In the assembly/operation manual I just purchased there is a second schematic and wiring pictorial which addresses the shock hazard posed by the transformerless power supply.
The chassi never was connected directly to the AC line. Rather there are bypass caps connected between the AC line (floating ground) and the chassi. This would allow a minor shock sensation but very limited current between the chassi and ground if plugged in backward.
However, the ground side of the RCA audio input jacks ARE connected to the AC line (floating ground) even though they are isolated from the chassi. This is where the shock hazard exists. When the AC cord is plugged in backward, the audio ground connection is connected directly to the AC hot.
The modified wiring disconnects the audio jack grounds from the AC line (floating ground) and connects them directly to the chassi. The audio ground return to the circuit uses the bypass cap connected between the AC line (floating ground) and the chassi. This restricts the 60 Hz AC current at the audio jack ground to a safe value. You may still feel a buzz but not enough current to electrocute.
I will post the modified schematic separately with the above text.
Not knocking anyone's attempt to make that thing safe, but even with that modification, that thing is still not up to today's electrical appliance safety standards.
If you really want to make that thing absolutely safe and up to today's electrical appliance safety standards, put a 1 to 1 isolation transformer inside the unit, or use an external one and then there wont be any concern about minor shock potential...that is if the unit is intended to actually be used and interface with today's far more modern and considerably safer audio devices.
This is why my unit sits on my TX history shelf and not in service. It was great for its time, started a lot of careers out there, and makes for a great conversational time piece.
It's fantastic to see the tube glow and light up and all..but IMO, for what is involved and the potential for hazard, I would rather spend the time watching the glow of the fireplace while I roast a few marshmallows. At least that fire is contained and controlled by the touch of a remote control..and keeps my breakers from tripping!
Enjoy it though. They sure were great transmitters and for their simplicity did quite well in what it was designed to do. Even back in the day, I always questioned why it never had a power transformer due to that shock hazard risk.
UL learned alot since then...we should too!
RFB
I know some of us are terrified to touch this transmitter with its transformerless design, but I keep tinkering.
I tried the "factory safety mod" and find that the audio suffers greatly so I'll opt to leave it as it was.
But while tinkering, always trying to improve it, I have made a change to the screen grid resistor of the oscillator/output tube. The resistor was moved from the B+ source to the plate and reduced in size experimentally to an optimum value.
The result, drastically reduced FM'ing to the point where I no longer have to tune the receiver off frequency to hear the audio. I think the FM'ing was so pronounced that it was more FM than AM. As such, tuning off to the side (called Slope Tuned Demodulation) was necessary. But now, tuning is dead on center and sounds reasonable.
To see the circuit changes take another look at my KT-195 WEBPAGE and look at the "Modified for Plate Modulation" schematic.
My original KT-195 FMed very badly. This was
in about 1969.
It also had another problem. It wanted to transmit
in the old Marine band. The transmitted signal
went about a half mile on 2100 kHz. A friend's
KT-195 tended to go the farthest in the 5 MHz range.
These emissions must have been harmonics of the oscillator,
that just happened to get out better than the fundamental
frequency.
As mentioned, the KT-195 I have now doesn't work. it just
sits on the shelf and looks good.
Looking back on this thread, I also see mention of the
Lafayette 991077 phono oscillator. I had one, and it
didn't go very far. It used a 35W4 rectifier and a 12BE6
for the oscillator. It was a dangerous unit, with voltage
on the meal chassis and exposed parts. It looked cool, though.
For UTTER SILLINESS, and for that great TUBE GLOW - just go and
build a 12AX7 tube preamp. Put one of two of them in a box
along with a Part 15 AM or (better yet) FM transmitter. Hey! While
you're at it, put in a mercury vapor rectifier! Heee Heee Heee!
I really wish I had the time, knowhow, and the money to do THAT!.
Best Wishes,
Bruce, DOGRADIO STUDIO 2
Like so many of you, I fondly recall the days in the 1950s when I played records into the Knight Kits, I ended up with about three different ones, and hearing the thump of a 45-rpm record come over the AM radio grill-cloth. I strung a long-wire horizontal indoor antenna and used 1630kHz, which most radios could receive.
I blew all the fuses in the house when one of the units touched a radiator, but somehow I am alive.
But looking now at all the modifications I see in the preceding posts, everyone of which achieves an improvement, why not bring back those glory days with a serious tube transmitter?
It would have a real power supply, it would utilize the crystal control like radio8z put into his Knight Kit (see LPH#3) and it would probably have more than just the oscillator. How about a buffer and a final?
How about meters and controls so the output could be daily checked for exactly 100mW to the final?
And, a proper output filter.
Tubes have a magic that cannot be achieved by solid state.
I remember an issue of PE that featured a really nice tube transmitter intended for carrier current operation. This was way back, maybe in the '70's? Nah, had to be earlier as I think I was still in school.
It was as Carl has described with separate oscillator, buffer, final and audio stages. It had a meter probably to monitor plate voltage and current. I don't remember if it was crystal controlled but probably not.
They even had a template to make the chassi from sheet metal.
I will look around to see if that article is still floatin' around somewhere out there or perhaps someone here already has it.
I believe I also remember the PE
carrier current transmitter article.
I remember a few things.
It used tubes. The circuit seemed to
be fairly small and uncomplicated.
It was five watts. (Input or output? I don't
know?)
I remember how it coupled into the AC line.
It just used small caps after the RF out -
then the RF went straight into the AC cord
of the transmitter. So basically - the deal
was to plug it into the AC outlet and let it
do it's thing.
As we know, this way of coupling is not the way
to go.
I could be completely wrong - but my brain says
this article came out in 1973. And the article
was small. Maybe two pages, the description and
the schematic. I don't think there was a picture
of a built unit.
Building a tube AM transmitter - even a small one -
is a major undertaking, I think. If you are doing
plate modulation - you need a lot of watts to go
into the modulator. You need an HV supply.
From my point of view, this is hard stuff to do.
If I was able, though, i would do it. But I wouldn't
use that kind of coupling system. And I know none of
you guys would either.
And, I thought I saw that schematic floating around the
net somewhere a couple of years ago.
Nest Wishes,
Bruce, DOGRADIO STUDIO 2
"I remember how it coupled into the AC line.
It just used small caps after the RF out -
then the RF went straight into the AC cord
of the transmitter. So basically - the deal
was to plug it into the AC outlet and let it
do it's thing.
As we know, this way of coupling is not the way
to go."
When those old units were on the market as well as the early LPB's, there were no safe or better ways to couple to the AC lines other than LC circuits, some complex, some very simple like the couple of caps straight into a fuse and straight into the AC cord.
When LPB came out with the TC-8 coupler, it was the first true real coupler on the scene, and the safest and most efficient, which also provided relief from what plagued CC systems since day one...60 cycle hum.
The real coupler also provided constant transmitter loading no matter what changes took place on the AC line side. The early LC couplers and simple capacitor couplers did not provide anything of the sort. Sure they coupled the signal onto the AC lines, but they also coupled back the load changes on those AC lines, the 60 cycle hum, and no constant load matching to the transmitter.
They worked..but worked very poorly and they were very dangerous.
RFB
Just saw the KT-195 (LA23) on ebay right now! $5.00 current bid and two days left, if anybody wants one!
Dave
KSOL RADIO-ALL SOLAR RADIO
KSOL 1000 AM
KSOL 92.1 FM
"All Solar Radio, Even at Night."
The schematics will be very interesting for legacy reference purposes, but what are the difficulties encountered by adding an isolation power transformer to these transmitters?
I will guess that the number one obstacle would be lack of chassis space to fit a transformer. Thus would be required a second power supply cabinet with an interconnecting power cable.
Given the availability of schematics, I have three Knight Kit versions, has anyone ever built one of these from parts, perhaps the largest problem probably being finding the electron tubes?
I think a newly packaged glowing tube transmitter could be popular with some members.
Frankly I would totally re-design that thing to incorporate today's NEC safety standards. Like use a different cabinet to include an isolating power transformer. I would also isolate the audio path while at it.
Those old designs are simply not really compatible with today's stuff, and they really were a safety hazard even back in the day when electrical safety was less than optimum.
They were fun to work with and watch. Though after about 2 minutes of staring at glowing tubes, it's time to change the channel. To me it makes no sense to have a device pulling X amount of energy to produce a -X amount of signal. At that point efficiency is file 13.
Anyone ever try putting one of those zap boxes in an outdoor setup? LOL!
RFB
My friend and I each built one
in about 1969.
This one is actually his. Mine
is long gone. I want it for a
photograph of the history of
some Part 15 equipment. It will
probably be a while before I take
the picture.
I love glowing tubes. That's why I
took the cover off of my tube CC
transmitter. True - though - I don't
look at them for very long. But once
in a while is a true pleasure for me.
Anyway, this particular KT-195 is not
in working condition. In fact, it's
a mess. If you have a working one,
I would do everything in the world
to make it safe, whatever that was.
I think MRAM modified his extensively.
I would love to see the one I have running.
However, this particular one never
will. It's just for a photograph.
Wonderful Wonderful Memories,
Bruce, W 60 HZ