I've built some AM microtransmitters, see my page tube AM xmit.

Virtually any signal generator with an external input will work, but one has to be careful.

One of the sig gens I have here and I suspect another which it is based on, is similar; runs on very low voltage and that places the operating point of the output valve right on the knee of its operating curve?

I have seen the HRSA one. I don't know where it is at, but I know modifications were suggested to improve its modulation. However, it does work.

I have used a dual gate FET in conjunction with a separate crystal locked oscillator made from 2/3 of a quad NAND gate.

The circuit of Peter Lankshears "Mate" could be worth a look. One must be able to control the level of Modulation lest it distort. That is where a few fall down badly.

Remember things like iPods, Record players etc. do put out an audio signal that is quite strong, relative to what is required to drive the modulator and therein lies a major potential source of distortion. I think a lot of users do not appreciate that, or the source signal is likely stereo.

As has been noted, commercial units are out there

Marc

The MATE is a two-valve set. I believe the OP is looking for a solid state solution, preferably.

In any case, here are some notes on the MATE:

http://www.philipstorr.id.au/radio/nine/mate.htmhttp://www.philipstorr.id.au/radio/nine/mate.htm.

... and a link to a PDF copy of the original MATE article:

http://www.vintage-radio.net/forum/attachment.php?attachmentid=15665&d=1204779434.

"a kit from Antique tubes supply in the US"

Do you have a web link for this outfit? Searching throws up dozens of similar names, can't find any kit-related ones.

Maven

a kit from Antique tubes supply in the US

It's possibly this one from Antique Electronic Supply (aka TubesandMore):

http://www.tubesandmore.com/products/K-488.

Thinking seriously about re-purposing the innards of a Pye receiver to make a transmitter, learning from various micro-transmitter schematics.

The circuit already runs on 8vdc.

There is an amplifier chip KA2209, volume controlled by pot, which could become the input amplifier with input redirected from original earphone socket.

The AM MW coil and tuning condenser are already set up for the right bandwidths. I think I could feed the output of the audio amp to the base of a new transistor (BC109C in a circuit I'm looking at) that also provides the oscillation using a feedback circuit that includes the tuning tank. If this works, modulation level will be controlled from the original volume knob, transmit frequency from the original tuning knob, and antenna will be the original telescopic stick.

All irrelevant parts of the receiver circuit would be removed. To complete the whole gender-bending process, I could leave the "speaker" in place but wired to the input as an optional dynamic microphone.

Any fatal flaws in this plan?

Maven

It must be serendipitous with all this talk on micro AM transmitters. I've been planning on buying or building one for ages. Yesterday on my way home from work I popped to my local op-shop. In there was a box of electronic bits & pieces including some Dick Smiths Funway with Electronics breadboards and circuits. One of the circuits is a very simple flea powered AM transmitter. I took it home and built it up, but alas, couldn't get it to work. All it did was make my radio squeal like a girl. I'll have another go at it tonight.

Monochrome - sounds like your radio is jealous of the attention you are giving the fleapower transmitter. You might have to keep them out of sight of each other.

Maven

All it did was make my radio squeal like a girl.

Perhaps that's why it was in the markdown dump bin.

Perhaps it was also too close to the radio & hetrodyned with the radio's oscillator?

You didn't power it with a switchmode PSU ..... did you?

Marc

I remember when the Funway gear came out in the 1980s. I managed to get through about half the kits (the ones I found useful at the time) through all three series. After the third series Dick sold his company to Woolworths and the 'fun' stopped coming. These days Dick Smith stores do the exact opposite of what was originally intentioned - they sell manufactured goods instead of kits, parts and hardware.

I think the only time I had any drama with a transmitter was if I hooked up a microphone and it screamed with feedback if the microphone was too close to the radio. Trouble is, hearing your own voice coming out the radio is a natural thing to try and do when experimenting.

There may be a coupling /radiation issue with that circuit. it is a feedback circuit.

Try putting a cap across the battery: There is nothing decoupling the power circuit.

Marc

"These days Dick Smith stores do the exact opposite of what was originally intentioned - they sell manufactured goods instead of kits, parts and hardware"

Sounds like Radio Shack in the USA. They used to have kits, parts (though the quality was poor), and such. Today a lot less parts and lot more cellphones.

Yeah, familiar story there too. Radio Shack was called Tandy Electronics here and there were over 400 stores in its heyday. Our very own (and much loved *giggle*) Woolworths bought that lot out too and closed each and every store in turn to try and increase sales at their Dick Smith stores. All they achieved was sending their punters to Jaycar, which remains to this day a proper electronics shop and Jaycar will most likely never be owned by Woolworths and we can all be thankful for that. Tandy was one of the last shops to sell valves before the supply of valves went dormant in the late 1980s and each store still had a working valve tester.

I still can't get my little girl to stop squealing no matter what I do. So I think I'll give up.

That said, it was never intended to be a serious contender for my domestic micro AM transmission needs. I just wanted to do a bit of experimenting. Anyway, the box of bits from the op-shop, for which I paid $5, had a few hundred brand new components, such as resistors, capacitors, transistors & LED's. There was also a brand new soldering iron with stand, solder, solder wick and sundry tools.

I too remember the Tandy stores as a kid. I used to take the valves I ratted from the hard rubbish there to test. Their valve testers could test Compactrons & Nuvistas, but couldn't test a lot of Australian/European radio valves. They couldn't test 6M5, 6AN7, 6N8, 6AD8, 6BH5 or 6BV7's. Nor could they test P-base or Rimlock's.
It was also reputed that their valve testers were rigged - testing good valves as bad - so as to try and make you buy one of their "Realistic" valves with their "Lifetime Warranty".

I'm sure the Tandy staff got sick of the teenage boy who kept coming over with his box of valves. So it was a blessing for both of us when I given a Paton Palec ET4A for my birthday. Then at least I could test any valve I wished without having to bother anyone.

Cheers.

P.S.
After 32 years the Palec ET4A is still in regular use.