[Boatanchors] BC375 and Dave's Excellent Tube Adventure, Part Two

[David Stinson]

(Posting this to non-military list because the idea of subbing expensive 
tubes
may be of use to someone in a non-military rig one of these days)

Since I've satisfied the restless spirits of my Celtic
and Confederate ancestors by a screaming, headlong-charge
into this project (with the predictable heavy casualties),
I decided that less bravo and more brain might be in order.
(Ya think??)

Note: All this work with the 1625s is "proof of concept."
I'm not building these assemblies to be permanent
or I'd make them neater and more rigid.
So don't laugh when you see the photos ;-)


Also- Yes; this is going to run at less than the specified
70 or so Watts out. The point is not to get exactly the same output
or the same performance as a set of 211s and a 10Y.
The point is to get the transmitter working on-the-air
with a set of tubes that cost $20 instead of *$1000*.

I decided to back the B+ on the 1625s down to about 500 Volts
and the screens down to something like "normal" for that B+.
Since the BC-375 needs a "floating B-" and I don't want to build-up
a new supply for a temporary experiment, I put a 5000 Ohm, 50 Watt
dropping resistor in the B+ lead from the dynamotor and that worked.
Installed 10 Kohm 5W resistor with a .004uFd 2KV cap across it
from the plate pin to the 1625 screen (pin 3).
This means the assembly is slightly taller than a regular 211,
and that means looong leads. So besides the parasitic suppressor in
the 1625 plate lead, I installed a .25x.25 in. ferrite bead in the grid lead
to help prevent parasitics. Here's a photo of the assembly before
installing it in the 211 base
(I did trim the leads shorter. Red is grid, green fils):

http://home.netcom.com/~arc5/BC375/375-1625B.jpg

When tuned-up with 211s in CW mode and the normal 1 KV B+,
the set rans about 200 mils Plate current for 70 watts out.
Running 500 B+ with these 1625s, I got just under 100 mils
for about 23 Watts out.
Neutralization was "touchy," but not very far at all from
the proper setting for the 211s.


One thing that shows up quickly- any gas in the 1625 oscillator tube.
Once a little of this ionized gas causes the tube to heat up,
the gas starts swirling-around and the oscillator wanders
all over the place. A good tube gave a solid note and as much stablility
as could be expected with the oscillator tube on stilts ;-).

http://home.netcom.com/~arc5/BC375/375-1625A.jpg

The 211s in the modulator at 500 volts don't like these and don't
modulate them well, but that's another problem I'll get to later.
For now and for the primary mission- a working CW 375 rig,
I think we're off to a good start.

73 OM DE Dave AB5S