Home AM Transmitter (again!)

[dtvmcdonald]

My idea for such a circuit would be to put the screen at B+ voltage
and use the grid and cathode for the oscillator (screen as plate
of oscillator, grounded-plate type circuit) Then modulate from
a low impedance source (i.e. speaker impedance) using a
plate transformer in reverse to plate modulate the tube. Gain
would be adjusted using a say 25 ohm wirewound pot before
the primary of the modulation transformer.

the basic idea is at

http://nmwilliam.tripod.com/el84.html

though this uses a crystal rather than a coil. However ...
that circuit generates several watts and needs watts of audio.

[jr_tech]

Is it time to throw out the bathwater and the baby?

I don't think so... before giving up and embarking on a re-design or totally different circuit, a couple of questions perhaps should be answered.

1.As designed can the transmitter work?

a. There are several arguments here that indicate that the tube possibly could operate (perhaps poorly) outside of its normal operating parameters.

b. Calculating from the voltage drop across the 10k resistor to the plate of the 117L7 (pin 5) indicates that the tube is drawing about 2 ma current which is about 300 mW of power. This would seem to be consistent with a typical power output of 100mW that is generally considered
permissible for un-licensed operation on the AM band if a very short antenna is used. More power might incur a visit from the FCC. A good reason, perhaps for this low-power configuration.

c. Phil says it works... there is no doubt that what he built indeed does work fairly well!

2.Does it work?

a. Perhaps, but not on the frequency tested... you might want to test at the high end and low end of the broadcast band just to make sure that it is not transmitting.

b. I still favour testing with a sensitive little portable held near the transmitter. Sony for example, sells a sensitive little analog portable for around 12 bucks that would be ideal for this testing. http://www.amazon.com/Sony-ICF-S10MK...y+pocket+radio

3.If not, can it be made to work?

a. The circuit is pretty simple, and appears to be wired correctly... a big question involves the coil phase and/identification of windings... perhaps the primary and secondary are swapped, or one winding is simply out of phase with the other.

b. I suggest disconnecting all 4 leads to the coil and measuring the resistance between all the pins... perhaps we can figure out what is happening here.

OBTW, your test radios are very cool!
Not affil with Sony,
jr

[Geist]

Hi All;
I have a copy of what I think is the origional article for this Transmitter, it is copied in jpeg, but I don't know how to make it show up here..
The Schematic is slightly different, but maybe it may help you find what the problem is with your unit.. It was origionally for a phonograph with a crystal pickup..


http://www.videokarma.org/attachment...1&d=1376186080

THANK YOU Marty

[Geist]

Hi All;
Winky, I have this file in PDF format, but the file is too large for me to upload here..
If you want I can email it to where ever you want If you cannot see the details or fine print..
THANK YOU marty