Mini-Mod MW 'Micro Transmitter'

[Ian - g4jqt]

The first circuit has a tuned output circuit so there will be some harmonic attenuation. Something from memory tells me a single tuned circuit is likely to give about 20dB of second harmonic suppression. But it depends upon Q. That's not much, but it's all the Minimod has!

The second circuit has no harmonic suppression - although it may not need it. But remember, these things have a range of a few tens of yards if you keep the antenna short, so nothing is going to get out very far anyway. But at least a basic LPF or tuned circuit is good practice if the output is more than say, 100 mW. The design I mentioned in post #12 had a 7 element LPF giving at least 40dB attenuation.

Ian

[Terry]

It would be rather nice (well I think so anyway), to have a  design for a micro-transmitter that uses valves, along the lines of those ubiquitous Chinese 'Little Bear' valved headphone/pre-amp amplifiers.

A bit like this sort of thing. They run in 12V and I assume they have a tripler or some such thing to raise the voltage for the HT:

At the bottom of the page you linked to is a "Mullard Vintage Valve Buying Guide" which contains, amongst other things, this priceless piece of advice:

'Vintage amplifier valves tend to generate enough heat to degrade other stereo system components. Because of the excessive heat, Mullard amplifier valve buyers should place the sound systems in a large room that encourages strong airflow, such as a basement with includes several open windows.'

[ppppenguin]

I looked at the date on that "Mullard Vintage Valve Buying Guide" hoping to find April 1st. My hopes were dashed. I wonder who owns the Mullard name these days, they might not like that piece very much. Liberal and relaxed as they are, I still can't really say much more without breaking GVR rules.

Here's the link in case you want to either laugh or cry:
http://www.ebay.co.uk/gds/Mullard-Vintag...277/g.html

Coming back towards the original topic, a valve bsaed pantry TX seems like a nice idea but for something that may well be running 24/7 I'd like to minimise heat and power consumption. Each watt drawn 24/7 from the mains costs about £1 per year.

[Yorkie]

I've got a couple of designs for valve pantry transmitters, one by our own MurphyV310, but have never built them as I don't trust them as regards harmonics.

— Joe

Thanks for posting those Joe.

Both of those designs are quite 'chunky'.

Trevor's scope traces  look good!

Peter Lankshear is a well-known Australian vintage radio enthusiast and keen constructor who's written many articles over the years, as well as books. EG:

http://trove.nla.gov.au/work/34943211?se...NBD9156751

I guess that given the limited range (on a short aerial), low power and the fact that so few people listen on MW these days, the risk of radiating harmonics to such an extent that they cause interference and annoyance to other spectrum users is fairly minimal. For now, I'm quite content with the performance of the mini-mod, which has the merit of being compact and running off a PP3, and with so many half-completed projects on the go,  my autumn/winter mission is to complete those before going off on a frolic experimenting with valved micro transmitters

Interesting topic though.

[Radio Fixer]

Coming back towards the original topic, a valve bsaed pantry TX seems like a nice idea but for something that may well be running 24/7 I'd like to minimise heat and power consumption. Each watt drawn 24/7 from the mains costs about £1 per year.

My bottle of beer of an evening costs more than a £1 a day but I'm not worrying about it Jeffrey   

We have been around this loop before I think but never mind. I have had the Peter Lankshear design running 24/7 (just two valves and silicon rectifiers) for maybe 5 years. Its pretty powerful and 2ft of wire will cover quite a lot of house for sensitive receivers. Performance sounds fine (how its constructed matters and I did find ways of lowering intermod hum) and in tests with an SSTRAN it was hard to tell the difference. Never had anyone knock at the door and think its most unlikely and cant find any trace of it outside with a portable receiver (maybe not sensitive enough).


Gary

[Radio Fixer]

Wow! time flies. Not 5 years but 9.

Gary