Tube voltage different under load

I was swapping around drifferent brand 12AX7 and checked there voltage under load , they all show a different voltage under load . one brand are show 75V other show 78V some show 120V , just all different ,why is that? can anybody tell me?

Also , I checked the schematic it said , B+ 105Volt  are required, then I put a tube in and I put 290Volt ( Unload ) , with a 12AX7 loaded it only read 75Volt ,,,,, hum............ is this normal? should I pump the voltage up to 105Volt underload?

All tube work is + or - 20%.

How do they sound??

However, good 12AX7 in normal circuits are fairly consistent. 75V and 120V is an odd spread.

> 290Volt ( Unload ) , with a 12AX7 loaded it only read 75Volt

This is of course normal. Look at how a tube works. The resistor pulls up. The tube pulls down. Usually you expect the idle voltage to be in the middle between ground and B+. Anywhere from 1/4 to 3/4 is usually a workable condition. Of course with the tube out, you only have the resistor pulling up to B+.

> the schematic it said , B+ 105Volt  are required

"B+"? Or Plate Voltage? They are different.

Thanks PRR! I was doing some experiment with the Plate resistor and the Cathode resistor. and I think the schemtic means plate voltage underload, but my AX7 does show very vary voltage underload , the lowest I seen are 70Volt and the higest are 125Volt when I put 290 B+ at them with a 100K plate resistor .........