Edmund H. Ramm wrote:
Can anyone please enlighten me as to why there is an 8BN8 heated
by only 6.3 instead of 8.4v, i.e. underheated by 25%, used in R2B?

What forced the designers to accept cathode poisoning instead of
using a 6BN8 valve?

73, Eddi ._._.

Beats me. You sometimes see detector tubes like the old 6H6s run at
slightly lower filament voltage because apparently the noise drops off
significantly and the extra filament voltage serves no actual benefit in
a detector.
This doesn't seem to directly apply in the 2B. Two of the sections of
the BN8 are simple diodes and could easily be replaced with their
silicon sisters. The triode section, however, must be operating at
significantly lower gain as a result.
If I have the story correct, the early model 2As used a 6AV6 in place of
the BN8. Later 2A models used the 6BN8.
Apparently the guys at Drake saw a need to keep working on whatever
concern they were having and going to the 8BN8 made sense from a
production standpoint. It "evolved" rather than being "planned" so to
speak.
Or it could have been an issue with availability of the tubes at the moment.
At any rate, I don't think 'cathode poisoning' would be an issue in this
case.

Pure speculation from WX4A.

-Bill