[TA Forbes]

Are we definitely speaking of the same Bob?

My experiences have been excellent with him.

RE: tube substitution, there is a time and a place for it. Personally, I recommend them (Guides) if for nothing else than expanding one's knowledge.

And while there are definitely cases where the substitution of a tube can introduce different parameters, there are (my opinion) just as many in which the tube will function in an acceptable manner.

Many subs are not "either, or." Example: you can use a 5R4 in place of a 5Y3, BUT you cannot use a 5Y3 in place of a 5R4. Not that I would waste a 5R4 where a 5Y3 belongs, BUT if I was out of 5Y3s and had a 5R4 sitting in the stash, I would certainly use it until another 5Y3 came along.

In some radios the Manufacturer listed different types of tubes that could be used. Example: The Truetone D-697 on the schematic lists either a 6U7G or 6K7 tube for the RF and IF amp tubes. Another WG set does the same for the rectifier (5Z4 or 5Y3).