.1 megohms = 0.248 mA

1435 ohms of meter resistance (as calculated above) in parallel with 5600 ohms (i.e. the resistor at the bottom of the voltage divider chain) => 1142 ohms.

0.248 mA of current flowing down the HV divider chain  x 1142 ohms (the parallel combination of the meter and the 5600 ohm resistor) = 0.284V => a full scale meter reading (as calculated above)

The cross check is okay.

Stu