Hi folks, want to put a nano into a tube amp mono block to monitor voltage and calculate current draws by the power valves. Voltages will be:
380 - 430 VDC x 1
~30-35 VDC x 2 (cathode resistors, one per output valve)
Current draw calculation. Cathode resistor 470 ohm, so mA will be the voltage drop...
Suggestions on ways to get these voltages into the nano? Doesn't have to be super accurate, rather it did to be consistent across the output valves so I can see if either is drifting.
Actual tube amps are a pair of Quad IIs I am rebuilding, and will have original KT66 or 6L6GC values put in.
Long term plan is to transmit these values to an Arduino in the Quad 22 box and drop some round LED displays into the old tone control positions (should drive the analogue purists nuts!). Along with a DAC Streamer to feed the Quad IIs
Thanks in appreciation...and having been around power for many years, don't need advice on the safety aspects of working with the amp HT
Trying to get a handle on whether to just measure the voltage drop and then get the nano to calculate mA using ohms law (as I know the value of the resistor)
Yes, the resistor has a capacitor across them 470uFx63V.
A voltage divider could work but the big problem is keeping the grounds separate.
The amplifier ground would be connected to the Arduino ground which is connected to your PC ground and/or some power supply ground. A good situation for creating a ground loop and introducing HUM in your amplifier.
You would need to run the Arduino from a battery or a good isolated power supply.
During software development you should use a laptop that is not connected to AC mains or a charger.
This should work well for the connections to the cathode resistors.
It scales the voltage by 1/8 and adds a 400K ohm resistance in parallel with the 470.
So the 470 ohm resistor is now 469.5 ohms
I'd use the supply - pole as GND to measure both the anode and cathode voltage. The Arduino can easily calculate the anode-cathode voltage and reverse the grid voltage.
So - and cathode should have the same AC voltage, with the AC sound signal.bypassed around the cathode resistor. Also check the cap for dry out over years.
Thanks Jim, have been doing some more research and perhaps the simplest method would be to use a ADS1115 in differential mode connected to the nano and looking at the voltage drop across a 1 ohm resistor that is in series with the cathode resistor/capacitor. Using a 1% 5watt wire wound and measure the actual resistance with no power and then use that value in the nano to get mA.
35V/470Ω = 75mA. That will produce a 75mV signal across a 1Ω resistor. A small signal to send through a cable and much less than the ADS1115 minimum range.
The current will vary depending on how much power the amp is putting-out, etc., so I'm not sure if it's worthwhile constantly monitoring it.
Constantly measuring the voltage might make more sense. But, if you don't have a voltage regulator, the voltage will vary when the AC varies. (I've only seen old tube amps and I've never seen one with regulated voltage.)
If you want to occasionally measure the current with no signal, or under test-conditions, you might want to figure-out a way to easily break the current-path and insert a multimeter. i.e. You could even have 5-way binding posts, with a jumper or a built-in switch for when the meter isn't connected, etc.
There is no truly non-invasive way to measure the current or voltage. You can still use the 1Ω resistor but I would put an opamp inside the tube amplifier chassis to raise it to at least 1V but that would require a power supply inside the amp chassis for the opamp.
The voltage divider will drop the 470Ω to 469.45Ω, that's less than 0.12%
You can use 1/4W or 1/8W carbon resistors.
Can you post the schematic for the Quad II. Maybe I or someone else will see a bettor way.
Note: The nano should not go inside the tube amp chassis, thats asking for noise problems.
Plan A, complete rebuild of the Quad IIs and just put external test point out the back with a suitable connector. HT and the voltage drop across the bias resistors. Measure with an external DVM and adjust fixed bias resistors to get the mA desired. Then leave the Quads alone.
Plan B, then focus on the Quad 22 build and stick suitable resistor divider network for the above into the Arduino and also install the DAC streamer. So the Quad 22 becomes the 'digital' box with separate power supply from the Quad IIs. Might need to look at the ADS 1115 differential inputs to remove the need to connect the Arduino to Quad II ground...
I'd be more interested in monitoring peak output-power. (1) (Peak because it's where you get clipping.)
And/or better yet, a clipping indicator that compares the power supply voltage to the output voltage. (With an output transformer, you probably have to do an indirect ratioed comparison.)
(1) Normally with a power amplifier you measure the voltage and then calculate the power based on the speaker's nominal impedance. Speaker impedance varies over the frequency range and it's just more useful to monitor the nominal power.
