Monitoring a tube amplifier: ~450VDC and ~40mA

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]
 |                 |        |
 |                 |        |
V(hi)             68K      V(lo)
 |                 |        |
 |                 |        |
 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?

Voltage divider looks good.

Question: I don't like the idea of tying the amp's ground to the Arduino ground. What else can I do?

Ya got to.

Question: Are the Arduino analog inputs accurate in the 20mV to 50mV range?

not very.

Is there a better (Arduino-friendy) way to compute the current?

doesn't your amp already have a ballast resistor in the cathode? how is the cathode current normally returned to ground? got schematic?

Some folks measure return current in the B+ rectifier.

I've attached a classic (and very typical) tube amp schematic.

You'll notice the 2 output tubes (6V6GT) have their cathodes tied to ground. This is typically where amp techs solder a 1 ohm cathode-to-ground resistor, to make it easier to re-bias a new set of tubes.

Measuring return current at the rectifier wouldn't exactly work, because that would measure the circuit's currrent, not the tube's.

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Too bad about the grounded "source" connection of your "high voltage vacuum FET", you dont wanna add much resistance in there. Probably best to use 1 ohm and amplify the result.

Use an op amp that goes to the negative rail and you can run it off a single supply +5V. LM324 works, other choices may have lower offset error, etc., but remeber you can trim offset in software I'd say about 1k and 50k would be good resistor choices for a +5v Arduino, that gives +5v for 100 mA.

Wouldnt hurt to add a series resistor for Vi maybe 10k just in case the input gets connected to anything bad.