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 Post subject: A single valve 9VDC Am Xmitter
PostPosted: Jan Thu 02, 2020 10:03 pm 
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Joined: Jan Thu 02, 2020 8:53 pm
Posts: 6
Location: Noordwijk, The netherlands
Dear forum.

Some time ago I received a box with several valves in it. Most were not so interesting, however, I did find a few ECH83 and ECH81´s in it. Now what could be built with that? Since I have a few valve-radios, and a single AM-only radio, I thought building an AM-Xmitter at low voltage. Could this be done? The ECH83 is defined at low voltages, A heptode/triode mixer for carradio´s.

So I read a few articles about the ECH83, about antenna theory and experimented with a homebrew Xmitter. Since that proved successful I thought to share it. Bought a few empty coil shells, designed a circuit board.

The inductor (actually transformer) was -for me- the hard part to get. I ended up making my own. I found an empty shell inductor base. The empty shells -with core- were too short for this application (I want 1MHZ, the shells are for 25Mz>). So I glued two together and wire wound a transformer.

I added a 2 meter (i.e 6 feet) antenna. I can receive the transmission at all my AM radio´s in my house.

The schematic has a few optional components. i used a voltage regulator for the filament voltage, but a 10 Ohm resistor (>1Watt) will do. The antenna output circuit is also optional, however you will need some impedance matching. I omitted L1 C8 C12.

The oscillator circuit is slightly critical at low voltages, so I deviated from the original Philips circuit. Lowered R4 to 12k and C2=100p instead of 47p. That doubled the oscillator voltage.

Pls look at the circuit, and if you have any comments or improvements let me know....


Attachments:
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File comment: Setup
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File comment: let´s call it the Beach II
schematic_AM.png
schematic_AM.png [ 345.63 KiB | Viewed 1469 times ]
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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sat 04, 2020 12:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
How much audio signal is required for full modulation?


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sat 04, 2020 1:30 am 
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Joined: Jan Thu 02, 2020 8:53 pm
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Location: Noordwijk, The netherlands
Good question.... I will need to measure that. As in the pic (2) I used an MP3 player. Connected the headphone output to the AM transmitter input.
I will get back to this...


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sat 04, 2020 5:49 pm 
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Location: Noordwijk, The netherlands
So I connected a sine wave at the junction of R1/R2/C10. At approx 400mVtt (140mV rms) I get 100% modulation. This might be adjustable by changing R10, but I did not test that.


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sat 04, 2020 9:47 pm 
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Location: Warner Robins, GA
Oh ok cool.

Interesting design though in that an audio preamp tube isn't needed at all.


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sun 05, 2020 7:08 am 
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Location: Vancouver Island, Canada
Clever use of an interesting tube. Simple circuit. 8)

https://www.radiomuseum.org/tubes/tube_ech83.html

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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sun 05, 2020 8:05 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
If you know anyone with a spectrum analyzer, get them to check
whether is creates any parasites in the commercial aircraft band.
You don't want your music to be heard in a plane flying directly overhead.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sun 05, 2020 4:31 pm 
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radiotechnician wrote:
If you know anyone with a spectrum analyzer, get them to check
whether is creates any parasites in the commercial aircraft band.
You don't want your music to be heard in a plane flying directly overhead.

I'm yet to see any tube produce enough signal at 9v to be detectable across a small room. The typical crystal osc at 5v can produce a signal that travels at least 15-20x farther.

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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sun 05, 2020 11:05 pm 
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Location: Warner Robins, GA
The tube is one designed for 12 volt plate voltage if that makes any difference.


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Sun 05, 2020 11:34 pm 
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Location: Powell River BC Canada
35Z5 wrote:
radiotechnician wrote:
If you know anyone with a spectrum analyzer, get them to check
whether is creates any parasites in the commercial aircraft band.
You don't want your music to be heard in a plane flying directly overhead.

I'm yet to see any tube produce enough signal at 9v to be detectable across a small room. The typical crystal osc at 5v can produce a signal that travels at least 15-20x farther.



It is what I did not see in the schematic that made me wonder. Like
how does it oscillate?

The output, power is needed.

Could it be 0.4 watts ?

Attachment:
ECH83.jpg
ECH83.jpg [ 242.7 KiB | Viewed 1015 times ]

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 3:15 pm 
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Joined: Feb Sun 02, 2014 9:13 pm
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Location: Roanoke, VA
For use in the US you need to measure the plate voltage and current (with no modulation) of the final tube. The product must be no more than 100 mW for legal operation in the US.

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


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 4:38 pm 
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Dale H. Cook wrote:
For use in the US you need to measure the plate voltage and current (with no modulation) of the final tube. The product must be no more than 100 mW for legal operation in the US.

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

And A Partridge In A Pear Tree.....

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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 5:55 pm 
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Location: Texas. USA
Dale H. Cook wrote:
For use in the US you need to measure the plate voltage and current (with no modulation) of the final tube. The product must be no more than 100 mW for legal operation in the US.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html
I doubt he's pulling even so much as 1mA plate current and at 9V that would be less than 9mW.


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 8:51 pm 
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Location: Roanoke, VA
The OP reports that he can hear his transmitter on all the AM radios in his house. After fifty years in broadcast engineering I can assure you that he is well in excess of 9 mW.

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


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 9:52 pm 
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Dale H. Cook wrote:
The OP reports that he can hear his transmitter on all the AM radios in his house. After fifty years in broadcast engineering I can assure you that he is well in excess of 9 mW.

Dale H. Cook, Radio Contract Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html
Maybe he has a really small house. :wink:

Btw, it he's putting out 100mW then he's exceeding the max allowable cathode current by almost 300%.


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Mon 06, 2020 11:04 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
It really looks like a nice project. Knowing the 9 volt
current levels on the three pins that feed it, as well
as what happens if any very high level audio, or any
AC signal gets in via the stereo jack, is of interest.
Attachment:
Similar.jpg
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Interesting read:

May 22, 2003 - Although the monitor was designed to work at

a frequency of 49MHz, a fault made it broadcast at the

restricted air traffic frequencies of 108-136MHz. It can easily

happen, says Tony Maddocks of ERA Technology in Surrey,

which recently assessed the threat of interference from

radiowaves to London's air traffic.

Interesting read:


https://www.theguardian.com/science/200 ... equestions

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Tue 07, 2020 2:44 pm 
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Joined: Jan Thu 02, 2020 8:53 pm
Posts: 6
Location: Noordwijk, The netherlands
Here´s the OP. My house is some 30*40 feet.
I don´t have equipment to measure the power output, however from the datasheet:

Heptode as a mixer:
Ia = 50uA @ 6V3
Ia = 170uA @ 12V6

Let´s say at 9V it draws 100uA, according to the US regulation you´d have 9V*2*100uA = 1.8mW output power AT MAX.

However, IMHO, that is a toptop value, RMS would be 1.8mW/2/sqrt(2) = 640uW. And that is if it actually would output 18Vtt.
It is more like 1Vtt, maybe 2Vtt max.

2Vtt--> 70uW

And finally, at best half of this is radiated through the antenna --> 35uW.

Are my calculations correct so far?


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Tue 07, 2020 4:55 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
Can you measure the current in the VCC line with a DVM line when the transmitter
is working broadcasting a tone, of 400 Hz. How many volts AC is the
signal fed into the audio jack ?

Attachment:
How much current is flowing here.JPG
How much current is flowing here.JPG [ 2.43 KiB | Viewed 874 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Wed 08, 2020 5:13 pm 
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Joined: Jan Thu 02, 2020 8:53 pm
Posts: 6
Location: Noordwijk, The netherlands
Quote:
Can you measure the current in the VCC line with a DVM line when the transmitter
is working broadcasting a tone, of 400 Hz. How many volts AC is the
signal fed into the audio jack ?


I would like to oblige, however that would mean breaking down my PCB and solder work..... And I had to cannibalize my original experimental setup, so I´m afraid I cannot answer this one.
:(


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 Post subject: Re: A single valve 9VDC Am Xmitter
PostPosted: Jan Wed 08, 2020 9:30 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 12437
Location: Powell River BC Canada
The dual aerial system is interesting. How long are these aerials ?

I you were to modulate your transmitter at a frequency of, 120 kHz
with a low cost function generator, do you think the signal would be
picked up on a common ham radio receiver ?

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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