[Milsurplus] Progress Report: Cheap Tubes for the BC-375

[David Stinson]

A short progress report on my Summer project-
reviving a BC-375 with affordable tubes.

  I abandoned "Big tubes with low
cathode emission which make power with High B+" (211, 811, 813 etc.)
and am now investigating "Medium tubes which make power with
high cathode emission and moderate B+" (6146, 1625, Sweep Tubes).
Much better results, though not fully workable yet.
The biasing resistors in the 375 are not far from what's needed
for 1625. 807 would be the same tube were one working on a 191.
I'm using lowered B+ (400V regulated) while experimenting.

A triode-connected 1625 in the oscillator and a Pentode-connected
1625 in the PA with only 400V B+ make 14W carrier out on 3880KC
and, when modulated (with original 211s at 400V; haven't worked
that problem yet) make 25W Peak out.

I pick-off Screen voltage at the top of the modulation transformer
secondary and supply it to the screen through 50K resistor, bypassed.
Has to come from there because we must modulate the screen in phase
with the plate modulation or we'll "ash" the screen.
Don't run the Osc as a Pentode because I tried that and it made
way too much drive for the PA.  Triode connection works fine.
So things look hopeful.

The problem with this configuration I'll bet you've already guessed:
Neutralization.  With the PA run as a Pentode, the original Neutralizing
cap is much too big and, no matter the setting, acts as a giant
feedback path, causing instability and pulling the MO with
voice peaks.  No matter the setting, it's a 6KC+ deviation
  FM transmitter.
Going to try disconnecting that cap entirely.
Taking vacation this week, hoping to catch-up on correspondence,
get some "honey-does" done and still find time to tinker with
the BC-375.

GL OM ES 73 DE Dave AB5S

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