Hi, you can get a circuit from here
, and the tube you need is a 6P1. They are rare, and a little hard to find try eBay
. Chinese copies, but will do the job. Found this interesting info regarding an upgrade until you can find an original
The 6P1 is a slight oddball. They do crop up for not too much money andif you have room for a larger envelope a 6P25 is a good alternative.These are both high slope valves and an American substitute such as a6V6GT will not work well here.
If you are forced to make an adapter, I would suggest using a 6CH6 (B9Abase) and adjusting the bias. It will need less bias than the 6P1 soyou can just shunt the existing resistor - taking out the errors at thesame time. This also makes it easier to refit a 6P1 when you find one.
There is more info
on changing out to an similer alternative for more obtainable NOS tube.