Valve AM transmitter (very very low power!)

[coopzone]

Hello everyone,

For anyone interested. I purchased one of those "valve pre-amp" kits, using two 6J1 valves (EF95) for the princely sum of £6.77 (inc postage from China!).

Well it arrived, and I re-used all the parts as a low power AM transmitter - to allow me to use my mp3 player on the various sets I have. Don't panic - it's not a radio 5 transmitter! the max out from these valves is 1W and configured as they are it's less than 250mw. So 10ft of wire aerial gives about 30ft range (at a push).

So I thought I would report success, it tunes OK from around 800kHZ - 1.5Mhz thats about 375m - 200m, so a good chunk of the MW band. Attached is the circuit and picture of prototype (by which I mean finished item, i'll never re-work it !)

Sound quality is really good. I did buy a ready made cheap transistor transmitter some time ago - but this knocks the socks of it.

If anyone wants more info just ask.

Have fun.

[paulsherwin]

Nice simple design. Most common pentodes should work - EF80, EF91, EF39 etc.

Watch the range though. You don't want to cause problems for your neighbours or receive a visit from officialdom.

[Biggles]

There may be harmonics present on the output too. 250mW is a "massive" amount of power to put into an aerial for this sort of application. Is there any way of checking the output frequency to check the relatively low range you are experiencing may be due to a harmonic you are receiving, instead of the intended carrier? Apologies for the questions, they aren't intended to be a criticism of any sort, I just would have expected more range from 250mW!
Alan.

[HamishBoxer]

A 1/4W is indeed quite a lot,maybe run into a dummy load and see how far it goes.

Pre War speak I believe "artificial Ariel".

[kalee20]

I really doubt it puts out 250mW of radiated power!

Interesting, and full marks for another design for a pantry transmitter!

As others have said, check the range, and also look for harmonics. A portable SW receiver will help - can you receive anything if you tune it around?

As you have your tuning capacitor from anode to 0V, and the aerial is taken from the same point, I'd expect harmonics to be fairly low. But worth checking.

The other thing, if you're so inclined, is to assess the degree of FM you're getting, by modulating the oscillator. As the modulation is by swinging the screen-grid voltage, can you set up a frequency counter, and see how frequency varies as you alter the screen voltage (by using a temporary potentiometer) each side of the mean? That'll give an indication.

Look forward to more posts!

A 1/4W is indeed quite a lot

I could get Reading universities 1W MW transmitter from here (about 12 miles). For scale, those FM transmitter thingies (the legal ones) are rated at a number of nW maximum (yes nano) and work for a few yards, mind you the antenna is much more efficient.

[boxdoctor]

"Neat, but not gaudy" as the saying goes. Well done.
You may find this circuit will run on a much lower H.T., possibly down to about 50 Volts or less if you find you need to reduce the output power.
An inductive coupling for the aerial (say 15 ? turns wound on the top of the coil) will reduce the amount of spurious radiations considerably.
Modulated oscillators always seem to produce some F.M. component in the output, but if the mod is not excessive, it usually sounds quite acceptable. Again, congrats. Tony.

[coopzone]

Oops, after the comments I re-checked and the output power is actually around 25-30mw I was out by a factor of ten. It's one of those fancy auto-range meters!

And have checked the frequency using a frequency counter, and it's ok.

I was thinking of adding a filter to reduce harmonics - just in case.

[orbanp1]

Great project, congratulations.

Simulations show that plate modulation would be more efficient.

PS: Added a 10nF bypass cap to U1_g2 in the original.
I used Q = 120 for the air core coil.

DISCLAIMER: It is just simulation!

Peter

[MrBungle]

Very nice work. I may attempt to reproduce this when I receive my HT/heater transformer from aliexpress.

[Ian - G4JQT]

Originally Posted by merlinmaxwell

I could get Reading universities 1W MW transmitter from here (about 12 miles). For scale, those FM transmitter thingies (the legal ones) are rated at a number of nW maximum (yes nano) and work for a few yards, mind you the antenna is much more efficient.

Reading University was one of the many Restricted Service License stations. I've built transmitters for a number of them.

It's a bit confusing, but the "1 watt" is effective radiated power, not transmitter power. The antenna specified is generally a vertical radiator not exceeding 10 metres. To achieve 1 watt ERP from such an incredibly short antenna for MW, it needs an RF power input of at least 15 watts from the transmitter and a good ground system.

The RSL I rigged for Reading College (or whatever it's called these days) got night-time reports from Scandinavia. So on a clear channel it can get out - although only DXers would listen to something so weak.

An AM transmitter output of anything under about half a watt connected to only a few feet of wire and a poor or non-existent earth really isn't going to generate a detectable signal more than a few tens of metres away. Its ERP is going to be in the microwatts range at best. Nevertheless, for our purposes a transmitter of a couple of hundred milliwatts, actually radiating only a very few microwatts should be more than adequate. Choose your frequency carefully and no one will notice.

[Boom]

Interesting. Just found this.

http://www.dxradio.co.uk/lpam/index.htm

[MrBungle]

Ordered one of those 6J1 kits. Only £5.97 including shipping via aliexpress! Miraculous. Will report back when I receive it and have knocked up an attempt. Also accidently on purpose ordered a tesla coil kit for £3.50 as well.

Interestingly the 6J1 appears to be an old RF/IF valve so I wonder if these will push up into the 40m band. Might make an acceptable very low power QRP CW transmitter too.

[coopzone]

Thanks for the input everyone, especially the bit with the emulation and info on 10nf cap from orbanp1.

I've revised the circuit and added 2 caps, a 22uf to decouple the audio for the AF amp across the 330ohm resistor, this definitely improves the modulation and a second (not 10nf but 6.8nf) on the g2 of the RF modulator. See attached.

I'm going to give the alternative modulation method a go during the week.

Thanks, any further thoughts let me know. And if anyone wants more info - just ask.

[Ian - G4JQT]

Originally Posted by Boom

Interesting. Just found this.

http://www.dxradio.co.uk/lpam/index.htm

Unfortunately I think most of those have now gone for one reason or another, although I know at least one of the Gurkha stations is still running.

Some RSLs are licensed for just a month, others like Blast 1386, have an indefinite licence.

From memory and without looking up the detail, the 1 Watt ERP over urban terrain has a theoretical range of about 5 miles, when the field strength falls to 1mV/m - the minimum field strength required for a service area. This was defined by the BBC in the 1930s I believe. In practice these days, a signal is lost under the noise in an urban environment well above 1mV/m - regardless of the sensitivity of the receiver!

Hence one of the reasons for the popularity of our little 'devices'.

[coopzone]

Good look Mr Bungle, it's a bit of fun. According to the data i dug up on these valves they should work up to 150mhz, so 40 metre band should be no problem.

Just as a matter of interest, I have also tried a secondary winding of aprox 20 turns to couple the antenna instead of the 100pf cap, it works ok but does not appear to offer any major benefit. But maybe it's worth a try for someone.

[MrBungle]

Excellent. I'll have a bash at that as well.

I've been reading that some people have used them as oscillators/mixers with only a 6v supply as well, for example as follows: http://projectcircuit4u.blogspot.co....-ef956ak5.html ... Interesting application.

[coopzone]

Here is another version of the circuit. Incorporating suggestions from this list plus a few extra capacitors I thought of later!

It's currently under test, but it's looking good so far.

[Murphy Maniac]

What gauge of wire did you use for winding the coil, or doesn't it matter?

[Argus25]

Originally Posted by kalee20

I really doubt it puts out 250mW of radiated power!

That is exactly right, even if the output stage could deliver 250mW into a matched dummy load there is no way the radiated power would be anything like 250mW, the radiation resistance of 10 foot of wire is abysmally low at MW band frequencies. Probably the radiated power would be less than 10mW from that wire if you were lucky.

Possibly with the right loop correctly impedance matched to the plate, you might get 50mW radiated power with a plate dissipation of 250mW to 1/2W. So you don't have to worry about upsetting anyone with the actual small output level.