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 Post subject: EL 504 single tube FM band transmitter
PostPosted: Nov Tue 07, 2017 12:54 pm 
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Joined: Apr Mon 06, 2015 5:31 pm
Posts: 83
Dear all hello,
recently i am working to a project that incorporate a single EL 504 tube at an amateur FM band transmitter.
I finish the construction and i have manage to hear it in band for few seconds probably 30 to 35 but by mistake i had forget the dummy load and as a result the tube didnt survive.
The transmitter was able to tune and i could hear the music input signal.
I have order a new one and after installation and connecting this time the dummy load in my attempts to operate it the tube inside arcing, immediately when i hit the switch. It looks like an internal short but i am not sure. Can someone provide me with some information regarding this issue? For info the anode takes about 395V and grid 1 has a 22K and grid 2 10 K resistors.
Thanks in advance.
Brgds to forum
Dimi


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Tue 07, 2017 6:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Beautiful Downtown Burbank CA
A schematic would help, if you have one.


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Wed 08, 2017 8:00 am 
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Joined: Apr Mon 06, 2015 5:31 pm
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Hi Mike, thanks for reply i have upload one please share your thoughts with me regarding this problem
Tks in advance
Dimi


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Wed 08, 2017 8:45 am 
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Joined: Apr Mon 06, 2015 5:31 pm
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attached as well the power supply i am using slightly different but i believe within limits


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Thu 09, 2017 9:34 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 2412
Location: Tucson, Arizona U.S.A.
The varactor needs DC bias to work properly. The signal at the grid of the tube is too large for the varactor to handle; they are small signal devices.

The oscillator feedback seems to be controlled by stray capacitance. A circuit where the feedback is controlled by components would be more predictable.

The EL504 is a horizontal output tube. It may not have short enough internal leads to allow it to operate at VHF. (What frequency are you trying for?) If it does work, careful layout and construction are required. It is probably prone to parasitic oscillation; most horizontal output tubes are. Since you can't use a parasitic suppressor at VHF, your only recourse is good layout.

I doubt that the frequency will be stable enough for anyone to copy it.

Transmitting music on amateur frequencies is illegal. Do you even have a license to use this?

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Jim Mueller


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Thu 09, 2017 10:25 am 
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Joined: Apr Mon 06, 2015 5:31 pm
Posts: 83
Jim, thanks for reply.
Yes it controlled from stray capacitance i was thinking to replace it by a variable capacitor but this on a later stage.
Varactor behave quite good although i can ajust a bit the bias voltage, the original construction was made back on 80s by a school team the transmitter been saved by one of them who ask to bring it back to life but only as a memory item, thats for the dummy load, the intention is not to really broadcast in between 88-108 MHz.
As i said the original problem was the arcing tube and that was the second as the first probably melt after 30-45 seconds at the first attempt after power up without the dummy load.
Now my fear is not to burn one more tube thats for i want to solve any problems of the circuit. To this any help is more than welcome.
Unless the arcing within tube was just because was not properly vacumed. What you think about this?
Brgds
Dimi


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 Post subject: Re: EL 504 single tube FM band transmitter
PostPosted: Nov Sat 11, 2017 5:17 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 2412
Location: Tucson, Arizona U.S.A.
To prevent future disasters, try starting with a low B+ voltage, say 50 volts. That may be too low for it to oscillate but shorts or other problems should show up without risking the tube (unless the 50 volts goes directly to the control grid!). If all looks OK, gradually increase the voltage while watching for signs of trouble. When it starts oscillating you can check the tuning. Keep the voltage low until you are sure everything is OK. And keep the plate and screen currents reasonable.

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