Discussion in 'Amateur Radio Amplifiers' started by N8VUL, Jul 2, 2020.
Did you price a meter from Ameritron?
wrong size. The newmeters are 2.5x2.5 in. Smaller. The original is 3.5 x 3.0 in.
Got a reply from Ameritron today. The service tech who works on the amps said a meter from an Al82 with a 3 ohm resister would do the trick. I ordered them and am waiting. Thanks to everyone who had good ideas.
Normally the AL82 has a 300 ohm internal resistance whether it has added resistors or not. The AL 80a manual shows the meter in it is either 430 or 450 ohms? Anyhow when you get the meter Ameritron sends you, check the resistance. Based on the 80a manual you’ll have to adjust the internal resistor so that the total is equal. IIRC the new meter will measure 300 ohms. Three ohms just doesn’t sound right.
Here is the reply from Ameritron....
We do not have the original meter for that amp, but the meter from the AL-82 will work. There is a scale difference. The 80A meter reads 0-300mA while the AL-82 meter reads 0-400mA. However you can add a 3 ohm 1/2 watt resistor across the meter to make it read correctly. we have both the meter and resistor in stock. You would need to call 662 323 8211 for ordering.
But I will check it.
OK, I ran the numbers based on both amps and now that I have what he told you to do he is correct. But he did not tell you that you have to swap the scale. You do put the 3 ohm resistor across the meter but you also have to put the 80A scale on the meter. Since the grid meter only does one function putting the 3 ohm resistor across it is fine. Here is why it works. The internal resistance of the AL-82 meter is 300 ohms and the FS current is .001A making the meter have a FS voltage of .3 volts. The AL-80a has a 1.5 ohm grid shunt resistor. Putting a 3 ohm resistor across the meter essentially parallels the existing 1.5 grid resistor. 1.5 ohms in parallel with the 3 ohms is a total R of 1 ohm. Being the 82 meter is FS voltage is .3v and now the effective grid shunt resistor is 1 ohm, when 300ma (.3a) of grid current flows the meter will be full scale. So that does work quit nicely. 3 ohms across the meter and swap the scale. that works.
Rodger Wilco. Thanks
I got the meter today. It is the same size and made by Prime instruments.(BTW, Prime wanted over $180.00 to rebuild old meter) After installing Face plate and installing 3 ohm resistor it should be good to go. Thanks for everyone's help. 73 de N8VUL Bill
And what was the price of the meter from Ameritron?