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Radio
Broadcast Technical Consulting and Sales
10032 Island Drive, Brainerd, MN
56401
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Radio
Broadcast Technical Consulting and Sales |
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Technical Tips from Mark W. Persons |
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Filament Adjust for Tube Transmitters |
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This is a Harris HT-5
FM Transmitter with the front panel swung out so you can look inside.
On the door, behind the plastic protective cover, is the filament
voltage adjust control.
The reason why you are seeing this is the filament control only gives 0.3 volts of adjustment downward from the rated 5.0 volts. That is just a 6% control range. To get the best possible life out of a tube, the filament should be adjusted down by more than that after the first 200 hours of operation. |
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This is a
modification which can give more filament voltage control range.
It involves putting a fixed resistor in series with the existing control
along with a switch to short the fixed resistor when full filament
voltage is required.
In this case, the filament voltage control is 10 ohms. I installed a resistor, which was on hand at the station. It was 8 ohms at 200 watts. To get the best possible range, a 10 ohm/200 watt resistor should be selected. With an 8 ohm resistor, the filament voltage was brought down to 4.5 volts, which is 9 percent below the rated 5.0 volts. The voltage is adjusted down until a significant drop in tube emission is observed and then cranked up just a bit. It is the sweet spot where best tube life is achieved. |
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Here is the switch on
the front panel. It is easy to install and it can extend the life
of the tube maybe by a factor of 1.5. This same trick can be done
on tubes used in other FM transmitters and on AM transmitters. A
fixed resistor value should be chosen to suit each particular situation. We are an Econco tube dealer. Story from 08-24-2011.
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The stories go on and on.
Stop in again sometime. I'll leave the soldering iron on for you.
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Questions?
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page last edited 11/02/2011
