Svante Nordenstrom wrote:
> A final note on this issue: 1625's are 807's with 12.6V filament, all
> other parameters are the same.
Except that the suppressor grid on the 1625's is tied directly to the
cathode internally (inside the tube base). This means that if you try
to use an unmodified 1625 in a GG triode connection, you have a big
problem! The 1625's used in such amps were modified so that the
suppressor grid could be grounded.
I recall trying to soften the glue on the base of the tube with a torch
so it could be removed. Another method involved drilling a hole in
exactly the right place, heating the appropriate pin and pulling the
lead out with tweezers. I think some brands of 1625 could not be
modified because the connection was actually inside the tube envelope.
In the 50's NIB military 1625's were essentially free, so this was quite