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 Post subject: AM, BCB design questions
PostPosted: Dec Sun 17, 2017 2:35 pm 
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Posts: 1001
Location: Santa Rosa, CA
Like others, not real happy with the lack of decent programming
locally (AM) and would like the use the old radios I've been restoring
for years now. So its time to build (another) transmitter.
Have been looking at the numerous designs out there and
most look promising (I'll be using tubes).

Since I'll be driving this transmitter from an FM receiver (we've got a
great local station with real DJ's), have been thinking high level
modulation would be the best and easiest approach. An audio
output transformer run backwards. Also thinking of using series
connected filaments and direct line power supply. The input and
output will be isolated and have lots of these types of tubes here.

Has anyone here built something along there lines?
Steve

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Sun 17, 2017 3:46 pm 
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zarco wrote:
Also thinking of using series
connected filaments and direct line power supply. The input and
output will be isolated and have lots of these types of tubes here.

Steve

That'll no doubt start 'em to a flappin' an a squawkin'...

I'll encourage you to use a xformer but it's your choice... I'm of the mind if you know what you're doing it's the builders choice of what circuit to use...

The easiest to build using high level modulation and performs well is Norm's 6888, but being that's a 6v tube with a 800ma heater, it really should be powered by transformer... You could use a 12CS6 with a dropper cap for heater, though I doubt you'll have one of those on hand either... The commonly used 12SA7, 12BE6 etc are remote cutoff so are basically junk in these type circuits...

Several years back I built a 25L6 xmitter that was high level, screen modulated... It used a 25v xfomer with B+ doubler, worked fairly well, though wasn't up to par with Norm's 6888... I'll check to see if I still have the schematic in my CAD program...


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Sun 17, 2017 4:08 pm 
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Location: Santa Rosa, CA
Thanks Norrn. And I do know what I'm doing and have been doing it
(engineering) for several decades. There are places where a
line powered design will work just fine and this seems to be one of
them. Good isolation at the input and output. Let 'em squak!

Just kicking around with some ideas now. Have a LARGE stash
of parts here so many options. A pair of 50L6's or 50C5's and a
a couple other tubes would be a good starting point.
There were many odd ball series fil TV type tubes that would do
the job too. Will use an xtal controlled osc. Time to play! ;-)
Steve

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Sun 17, 2017 6:15 pm 
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Here's a link to page with Norm's 6888(BTW I'm Tom)

viewtopic.php?t=63310

BTW if you have TV RF amp triodes on hand(6FQ5, 6HA5, etc) I have a good circuit that uses them as AF preamp..


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Sun 17, 2017 9:20 pm 
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I built this one:

http://antiqueradios.com/forums/viewtopic.php?f=12&t=292628

Still working fine for me.

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 3:04 am 
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35Z5 wrote:
Here's a link to page with Norm's 6888(BTW I'm Tom) ...

Unfortunately PhotoBucket has deprived us of the schematic and instruction sheet for Norm's transmitter that were posted in that thread. Are those available anywhere else online?

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 3:30 am 
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Location: Santa Rosa, CA
Thanks for the suggestions guys. After some quality time with
an RCA tube manual and looking at what is available in the junkbox
I've decided on an original design.

Three tubes; two 50C5's and a 35W4 rectifier. One 50C5 will be
the xtal oscillator driving the other which will be a class C amp (PA).
Drive to the PA will be variable. Since I'm planning to drive this
from the FM stereo in my shop I'll use a reverse connected
audio output transformer in the plate circuit of the PA for high
level modulation. Audio processing and level will be with the FM
stereo. I'll include a VU type meter connected across the secondary
and calibrate it so I can set 100% modulation. May also use the
meter to read output RF power so I can tune the Pi output matching
network to whatever antenna is connected.

Should have prototype done this week. Will post results and
schematic if its a worthy design.
Steve

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 5:25 am 
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I didn't realize 6888 schematic was hosted on the "bucket"... This computer has a hack that sidesteps the issue('mazin what you can find on net)... Don't think anyone will mind if I post it at postimage...


This is the basis for probably 75% of my exploits... I'm partial to line input so added a preamp, gain control, etc...Other than the plate modulated 6AQ5 xmitt I was working on early last month, don't really care for audio transformers... That one is now back to being a 6GY6 Tx...

Image


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 8:22 am 
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Location: Saskatoon
Also the instructions for Norm's 6888 transmitter:
Attachment:
Instructions_6888.jpg
Instructions_6888.jpg [ 95.9 KiB | Viewed 863 times ]


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 3:08 pm 
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zarco wrote:
I'll include a VU type meter connected across the secondary and calibrate it so I can set 100% modulation

True VU meters and, in general, VU-type meters, are quasi-peak devices that cannot display some of the fast peaks responsible for overmodulation. That is why broadcast modulation monitors demodulate the transmitted signal and use flashers driven by peak-detection circuits to display peak modulation. When setting up your transmitter I would suggest looking at your signal with a 'scope and a variety of modulation to determine what maximum VU-type meter reading will prevent carrier pinch-off.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 3:29 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
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Location: Santa Rosa, CA
Dale H. Cook wrote:
zarco wrote:
I'll include a VU type meter connected across the secondary and calibrate it so I can set 100% modulation

True VU meters and, in general, VU-type meters, are quasi-peak devices that cannot display some of the fast peaks responsible for overmodulation. That is why broadcast modulation monitors demodulate the transmitted signal and use flashers driven by peak-detection circuits to display peak modulation. When setting up your transmitter I would suggest looking at your signal with a 'scope and a variety of modulation to determine what maximum VU-type meter reading will prevent carrier pinch-off.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


Had planned to use a 'scope to calibrate the meter to indicate
100%. The meter I'm using is the same one used in the HP8640.
A 1mA full scale with a 'dBm meter scale'. Will use a diode/RC circuit
to drive the meter so I can set the delay to a resonable level.
So its not really a VU meter.

Suprised other low power AM BCB transmitters people have built
don't have a mod indicator of some sort. Very easy to go over 100%
with most designs I've looked at.
Steve

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 6:07 pm 
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zarco wrote:
Suprised other low power AM BCB transmitters people have built don't have a mod indicator of some sort.

Most of the designs that I have seen, and most built and sold back in the day as wireless phono preamps, use directly modulated oscillators, and those cannot be modulated beyond about 30% before generating significant envelope distortion and other artifacts. The same is true of external modulation of service-grade signal generators using directly modulated oscillators.

I am tempted to build the 6888 transmitter just to see how it specs with my commercial AM modulation monitor and distortion analyzer.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 6:10 pm 
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Steve

your 50C5 idea sounds somewhat like the old Knightkit "Broadcaster."

http://www.crompton.com/KnightBroadcast ... ematic.jpg

Rich


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 9:28 pm 
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zarco wrote:
Suprised other low power AM BCB transmitters people have built
don't have a mod indicator of some sort. Very easy to go over 100%
with most designs I've looked at.
Steve

I use a scope, have a 2236 Tek with freq counter that transmitter monitor has been its sole duty for several years...

At close range I find most older radios are happiest at around 80% modulation...


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 10:27 pm 
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Location: Arlington, TX, USA
Dale H. Cook wrote:
zarco wrote:
Suprised other low power AM BCB transmitters people have built don't have a mod indicator of some sort.

Most of the designs that I have seen, and most built and sold back in the day as wireless phono preamps, use directly modulated oscillators, and those cannot be modulated beyond about 30% before generating significant envelope distortion and other artifacts. The same is true of external modulation of service-grade signal generators using directly modulated oscillators.

I am tempted to build the 6888 transmitter just to see how it specs with my commercial AM modulation monitor and distortion analyzer.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


+1, this kind of testing is something I'd really like to see. I'd do it myself but don't yet have the equipment to do a really good job of it. I want to see something supported by measurements instead of just "sounds good on my Belfonic 3-transistor radio". Some of us here have really good AM radios that call for a quality source. Absent test results to the contrary I remain skeptical of the signal quality from the types of transmitters that get the most attention on this forum.

I also wonder how many of these tube type transmitters comply with the Part 15.219 input power limit. Although it's somewhat moot if the signal is within 15.209 limits anyway which only concern field strength. Theoretically one could run 100W into a dummy load as long as the signal doesn't get out far enough.


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 11:08 pm 
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Something I forgot... It's more important to prevent modulation from going below 0% than exceeding 100%. If I was going to add an LED circuit I would have it turn on when modulation gets close to the 0% point. A bit of signal past 100% isn't a real big deal, but completely cutting off the carrier is.


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 11:11 pm 
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35Z5 wrote:
hosted on the "bucket"... hack that sidesteps the issue

Thank you so very much for the clue.
I had to do a browser update to use, but very well worth it

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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Mon 18, 2017 11:30 pm 
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Erich Loepke wrote:
I also wonder how many of these tube type transmitters comply with the Part 15.219 input power limit.

I would think very few - how many do you see equipped with meters to measure the DC voltage and current in the final stage?

Erich Loepke wrote:
Although it's somewhat moot if the signal is within 15.209 limits anyway which only concern field strength.

A transmitter meeting 15.219 should meet 15.209, though I have not tried measuring one (I do have access to a Field Intensity Meter at work).

Erich Loepke wrote:
Theoretically one could run 100W into a dummy load as long as the signal doesn't get out far enough.

I would think that Enforcement Bureau would say otherwise - the general rule is that operation into a dummy load is for testing and maintenance.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Tue 19, 2017 12:17 am 
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Location: Saskatoon
Keeping the modulation within limits is definitely an important concern. I initially found it very difficult to get good results, because of the widely varying levels between different program material. It meant constantly readjusting the input level or else permanently setting it to a very low level to prevent over modulation. However, setting to a low level is a poor solution.

Many people who feed their transmitters with computer audio, also use audio software that provides limiting and compression.

I eventually came up with a different solution: a controlled carrier transmitter circuit that is very resistant to over modulation. On sudden audio peaks, the carrier level just goes up to handle the transient audio. Average modulation is over 100%. I also made a simple mod to Norm's 6888 transmitter to convert it to controlled carrier. It only requires a diode, a capacitor and two resistors.

As for concerns about going below 0% modulation (i.e., negative modulation), this can only happen with a balanced modulator circuit. Neither the tube transmitter circuits nor the LM386 circuit mentioned here use a balanced modulator. So, this is not an issue.


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 Post subject: Re: AM, BCB design questions
PostPosted: Dec Tue 19, 2017 1:04 am 
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Location: Arlington, TX, USA
Dale H. Cook wrote:
Erich Loepke wrote:
I also wonder how many of these tube type transmitters comply with the Part 15.219 input power limit.

I would think very few - how many do you see equipped with meters to measure the DC voltage and current in the final stage?

Erich Loepke wrote:
Although it's somewhat moot if the signal is within 15.209 limits anyway which only concern field strength.

A transmitter meeting 15.219 should meet 15.209, though I have not tried measuring one (I do have access to a Field Intensity Meter at work).

Erich Loepke wrote:
Theoretically one could run 100W into a dummy load as long as the signal doesn't get out far enough.

I would think that Enforcement Bureau would say otherwise - the general rule is that operation into a dummy load is for testing and maintenance.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
http://plymouthcolony.net/starcityeng/index.html


I have read stories on several Part 15 AM forums where the FCC tried to cite certain part 15.219 AM transmitter installations for being above the allowed field strength under 15.209. Once the FCC inspector found out the transmitter was indeed compliant with 15.219 they backed off. Apparently with a well designed and constructed antenna system and favorable ground conductivity it is possible to get a mile or more usable range from a 15.219 compliant transmitter/antenna system which is above the allowed "about 200 foot" range you'd get under 15.209 field strength rules.

I'll defer to your knowledge about dummy load rules. It'd be pretty stupid to run that kind of power and turn it into heat. The point I was trying to make is as long as the field strength is within limits it really doesn't matter what the transmitter power is. From what I understand, 15.219 rules were given as an simpler to measure alternative to making measurements with an expensive calibrated field strength meter, at least for the AM broadcast band.


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