807 valve in place of the KT 66 valve

Nowadays NOS and used original GE KT 66 valve are becoming very scare and also expensive.

So why do not use a 807 valve in place of the KT 66 one.

The 807 has more or less same characteristics as the KT 66 valve.

Both are beam tetrodes except that the 807 has a higher anode
dissipation (807=  25 and KT 66 = 21 Watts) and higher anode
voltage (807= 750 and KT 66 = 400 volts).

The 807 has an UX5 base whereas the KT 66 has an octal base. 

Because the 807 has a top cap anode it is not such a popular audio valve.

Somebody I know had replaced the the KT 66 in a Quad II with 807 ones
and the set worked well with the replacement valves.

In the original publication "The Williamson Amplifier.  A Collection of Articles
reprinted from "Wireless World," on "Design for a High-quality Amplifier" by
D.T.N. Williamson (Second edition)"
On page 36 under the heading "Replies to Queries Raised by Constructors"
regarding the use of other valves when KT66 valves are difficult to obtain:
"The 807 may be used at the full rating of 25 W, with modifications to the
valve connections."

The 807 valves are not so cheap anymore but relative easy to obtain.