AM microtransmitter with internal feedback loop

[wa2ise]

I converted an old hobby grade tube RF generator, a "Superior Instruments" model TV-50A (it's not that superior ), to an AM micro transmitter. It has 6 bands of RF frequencies, including the MW AM b'cast band.

The 12AT7 RF osc and modulator are original, the rest I built new. The 6AQ6 triode, similar to half a 5751 triode, with a mu of 70, serves as the audio gain amp and feedback mixer. The 6C4 is just a cathode follower, to feed the 12AT7 modulator triode, which has a low impedance input circuit. I used geranium diodes instead of the 1N4148's shown on the diagram.

The feedback loop is described at http://www.wa2ise.com/radios/amxmit.html

[bob91343]

That's not just an AM transmitter. It's an FM transmitter too. Modulating an oscillator is NOT the way to get AM.

[dieseljeep]

Originally Posted by wa2ise

I converted an old hobby grade tube RF generator, a "Superior Instruments" model TV-50A (it's not that superior ), to an AM micro transmitter. It has 6 bands of RF frequencies, including the MW AM b'cast band.

The 12AT7 RF osc and modulator are original, the rest I built new. The 6AQ6 triode, similar to half a 5751 triode, with a mu of 70, serves as the audio gain amp and feedback mixer. The 6C4 is just a cathode follower, to feed the 12AT7 modulator triode, which has a low impedance input circuit. I used geranium diodes instead of the 1N4148's shown on the diagram.

The feedback loop is described at http://www.wa2ise.com/radios/amxmit.html

I always try to pick up some of that Superior Instruments equipment for some of my projects. They're a good source for parts and enclosures.
It's amazing to see what kind of designs, they came up with nothing but a bunch of junk and surplus parts. I have a pattern generator, using two acorn tubes and a 7193 high frequency triode as a rectifier.

[old_coot88]

Originally Posted by bob91343

That's not just an AM transmitter. It's an FM transmitter too.

So? How big is the FM component compared to the AM? And is it big enough to be objectionable in the recovered audio? Might be a lively topic for discussion.

[wa2ise]

You're right, it did produce FM. So I reworked the modulator stage, taking it away from the oscillator. I used a 6CS6 pentagrid tube (a sharp cutoff relative of the 6BE6) as the modulator. No more FM (or it's greatly reduced). According to my ham radio TS440SAT transceiver, which can be set to AM, FM, USB, LSB and CW modes on every frequency and band. I repurposed the old modulating triode to become a cathode follower to produce unmodulated RF, to feed the "sync" jack.
The revised schematic:


But it doesn't work on the top 2 bands, 15MHz and up, but I'll worry about that later. Actually it does, it's just hard to tune it to a specific frequency. Also I had a 6211 instead of the 12AT7 plugged into the osc circuit.

[bob91343]

That is an improvement. We don't need FM unless we need FM, if you get the drift (pun intended).

My Measurements Model 80 modulates the oscillator also. The benefit is that you can get FM from a signal generator that should have an FM function but doesn't. I found that a modest amount of AM produced plenty of FM for aligning VHF ham radios and such.

The Marconi TF1066/4B has separate AM and FM functions but since it modulates the oscillator for AM, it gives FM even if not asked for. And my Precision E200-C also is a modulated oscillator.

I wonder what was going on in the minds of the engineers. The HP606A goes to great lengths to eliminate incidental FM, and does a good job of it.

I have all these generators, trying to sell them off. My recently acquired HP synthesized unit puts them all to shame with its accurate and stable frequency; I can calibrate my ham transceivers with it, with 1 Hz resolution and a crystal oven.