Questions on 160/80 metre AM transmitters.

[G6Tanuki]

Originally Posted by Bazz4CQJ

How critical would the matching be of the centre-tapped winding on the transformer used for the modulator? I'm wondering about whether the transformer from a Pye Ranger could be used, using the centre tapped winding originally used by the push-pull EL90's in the Ranger modulator ?

B

As a followup to this, and noting another comment where it's mentioned that the AT5 transformer is _not_ symmetrically centre-tapped, the modulator of the RSGB Handbook AT5-alike is shown using a modulation-transformer with a 1.2:1 ratio.

See below.

But, like I said, in practice you will probably get away with a 1:1 effective ratio. Impedance-mismatches in this sort of situation can generally be dodged by making sure there is enough audio power available so you can still get the necessary voltage-swing in the absence of a perfect match.

[Bazz4CQJ]

I've never used an AT5. Does it achieve a good modulation level using just the two sections of the 12AX7 prior to the 6BW6?

B

[G6Tanuki]

The audio can get a bit rough if you talk it up too much; but remember it's for communications-purposes not hifi. A bit of 'flat-topping' can actually improve readability.

The original design was intended for use with a high-output crystal microphone [usually Acos was the brand everyone had] so a couple of triodes ahead of the 6BW6 was all you really needed. These days I guess you could achieve the same using an Electret mic insert and a little bias battery in place of the crystal mic.

The RSGB design uses a 12AX7 double triode [both halves] followed by a Z77/EF91 pentode to feed the 6BW6. It's still specced as needing to be drivn from a crystal microphone.

[Bazz4CQJ]

My original 2m rig had a pair of double triodes, so three stages of amplification and then the phase splitter for the PP EL90's. That achieved full modulation even when Bob Harris was using it .

B

[AdrianH]

Still here and reading comments, after frsimen's comments on LTspice I have finally installed it on my Ubuntu system with Wine!Not that I will be able to use it yet without serious reading and playing, but that is later. Still waiting for chokes and things to turn up so can not get much further yet.

Adrian

[Cruisin Marine]

I reckon the mod transformer is a 0-110-240 volt auto transformer.

On a slightly different but related note: Does anyone remember a similar rig, possibly earlier than the AT5 which was a similar size but darker colour, probably either dark Green, dark Grey, dark Brown or Black in colour?

I had one way back but can hardly remember much more about it, except it had to be left on for an hour for the VFO to settle down..

[frsimen]

The AT5 VFO uses polystyrene capacitors for the frequency setting components. The trimmer is a small mica compression type.

The cause of the drift problem is that when transmitting there is a lot of heat in a small place and some components heat up faster than others, so the temperature compensation doesn't really have a chance. Stop transmitting and it drifts back to somewhere near the original frequency. The VFO alone drifts steadily but nowhere near as fast.


There were two other small top band transmitters that come to mind. The TW Communications Topbander and the KW160. The TW was quite compact from the photos I've seen of it.

Paula

[G6Tanuki]

The AT5 also has the drift-issue that when switched to "receive" you lose the HT to the local oscillator [and the rest of the valves] so it cools down.

OK, so you need to do something about the LO being active when on receive so its signal [or harmonics] don't leak-out and desensetize the receiver - I get that. Which is why my "Solid-state AT5-alike" FrankenFunker runs the VFO continuously but includes a diode-switch to drop a 50pF capacitor across the VFO on receive, so shifting it a few hundred KHz away from the receive frequency.

Keeping the LO running at all times is a big thing for stability, as is sensible component choice.

[A Stateside friend had an EICO 753 transceiver, which as originally designed had a good thermally-stable VFO; alas it got reworked when it went into series-production and the cheap-but-affordable-to-your-average-Ham components were substituted with non-thermally-matched parts, hence the radio became known as the Seven-Drifty-Three or the Drifty-Fifty]

[Bazz4CQJ]

I'm sure that Codar thought it was a good selling point to make it so compact, but that was always going make the thermal side tricky. It will probably be a challenge to make a clone with same valves and in the same volume that does much better?

B

[Cruisin Marine]

I'm sure that Codar thought it was a good selling point to make it so compact, but that was always going make the thermal side tricky. It will probably be a challenge to make a clone with same valves and in the same volume that does much better?

If I were to make a present day version I would def use a PLL/Synth to eradicate that problem. It may not be original, but what the hell- the rest can be.

There were two other small top band transmitters that come to mind. The TW Communications Topbander and the KW160. The TW was quite compact from the photos I've seen of it.

Paula

Sadly, it is neither of those I refer to in my post above. It also had 80m capability (the O/P was about half on there), I think the PA was a 6BW6 or similar, looking back, I never did know the make, although it had a model number on it somewhere. It was very similar in size to the Codar though. It had a Belling Lee antenna socket I recall (which I changed for BNC).

[Aub]

There is a design in Practical Wireless, from the late sixties ,for a similar transmitter, both in size and performance. I think it was called the Five Ten, by a man called T. Simon. Probably worth looking up on the World Radio History site.

Cheers

Aub

[Bazz4CQJ]

Originally Posted by Aub

There is a design in Practical Wireless, from the late sixties ,for a similar transmitter, both in size and performance. I think it was called the Five Ten, by a man called T. Simon. Probably worth looking up on the World Radio History site.
Cheers

Aub

Aub, was it T.Simon or A.S.Carpenter? In front of me is an article by Carpenter, a modulator for his "Ten-Fifty" transmitter which had been described in Jan '67. I think the 5-10 had a 6BW6 PA and the 10-50 maybe a 6146?

As I recall, I was planning to build the Five Ten, which was very AT5-like, but then failed the Morse test and succumbed to the lure of being a G8 on 2m .


PS the ten five is by Carpenter and is on p661 on jan 67 PW. ECF80 very conventional osc, 6BW6 and ECC83 in to 6BW6 mod. Same edition of PW as the dreaded "Explorer" VHF radio .

My flipping sister threw away all my old PW's and SWM's when she cleared my mums house out. We don't speak.

B

[Aub]

Baz, it could be carpenter. It was dual band, 160/80 and used an ECF80, I think for the VFO and buffer/doublet. I'll dig it out, unless someone else does it first. Shame about your sister.

Cheers

Aub

[Bazz4CQJ]

Overlapping posts Aub.

B

[Aub]

I found it. It was Carpenter:-

https://worldradiohistory.com/UK/Pra...PW-1967-01.pdf

Cheers

Aub

[Bazz4CQJ]

Originally Posted by Aub

I found it. It was Carpenter:-

https://worldradiohistory.com/UK/Pra...PW-1967-01.pdf

Cheers

Aub

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

B

[Graham G3ZVT]

Originally Posted by Cruisin Marine

I'm sure that Codar thought it was a good selling point to make it so compact, but that was always going make the thermal side tricky. It will probably be a challenge to make a clone with same valves and in the same volume that does much better?

The AT5 was a popular choice for mobile operation. From memory, the valve heaters were wired in series-parallel for 12v operation, so one only had to add an inverter, or more likely a war-surplus dynamotor for HT.

[IanG3XYV]

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

Thanks for the reminder of that PW design. I built it and used it on air when I got my licence in late 1969. It worked well on AM and CW but I lived in a fairly remote area and there weren’t many locals. Found I could double up to 40M and the world seemed to open up on CW at least.

Pushing that further I added an 807 amplifier for use on 20M which brought me over 100 countries. Heady stuff for an 18 year old. Don’t recall VFO stability being a problem once warmed up. Wish I still had that TX but long gone sadly.

[AdrianH]

So another circuit to compare with and I like the idea of a Dynamotor as I have one 12 Volt input, Volts may be a bit low on the output at 225 V at 100 mA, but will see what my options are when completed.


Adrian

[Radio Wrangler]

You may not like the current those things take at 12v. Also, the noise gets on your nerves after a while if you're not using it /M. Correct, in-period but not the most fun to live with.

The 19 set Dynamotor was one of the most common. It was quite understandably not a noise problem in a tank with an engine revving and a gun firing.

David