The "Linear Power TX"
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This information is provided for educational purposes and/or authorized usage only.
This TX is a modified version of
Evan's comments on the original design
First - Bob's one tube transmitter is one of the finest I've come across. It's a controlled carrier transmitter which helps prevent over-modulation and under-modulation. It's amazingly linear and now, with the addition of the RF output stage shown below, it's powerful enough to hear throughout your house using a simple wire antenna.
After building and working with transmitters using a pentode as the oscillator, I can say that this TX is unmatched in its simplicity and performance.
Keeping a tube type TX linear is the challenge and of course the more linear the more true to the original the broadcast will be. Bob's done something special here.
Now, if you are a visitor to this site and you are thinking about building a transmitter from a print found here, I'd recommend you build this one. It's easier to build, with only two vacuum tubes (I chose to use a 1mhz glass crystal from the Ukraine so it looks like 3 tubes) and yet the quality of the audio is truly amazing, just as Bob mentions on his site.
Bob also offers a 3-Tube AM Stereo Transmitter project, maybe that will be the next one I attempt to build?? A bit more challenging. But we shall consider that in the future.
How does the 'Goldberg' transmitter compare to this TX? Well, I must say I enjoy listening to both. There is a slight difference, and both are fantastic. But if I was to describe it, I guess I would say:
1. The Goldberg is a beautiful, sonically
rich transmitter. Warm and mello, yet linearly accurate.
Click on the image below for a larger schematic.
RF Tuning - Both Power TX Models
Tuning this circuit for best performance is best done using an oscilloscope, a basic scope is just fine.
To make it easy, install a 50k RF gain potentiometer (as shown below) between the Audio Output tube (6GH8A) and the RF Output tube (EL84). Adjust it so that the full 50k is seen between these two sections.
Connect an adjustable sine generator (1 kHz) to the input of the circuit. This can be done by installing a signal generator 'App' onto your iPhone or Android audio audio 'source'. These Apps are typically free of charge.
Using a jumper wire with shielded alligator clips on both ends, wrap the jumper wire around your antenna and connect them using a couple of signal diodes (1N34A x2) as shown above.
While applying a sine wave signal (1 kHz) to the audio input, turn on your TX and once it's transmitting, obtain your transmitted signal (..check the 5v scale) alternately tune C9 and C11 for maximum output. Adjust C4 to it's maximum output, then repeak C9 and C11. Now, stop the signal generator and play some music through the TX. While listening, adjust C4 - 1/4 of a turn either direction. Leaving it where it sounds the brightest (crisp). Listen carefully, there may be little variation.
Now gradually turn the RF gain, 50k potentiometer, to minimum resistance. The scope might be off the scale now, unless you have a 10x probe. No worries. However, if you do have one, flip it to 10x and obtain your transmitted sine wave signal once again (..click to the 2v scale) repeak C9 and C11, ever so slightly.
You might wish to leave this RF gain potentiometer in place. Setting it just below the point where the waveform distorts at higher input levels.
Now, check out the sound quality and range.
The schematic/s above always show the latest updates.
The pictures below may not show every update.
Click on any image below for a larger picture.
The ribbon cable you see goes to the LED's installed in the tube sockets.
All on a single 5" X 7" top!
To answer your FAQ's
1942 Philco Radio - Model 42PT-7