OK. I have some good news and
some bad news about the frequency
change for my LPB RC-6A.
The 1020 KC crystal works. I don't
know how robust it is, because it
is an FT-243, but the transmitter
is on 1020 kHz, now, and not too
far off frequency. Only a couple
of hundred Hz. Maybe I can get it
closer to 1020.00 by doing something
with an external cap - maybe.
The bigger problem is - I can only
get 2 watts out of the transmitter
instead of 6 watts, with a dead carrier
and no modulation. No matter what I
do, or what I try with the adjustments,
that is the best it will do.
If I run modulation, the watt meter jumps
up to 4 or 5 watts, but I believe it's not
supposed to operate like that. The watt
meter is not supposed to move with modulation,
right? When this happens, the neon light
lights up also, but again, this does not feel
right. This tells me that things may not
be too far off, but I do not want to hurt
the 6AL11 final RF section.
RFB, you are familiar with the coil and the
2 caps that are in there. It is my belief
that this RC-6A is really just for the lower
half of the AM BCB. 1020 is too high, but it
is the best I can do right at the moment.
1020 is a very good daytime channel.
Even for me, with my poor eyesight, the caps
and coil do not seem impossible to reach.
I have not been really able to get a good look
at the LC network without a good light, but
is there anything I can do - such as changing
the inductance or capacitance, as needed, with
some external parts? I MIGHT be able to do this.
RFB, I really need your experience now.
What do you think?
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
Unfortunately my manual to the RC-6A was lost some years ago during a move. The RC-6A unit I owned had an inductor coil with 4 taps on it and was a bit longer than the band specific inductors. The tap closest to the ground point of the inductor was for the high band and the tap furthest away from the ground point was for the lower band.
It may be possible to re-tap the current inductor coil to bring the LC circuit closer to resonance. The two caps may have to be changed as well, but the main part is that inductor.
I know that's not much to go on but this is all from memory and a few old notes I found in my file archives.
One thing I do remember vividly is when I changed frequency from 750Khz to 1150, the tap moved from the point furthest away from the ground point of the coil to the tap closest to the ground point. Which specific points along the coil where the taps were in relation to coil turns I can't remember. Once the tap was moved, it only took about a turn or two of the slug to tune it up, and about a quarter turn on the two variable caps.
RFB
Thanks RFB.
Well, I would definitely START with
the coil.
Maybe I can use an external component
along with the coil to reduce the apparent
inductance.
This is going to take me a long time.
I guess it's not a good idea to run
the transmitter in this condition -
is that correct?
Best Wishes and Thanks For Being There,
Bruce, DOGGRADIO STUDIO 2
Thinking about Bruce's results at 1020 compared to earlier results at 860, it does seem that the transmitter is not optimized for the higher frequency. That seems like the most likely reason for the lower output wattage.
But could the crystal itself have less output or some other mis-match that would cause the weakened performance?
How can the safety of the tubes be estimated while operating in this out-of-ideal condition?
Carl, your point about the crystal is well
taken.
However, (and this is only an educated
guess) after dealing with lots of funky
old basic vacuum ham transmitters, this
is what I think:
If the crystal IS oscillating - it is
probably enough to sufficiently drive
the next stage. (That's just my guess.)
The immediate question is: if I run the
transmitter in this state, will it hurt
anything. I looked up some specs
regarding plate dissipation of
the final 6AL11, which is a
double pentode. The two pentodes in the
envelope are not the same. There is a
beam pentode and a dual control pentode.
I am going to assume that the beam pentode
section is the final RF amplifier. (I really
don't know for sure, but the plate dissipation
of that section is 10 watts.) The other pentode
section (the dual control pentode) has a plate
dissipation of only 1.7 watts.
The transmitter is supposed to put out 6 watts
under the correct conditions. If the final amp
is running at 50 percent efficiency, then we
are looking at 12 watts input. Maybe.
Under this present off resonance scenario, two watts
are coming out of the RF connector on the back
of the RC-6A. If 12 watts of DC input is going
into the tube, and 2 watts RF is coming out, then
10 watts is staying inside the tube. That is
the maximum amount a 6AL11 can dissipate. So
I would call it borderline, depending on unknown
variables that exist. And, there are plenty of
real world unknown variables. So, it might be OK
to run the RC-6A in it's present condition.
I REALLY WANT TO KNOW WHAT RFB THINKS ABOUT MY
ABOVE OBSERVATIONS.
I still have the old 6AL11s, I could just put
one of those in the final socket, and if it
gets soft, well, who cares?
I know energy is staying inside the transmitter,
because the transmitter is "Singing." In other
words, I can hear the input test audio coming
from the inside of the transmitter itself. It
is coming from SOMEWHERE, you know - without
a speaker. I haven't gotten my ear close enough
to tell from where. But maybe the music sound is
originating from RIGHT INSIDE the final 6AL11. From
the beam pentode plate.
I have to get back in there and stick my ear right
into the transmitter deck. This will be interesting.
Please let me know what you think.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
"But could the crystal itself have less output or some other mis-match that would cause the weakened performance? How can the safety of the tubes be estimated while operating in this out-of-ideal condition?"
Well fortunately the RC-6A isn't a suitcase nuke and won't bring about the end times due to a new crystal and off-resonant final! Worst case that could happen...well what is happening at this moment, low output power.
I've never heard of a crystal making the oscillator oscillate in a "weak" state, it either begins oscillation or it does not. A crystal however can be inadequate in handling the current flow within the LC oscillator tank circuit and be damaged. This is more true with the tube based oscillators, but in either case, the oscillator either does it's thing or it does not.
The RC-6A uses one half of the same tube as the oscillator and the second half as the final. With the output LC tank out of resonance, it could be presenting an unusual loading characteristic within the tube itself but not so much as to worry about the quickest way to the underground shelter! :p
"I guess it's not a good idea to run the transmitter in this condition - is that correct?"
I would not run it for any long periods of time without that LC output tank tuned properly. The off resonant LC circuit will cause the final section to draw more current than it would on resonance (dip in plate current at resonance) and is not healthy for the tube..will cause accelerated degradation and effectiveness of the tube if ran this way too long.
RFB
Putting a path through a wall for the stranded wire out to the ground rod.
Wall = 8" thick;
Present hole as drilled in 1-hour = 6.5";
Distance remaining = 1.5".
No rush.
Sounds like you are on your way to making some ground!
RFB
"I REALLY WANT TO KNOW WHAT RFB THINKS ABOUT MY
ABOVE OBSERVATIONS."
Absolutely correct. The power missing from the RF output connector due to the off resonance LC output tank is bouncing around in the final tube, as I noted earlier that this off resonance will present unusual loading effects within the final tube...and yes it will sing and sing and sing with every tone and note put through it in modulation.
Not to mention some of that wattage will bleed over into the oscillator section of the tube. Basically the final is acting like a blender set on high and creating a mix mash of off-resonance mush within the final.
RFB
1) Carl! Congratulations to you!
You are getting very close!
2) RFB, you sure are right about the
"singing" coming from in the transmitter.
It is definitely coming from the
modulation transformer. I'm pretty sure
I can hear it from inside the final tube,
also. It's just hard to get my ear in
there.
3) RFB, if I break the connection at the
hot end of the coil (not the cold end, and
not the tap for the neon bulb) and put a
small cap in there, such as 10 or 20 pF,
will that bring the transmitter up in the
band (if the tuning caps ARE NOT in the
equation) THEORETICALLY??? Again, is this
a place to start? It's easy to do. Do
you think this is a valid idea? Again,
this is all theoretical. I am not asking
for any specific component values. I think
it's just good to start with a small cap
value.
Best Wishes,
Bruce, DOGGRADIO STUDIO 2
Ladies and gentlemen, by your feet you will notice some dirt. If you look closer you will notice a ground wire coming up out of the dirt and clamping to a copper ground rod just poking above the dirt. That was not as easy as it looks.
RFB makes things sound easy. "Just sink a ground rod", "just run a stranded copper wire out from the wall connected inside to the neutral connection on the TCU-30 coupler". Easy as pie.
Well I do not find pie easy. It took days of planning, strategy, tools, measurements, strength and time away from normal things like sitting somewhere and wondering what it would be like sitting somewhere else.
But now it's all there. The line output is firmly attached to the panel neutral buss and we are about to test the neutral injection method of carrier current part 15 broadcasting.
The results will be ready just as the part15.us site switches over to the new super part 15 site.
Making pies, especially from scratch is mostly a messy task where 90 percent of all the ingredients end up on the counter and floor and the remaining 10 percent is in the pie pan cooking in the oven.
As I mentioned, it took me more than a month to discover that "easy" solution. The reason for that is because, and I restate it once again, that not every site is going to be exactly the same or require the same approaches to making it work. Every single location will be different, count on it.
You discovered that hot coupling was..well not so hot after all. The vswr never got anywhere close to the upper green of the meter, and that in of itself is very odd.
However this is a perfect example of how things vary from one site to another. Theory on paper is just that, theory.
The real world is gonna tell you something completely different and will require some real world thinking and real world solutions.
Your doing fine and making giant leaps for all mankind here! Since it's been 42 years since the great gig trip, your progress is the most exciting thing happening today!
Look forward to the drive test results.
RFB
"if I break the connection at the hot end of the coil (not the cold end, and not the tap for the neon bulb) and put a small cap in there, such as 10 or 20 pF, will that bring the transmitter up in the band?"
Possibly. However the tap really should have a direct connection onto the inductor coil. You could try it and see what happens, but I think that will throw out the LC network from functioning all together.
See our problem here is that we need to be going up and the inductor is wired for the low band. But that could work to our advantage. Everything is there already to shift this thing up the band using the existing coil inductor. The way to do this is to simply reduce the number of turns on the coil, or re-tap it further down the coil windings..ie closer to the cold end.
Usually what I do with an unknown inductor in a situation like this is to imagine dividing the coil up into even segments. Then carefully place taps at each of those segments. This is a shot in the dark approach, especially for non-variable inductors. But in this case we have a variable inductor so we have some elbow room for the variables of the unknowns. It's a place to start and with
a little luck, just might hit the bull's eye the first shot.
What would help is finding the schematic to this puppy. I had no luck trying to find my old manual, that probably got lost way way back in the 70's after getting the TX 2-20 replacement.
The extreme approach would be to remove that inductor, and start unwinding it and then re-wind it and adding the tap points during the re-wind. I knew a guy who used to rebuild 1930's radios and actually did just that, re-wound inductor coils, transformers, and even replaced no longer available tubes with solid state circuits inside the tubes with little light bulbs to simulate the tube glowing.
Sometimes the extreme approach is necessary..but let's see if that can be avoided here with your idea Bruce. It's worth a try, and may or may not work, but we will never know until you do try it. Worst that will happen, it doesn't work and you learned a new way of not doing the fix. It's all a learning process...regardless if the approach is successful or not.
Just remember that you got plate voltage present in that LC area so be careful probing around while it's fired up.
RFB
With the dashboard tuned to 670kHz and a low level 440Hz tone we headed west.
The signal was there for the 1st block. The signal was there for the second block. In the 3rd block the signal was suddenly gone.
That's about the same result we got when the coupler was wired into the hots, but of course neutral is safer to work with.
And now we have the Standing Wave (SWR) in a healthy green range.
All this with only 2-Watts. Soon we'll try 5-Watts.
Carl! Keep going man!
We'll all be waiting for
the 5 watt carrier current
test!
RFB! Thank you for the
words! I'm going to go
real slow at this LC thing!
330 volts is nothing to
sneeze at!
Thanks again,
Bruce, DOGGRADIO STUDIO 2