I work on tube amps alot, but I'm just getting into the world of Arduino. I'd like to build a monitor to display operating information for a guitar tube amplifier. Ideally, I'd be able to display the following information in an LCD output:
Plate voltage: typically 350 - 460 VDC
Cathode current: typically 20 - 40 mA
Watts: (V * I), typiically 5 - 40 W
Temperature: not sure what the typical tube socket temp is
Plate Voltage
I'm thinking I can use a voltage divider circuit to bring the high VDC down under the 5V range. The maximum VDC I ever realistically anticipate is about 525VDC, so I've spec'd out the voltage divider to handle up to 750VDC and put a very tiny (<.08mA) demand on the circuit. The voltage divider circuit needs to be located at the tube socket to keep the high VDC away from the Arduino.
o------- 10M -----o--------o-----> Arduino [A0]
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V(hi) 68K V(lo)
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o-----------------o--------o-----> Arduino [GND]
V(lo) x (68K + 10M)
V(hi) = -------------------
68K
Example: 3.1V is read by the Arduino [V(lo)], then V(hi) = 459V.
Question: I don't like the idea of tying the amp's ground to the Arduino ground. What else can I do?
Cathode Current
The easiest thing to do is to place a 1 ohm resistor between the tube's cathode and ground.
Measuring the voltage drop across the 1 ohm resistor makes it trivial to compute the current -- 1mV = 1mA.
Question: Are the Arduino analog inputs accurate in the 20mV to 50mV range? Is there a better (Arduino-friendy) way to compute the current?