• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

will E80CF fit into yaqin 100b

How could it possibly?
12AX7 is a dual-triode and E80CF is a triode-pentode.
At least you need to rewire the socket an strap the pentode for triode-use.
Since the 12AX7 contains hi-mu triodes and the E80(C)F triode is a low-mu you'll loose a lot of gain.
I strongly suggest you drop the idea....


rgds,

/tri-comp