Questions on 160/80 metre AM transmitters.

[Bazz4CQJ]

We used to "play" with an old dynamotor at school. Damn heavy chunk of metal. The start-up current was more than the lab PSU could really provide and they would trip. We had to get them going by raising the volts slowly. This entertained us greatly as the noise coming from it was similar to that made by the Starship Enterprise undergoing rapid acceleration!

Strongly recommend an inverter for practical use.

B

[Aub]

Originally Posted by Bazz4CQJ

Might Carpenter have got better spacing and shielding of the VFO in his layout than Codar got with the AT5?

B

I thought I'd replied to this point earlier today but seems to have gone missing. I think the Carpenter layout might be a little larger than the Codar, so that might help with stability.

Aub

[Bazz4CQJ]

Originally Posted by Aub

Might Carpenter have got better spacing and shielding of the VFO in his layout than Codar got with the AT5?

B

I thought I'd replied to this point earlier today but seems to have gone missing. I think the Carpenter layout might be a little larger than the Codar, so that might help with stability.

Aub[/QUOTE]

The objectives of a lone constructor and a commercial producer are different and it seems to me that even if you clone the AT5 circuit, you might well make it a little bigger and see what you can do to "protect the VFO".

Seeing that Ten-Five again certainly brings back memories; the summer of 1968 - piece and love - but that was just on the telly and never got as far as West Yorkshire; unless Wrangler was doing it

B

[Aub]

While we're at it, there's another interesting project from the same period. Practical Wireless, March 1967. FM is used here.

Cheers

Aub

[AdrianH]

The dynamotor I am thinking of trying is 12 Volts at 3.8 Amps for 225 at 100mA so about 50% efficient if my maths is correct. That idea is just for a bit of fun, it is unlikely that my unit will ever get on the air and just be a bulb test. The aerial space is minimal and although I could tune up a 30ft of wire I doubt there is going to be much activity on Top band AM around here.

When did the power increase on Top band by the way?

Adrian

[Aub]

Not sure about the power increase. I know it changed from the old 10 watts input years ago and was 9dbw output ( 8 watts for AM ) and 15dbw PEP for SSB. I think there may be some spot frequencies where 26dbw is allowed but I'm not sure. Need to check .

Cheers

Aub

[G6Tanuki]

For reference, here is a F.G. Rayer 160M 3-stage transmitter circuit from April 1970 SWM.

5763 as the PA, and I note that it uses a coil in the anode of the 2nd stage, apparently tuned by stray capacitance as there's no separate parallel capacitor.

And another circuit from Pratical Wireless: http://www.g4kfk.co.uk/7108_PW_QRP_T...r_LF_Bands.pdf

[G6Tanuki]

Originally Posted by Aub

Not sure about the power increase. I know it changed from the old 10 watts input years ago and was 9dbw output ( 8 watts for AM ) and 15dbw PEP for SSB. I think there may be some spot frequencies where 26dbw is allowed but I'm not sure. Need to check .

Cheers

Aub

See attached [applies to 'full' licence-holders; novice and intermediate have lower limits]

[Bazz4CQJ]

In terms of stability, both of those designs use separate valves for osc and buffer; maybe that helps keeping the VFO at a steady temperature. They both have V stabilisers, which I don't think the Codar has, or is that in the PSU?

B

[G6Tanuki]

AT5 mains PSU does have an OA2 regulator for the VFO.

[Bazz4CQJ]

Not quite sure why, but I had it in my mind that best practice was have the stabiliser tube as close as possible to the oscillator?

B

[Radio Wrangler]

Originally Posted by Bazz4CQJ

1968 - piece and love - but that was just on the telly and never got as far as West Yorkshire; unless Wrangler was doing it

In those days I was buying a quarter of Riley's chocolate toffee rolls on my walk home from school for tea and an hour or two working on my first oscilloscope. Peace and love may have passed my backwater by, but I enjoyed the great music that went with it - whatever "it" was!

David (dark chocolate ones, of course)

[Radio Wrangler]

Originally Posted by Bazz4CQJ

Not quite sure why, but I had it in my mind that best practice was have the stabiliser tube as close as possible to the oscillator?

B

A lot of people say this, but it's probably an old wive's tale. Regulator tubes are very noisy, and before fancy semiconductor diodes were at the heart of noise sources. So they need filtering before their voltage reaches a VFO else you get random noise FM. So they look after DC voltage control and slow variations, so you can have a longish wire to the VFO without fear. Also when you consider the current a VFO designed for stability is going to take and the ESR of the reg tube at its operating point, and you can allow several ohms worth of wire so long as you don't also have it feeding anything greedy.

David

[G6Tanuki]

Yes, the inclusion of a regulator-tube doesn't absolve you from the need to include decoupling [series L or R, parallel C] in the various feeds. The problem being that if you're not shoving enough current into the regulator-valve any capacitance that appears in parallel with it can create a 'relaxation oscillator' when the regulator goes at all current-starved.

And you need to isolate the frequency-controlling parts from the variable apparent-source-impedance of the glow-tube too.

[Bazz4CQJ]

Originally Posted by G6Tanuki

The problem being that if you're not shoving enough current into the regulator-valve any capacitance that appears in parallel with it can create a 'relaxation oscillator' when the regulator goes at all current-starved.

And you need to isolate the frequency-controlling parts from the variable apparent-source-impedance of the glow-tube too.

.
Are you saying lots of decoupling on the High volts side, but much less on the Low volts side of the regulator ?

B

[turretslug]

Seems a curious notion- as mentioned, its typical ESR will be far more than even a generous dollop of typical interwiring. They also run warm, even comfortably positioned in the operating window, so keeping them remote from tuning components seems wise. The AR88 has shunt stabiliser and LO, BFO valves at opposite ends of the chassis- now there have been umpteen ideas over the years for "improving" these stalwarts but I've never seen re-positioning the VR150 as one of them!

[Aub]

Originally Posted by Aub

While we're at it, there's another interesting project from the same period. Practical Wireless, March 1967. FM is used here.

Cheers

Aub

Here's the PDF. Couldnt do it before from my mobile phone. on the laptop now:-

https://worldradiohistory.com/UK/Pra...PW-1967-03.pdf

[Bazz4CQJ]

I remember that front cover, barely recall the design but don't recall anyone ever using FM on topband. Certainly a compact rig.

B

[Bazz4CQJ]

In the context of valve VFO, I've always wondered about the use of a battery-rated valve; 1.5 or 2V filament and maybe 60-90V HT probably taking just a few mA.

The unit is still going to have a couple of hot valves at the far end, but maybe some old Space Shuttle tiles (or similar) could offer good insulation?

If the use of a FET(s) crosses the line, it might be worth consideration?

B

[G6Tanuki]

Originally Posted by Bazz4CQJ

In the context of valve VFO, I've always wondered about the use of a battery-rated valve; 1.5 or 2V filament and maybe 60-90V HT probably taking just a few mA.

If the use of a FET(s) crosses the line, it might be worth consideration?

B

Pragmatically, a sensibly-designed free-running VFO shouldn't suffer enough drift to be a problem when used with 1.8/3.5MHz AM.

Keep the heaters on all the time, keep the VFO running all the time, and use a diode-switched capacitor to shove the VFO up a few tens/hundreds of KHz when on receive.

FET and transistor VFOs - in the absence of the always-on-running mitigations - have low thermal-inertia and so just swap slow frequency-drift for fast-drift.

Klaas Spaargaren PA0KSB's "Huff and Puff" stabiliser from the 1970s is an interesting fix for the problem; it will happily lock to a VFO-tuned frequency and hold it to within 50Hz.

http://www.hanssummers.com/huffpuff.html