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having looked up 9d in the OED, i know what it is but that does not help me solve the clue!!
6 letters, thermionic valve: t?i?d?

http://en.wikipedia.org/wiki/Triode

triode

thanks.

happy to learn something new today; looked it up in OED (after Wiki); i notice that there is no reference to a triode in the OED definition of thermionic valve; would still be looking; many thanks; now complete.