[Boatanchors] Crawfish Boil: BC-375 on 30 Back in Business

[David Stinson]

(Note: Copied to the "boatanchors" lists because the principles
involved may be of use in other rigs)

Re: BC-375 Tube Substitution 
and Keying Progress Report 11-04-2013

Finally replicated the success with the BC-375 on 30 meters.
I was going to address the drift/chirp problems separately 
from that of the tube substitution, but the two issues are linked.

As you know, I had some success in making the 375 useable on 
both 40 and 30 meters. The original problem is both fast drift
and chirp when trying to use the rig above 75 meters.
Here is what a BC-375 with 211 tubes in "all original" mode, 
using the PE-73 dynamotor at 1000 V B+
and with a stock TU-10 tuning drawer tuned (at first ;-) to 
10.108 KC delivering 50 watts into a dummy load:

http://home.netcom.com/~arc5/BC375/375baseline.mp3

My investigation has revealed several causes for this behavior:

1. A+ Filament Buss Sag.
The 211 Oscillator is a directly-heated filament tube. 
When the transmitter is keyed, the A+ buss will sag 
a small amount due both to the extra current drawn by the 
dynamotor and from I/R losses in the A+ supply cabling 
to the transmitter- a good reason to use the larger wire GE 
said to use there. Even a 1% drop in A+ (about a quarter of a volt) 
will create a marked frequency drift when on 7 or 10 MC.
I proved this by tube substitutions. The 211 has a large filament
that has a good deal of "thermal inertia." The 1625 (cathode to heater)
has much less of this property. It will drifter further and faster
if the A+ buss sags. The 837 tube, with a much bigger filament/
cathode at near twice the current and thus, with more thermal inertia 
than the 1625, falls between the two, strengthening the hypothesis.
As soon as I can fit it in the budget, I'll be ordering buck regulators
to fix the A+ buss sag and see if this helps.
Until these arrive, 
I'll concentrate on the other problems while using the 211s.

2. Thermal Drift
At full 1000 VDC B+, the 70-year-old frequency-determining 
components heat-up and drift, so the oscillator drifts.
Some of the old transmitting micas have started leaking a little
(some a lot). When hit with 1000 volts, they heat and drift.
Even a small amount of heating and drift at 10.1 MC will walk
your rig right down to WWV. 
Reducing the B+ to 500 VDC cured that problem.
The BC-375 uses a floating B- supply, keying by grounding
part of the grid resistor, thus removing cut-off bias from the tubes.
Here is a simplified diagram of 375 keying:

http://home.netcom.com/~arc5/BC375/375keying.jpg

I pulled the fuses on the PE-73, lifted the B- brush on 
a DY-8/ARC-5 transmitter dynamotor 
and jumpered it into the circuit box of the PE-73. 
This worked a treat. Here is the BC-375 on 10.108 KC, 
delivering 20 watts to a dummy load, 
using the 211s and 500 VDC B+. 
Chirp was still a problem, but 
there's no longer any chance of QRMing WWV.
Note that the clicking noise you hear is not "key clicks,"
but the keying relay.
Note that the frequency jumps you hear are a Hi-Z intermit in the 
keying relay I haven't yet addressed, so please try to ignore them::

http://home.netcom.com/~arc5/BC375/375origkeying.mp3

3. Keying Sequence.
When adjusted according to the GE 375 manual procedure,
the keying relay brings-up the transmitter first, then connects
the antenna. This is a certain recipe for chirp. 
The GE engineers were smart men- they had to know this.
Was it done this way to make the 375 easier to pick-out in
a noisy receiver? Who knows? I thought about re-adjusting
the relay to attach the load *before* turning-on the transmitter.
But that would be assuming I know more than the engineers
and I don't. Whacks and hacks are verbotten, so what to do?
The solution was to use the original keying line as a "transmit"
switch and key the B- lead, similar to what is done
with an ARC-5 transmitter. While there is still some 
residual chirp, I think you'll like what you hear from 
this this BC-375 on 10.108 KC, 211 tubes, 500 VDC B+
keyed via the B- lead, delivering 20 watts to a dummy load.
No keying shaping or spark suppression were used for this test 
(will work on that):

http://home.netcom.com/~arc5/BC375/375biaskey.mp3

No; it's not "break-in keying," but come one, guys-
are you really talking about running a traffic net with a BC-375?
"Can't have everything" ;-)

So a 500-600 VDC supply (the higher you go, them
more drift you will introduce) able to deliver 250 mA 
without fainting, a good keying relay with spark/click suppression 
and 6-8 amps of 24 VDC (plus the right connectors from Steve ;-)
and you're on your way with the BC-375 even on 30 meters.

Next:
Regulating the filament buss and trying the 7984 tubes,
which will have to wait until next paycheck.
Nice thing about pentodes/tetrodes in this service:
I can disconnect the screen lead (like in Drakes) and
watch the output scope while tweeking the Neutralizing cap
for minium. Works great. 1625s are about 45 degress from 211s.

Gonna work me some 30 meter DX with a war vet
designed in 1928.... sweeeeeeet....

GL OM ES 73 DE Dave AB5S