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

[Richard Brunner]

Nice work!  I had a similar experience with the GRC-9.  The filaments were 
regulated by selenium rectifers forward conducting, but with age they become 
more or less equally non-conducting in both directions, and, key-down the 
filament voltage went UP!  The filaments were supplied through dropping 
resistors, and the tube space current looked like a resistor from the 
filament line to B+, and naturally the filament voltage went up.  I 
regulated all the filament voltages, and it works fine now.  It was quite a 
change, from sick Canary to a usable transmitter.

Richard, AA1P

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: <milsurplus at mailman.qth.net>
Sent: Monday, November 04, 2013 9:07 AM
Subject: [Milsurplus] Crawfish Boil: BC-375 on 30 Back in Business


> 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
>