10-12-2016, 05:29 PM
(This post was last modified: 10-12-2016, 05:38 PM by Murphyv310.)
Hi.
Here is the finalised circuit for the two valve TX.
This works just about as good as the three valve TX that I will post up in a day or two.
Some interesting facts, the current consumption, HT 0.3ma LT 24ma. You'll notice the anode load on the audio preamp is 120k, the anode volts on my Fluke DVM is 3.6v and this stage gives 3 times gain, I haven't fitted a volume control as most mp3 players have a volume control anyway, Line output seems to be around the right level but a volume control could be fitted prior to the audio preamp. One thing to bear in mind is these valves don't like a low value grid resistor so if you fit a conventional pot of 4.7 meg etc you'll need to put the top end to grid rather than slider to grid, strange but true and this seems to be across the board with rod pentodes.
The pi tank circuit is conventional, I've used an old former from a MW coil off a ferrite rod, I have 65 turns of 24swg cotton covered enamelled copper wire, The two chokes on the modulator valve are not critical but the values I have used seem to work best. I am using a 1566khz crystal but any in the AM band will work fine, although the top of the band crystals will work best, the oscillator works fine up to 8mhz, above this frequency it gets a bit tetchy and doesn't always start, The tank circuit would need re-designing if a short wave micro TX is required.
The three valve TX only gives a marginal improvement, possibly a little better audio quality but it does need more drive due to the NFB loop so it could go to four valves.
If you have a MP3 player or other signal source with around 2-3v p-p drive then the one valver is fine.
I do hope others build this and give me feedback.
I'm happy for anyone to reproduce this circuit elsewhere but please mention who did the donkey work.
Have fun.
PS, the audio input cap isn't critical, frequency response is good anyway but if it's found to have too much bass then this cap could be dropped to 1nf or below, I'm using a 4.7nf here.
Oh, on the picture of my prototype you'll see the tank coil has a ferrite rod, with this left in and the "aerial" tuning cap fully meshed the rod will radiate the signal about three metres, the quality is great so good in fact I knackered the speaker in the DAC90A listening to Kraftwerk at high volume!!!
