There is a couple of ways to determine each individual tube of a quad's plate current. The most accurate way is to "break" the plate circuit & insert a Milliamp meter (between the valve & it's OT primary lead). I use one of these for this purpose
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It is simply an octal plug & socket that is inserted between the valve socket & valve. All pins are connected between the plug & socket, except the plate pins (pin 3), which have the one of the two leads connected to each of the pins 3, thus breaking the circuit. Connected to a mA meter, this gives a direct & very accurate reading of that tubes plate current. Note, you "must" have the Milliamp meter connected when you power the amp up, otherwise the plate won't conduct as it won't have the B+ voltage on it, but the screen grid will conduct & maybe over-dissipate (red-screen or even achieve meltdown).
Another way is to insert 1 ohm (use minimum 1 watt) resistors between each power tube cathode & ground. They need to be 1% tolerance or better for accuracy. Reading the millivolt drop across these will give each tubes "total" current draw (combined plate & screen current). 1 Mv voltage drop = 1 Ma current. As it's giving the tubes total current draw you will need to calculate the screen current & subtract it from the total current draw to get the plate current. To calculate screen current, first measure & note the actual resistance of the tubes screen grid resistor (amp not powered up), then, with the amp powered up & stabilised, read the voltage drop across the screen grid resistor. Now divide the voltage drop figure by the resistance of the resistor to get the screen current.
Once you know all tubes plate current you can work out which tubes to pair up to get the best side to side plate current balance. Hope this helps. Cheers
Edit:- Some people actually prefer somewhat mis-matched power tubes as they say it gives better harmonic content. Cheers
