yes all you need is a volt or multimeter. Make sure you know how to do this first.
You don't need to faff around with signal generators for distortion,
its frowned on, 70% power dissipation is a very common amount. See below
2 kinds of power valve bias..
1/grid or fixed bias american brute force technique
2/ cathode bias ince polite british method
I have the feeling you have grid bias, which is actually variable, tho' its also confusingly called fixed, cause you wouldn't need to bias if it wasn't grid bias!
all you do is measure the voltage across the resistor at the cathode of the power valve.
Find this by looking at the valve socket, and checking with a bottom diagram for your valve on the web.
Usually the resistor is about 10 ohms for fixed bias, and it will be a big high watt type.
Then calculate like this....voltage= whatever you find it to be.
current thro valve= volts you measure/ the 10 ohm resistor.
Find the maximum anode dissipation for your valve. el34=26 watts, 6550=35 I think, look on the web.
Measure volts at the anode, you have the current, so anode volts multiplied by current must equal less than 100% of the power or the valve will go kaput.
Typically adjust the pot for your grid biased amp so it gives a value 50-70% anode dissipation for your valve. vary it and see which sounds best, you can go to 95% if you wish, but valve life will be less.
great care, as there could be 600 volts at the anode, have your multimeter on dc volts . Use rubber gloves and don't touch!!
lets say its a 6550 which the yanks use a lot, as its big sounding and they like big
say the anode diss. is 35 watts, anode volts is say 500, so current is from
power=voltsxcurrent
35=500x current,, thus current= 35/500= 70 milliamps..this is the maximum, so bias to say 70%= 0.7x 70 mamps
=50 milliamps.
If your cathode resistor is 10 ohms, you tweak the pot until the volts is..
v=current x resistance.
v=50 mamps x 10R
v=half a volt measured across the resistor.
if your cathode res is a different value, you will need a different voltage. don't go adjusting for .5 volt, it all depends on the values in the circuit.
That is how to do it without any circuit info. anything from 50-90% power dissipation is what the bias is for.
aren't valves lovely?
If you can find the value of the cathode resistor, and the anode volts, I can tell you what your measured voltage should be...simple as that. Alt, check on the web for this amp, or phone the maker for info.