What do you think you want to know about the valves Frank and do you have a lot of them to sort?
As Rob says, those bias testers just measure the voltage across an inserted cathode resistor, invariably one Ohm which means mV become mAs.
That allows you to set the cathode current by adjusting the bias voltage but (Rob again) tells you nothing about the 'quality' of the valve. Even if you find you cannot set the bias to the maker's value that does not mean the valve is a dud. I have had to mod bias circuits because they had too little range (both ways) But setting Ik does not tell you the dissipation of the anode (Pa). You need the anode voltage for that. To be totally accurate for Pa you should extract the G2 current from I k but since guitar amps should never be biased hotter than about 60% of the maximum value, that hardly matters.
You could of course check Ik at various bias voltages and get a family of Ik/Vg curves the slope of which will give you a ball park figure for 'mu' but since the AF amplification of any valve stage is more complex than simply mu not a lot of point? Have you delved into valve theory? The three, integrated parameters (for a triode) are mu, gm and ra. That is anode voltage change for grid voltage change with an infinite anode load. Mutual conductance and anode slope resistance. The last two are determined to an extant by circuit design and change with age. Mu is fixed by the mechanical spacing of electrodes so is not affected at all. (there are valves with a mu of 1000+!)
Pentodes and beam tetrodes are of course more complicated by the presence of the two extra grids.
If you want to check a rake of valves the best solution is a "test chassis" an old 50W say amp that you can bodge with sockets to check cathode, anode, G2 and bias voltages plus a dummy load and a sine signal sources to test AC gain. The analogue meters you get from car parts shops are quite good enough for the purpose (mains V won't be stable anyway!) Buy several.
OF COURSE!! To construct and use such a thing you must be well up on HV safety procedures!
Dave.
