12DT8 Valve

[Mike Watterson]

12DT8 has a 12.6V 0.15Amp heater.  Pin 9 is the intersection screen connection.

From the Radiomuseum:  https://www.radiomuseum.org/tubes/tube_12dt8.html

12DT7 characteristics are more similar to 12AT7/ECC81 than 12AU7. 

Disconnect pin 9 from chassis.

In fact the 12AT7 /ECC81 seems identical except that 12DT8 screening is to pin 9 and has no centre-tap heater, but 12AT7 has the heater centre-tap to pin 9 instead.
https://www.radiomuseum.org/tubes/tube_12at7.html

The 12DT7 may only be preferred in a very few applications where additional RF isolation between the two triodes is needed. The two triodes are well spaced apart in the 12AT7/ECC81

Do what Geordie writes, simply cut pin 9 connection and use an ECC81. Even if it's in the VHF it's unlikely to be any more effect than changing a 12DT7.