Tuesday, March 1, 2011

Perhaps someday ...

I've been slowly acquiring parts for a home built 'something' with vacuum tubes.

As I already have some tube receivers perhaps I should be looking at a transmitter ... what to build though?

I'm hoping the next ham swap-meet will provide some inspiration in the form of parts or perhaps something that can be rebuilt. I would like to put together a low power AM rig to experiment with & later use as an exciter in conjunction with a power amplifier.

My ideas are a bit fuzzy at the moment but I found a series modulated low power AM transmitter circuit diagram on-line which looks promising. It does without the modulation transformer which can be a very hard to find component for home-brewers.


 Five Watt Series Modulated Rig by Tom, K1JJ

No modualtion transformer is required. This rig is capable of clean audio from 10 Hz to 15 kHz. The VXO can "pull" the crystal up or down a few kHz with good stability. Only 3 or 4 crystals could cover the entire AM Window on 75 Meters. For complete crystal stability short out the VXO cap and inductor to ground. The crystal will now determine the frequency and not be able to move crystal 3 kHz as before. Recommend using a filament transformer for the final amp tube rated at 1500 VDC insulation. Be careful. Remember, the cathode of this tube is at 300 volts or more.


Note on diode string: Start with 35 diodes.Use a three position toggle switch for S1. The High setting shorts out about 1/3 of the diodes in the string. Thi setting yields about 5 watts output with 100% positive peak modulation capability. The Lo position shorts out about 2/3's of the diodes. This setting yields about 2 watts output with 200 % positive peak modulation capability. The third position (not shown) shorts out none of the diodes. This setting yields about 500 milliwatts output with 300 % positive peak modulation capability. You SHOULD play around with the numbers of diodes shorted to suite your tastes.
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