Top band transmitter

[mictester]

Originally Posted by ms660

Part of a PW article...Top Band Tx....807:

https://www.americanradiohistory.com...%22transmitter 807%22

Lawrence.

That was the first transmitter I ever built! It was modified down in frequency into the higher end of medium wave, and used to broadcast for a few hours each afternoon after school! It was impounded by the PTT on their second visit.....

The next rig had a pair of 807s!

[Bazz4CQJ]

All these references to 6BW6's and 5763's are stirring up memories. Shall we all build top band rigs to help get us through the coming months?

There was a design published in PW around '68,

RF........ECF80 osc/buffer in to a 6BW6 PA, and MOD.....ECC8x in to an ELL80.

I'm sure I have the pages in the filing cabinet. I bought the ELL80 (brand new) then never used it - but it's only a few feet from where I sit and I'm sure the other valves all lurk in my junk box. I have a Pye Cambridge case which has no innards and would offer a nice chassis and there's a PSU which only needs a fuse or two putting in it to power it up quite nicely.

B

[Aub]

Do it Bazz. I've also used a QQV03-10 as a push pull modulator. Got a few of those lying around.

73

Aub

[Bazz4CQJ]

I think I should do a bit of listening first. Last time I took a look at 2m it was totally dead here. It's ages since I last listened to top band.

B

[Bazz4CQJ]

Re my comments in post #22, some clarifications. The design I was thinking of appeared in PW in Jan ’67, called the “Ten-Five” transmitter by A.S. Carpenter and used 6BW6’s for both the PA and the single-ended mod. The construction was notable in that it’s base measured only 8 x 4.5 inches.

In Sep ’68, Carpenter published a “QRP Modulator” design using an ELL80. Another option for the modulator would be the Pye Ranger design using a pair of EL90’s and the “bean can” transformer.

Can anyone tell me about AM activity on 160; are there any particular frequencies worth checking or anything like a calling channel?

B

[David G4EBT]

Originally Posted by Bazz4CQJ

Re my comments in post #22, some clarifications. The design I was thinking of appeared in PW in Jan ’67, called the “Ten-Five” transmitter by A.S. Carpenter and used 6BW6’s for both the PA and the single-ended mod. The construction was notable in that it’s base measured only 8 x 4.5 inches.

B

That was almost identical to the CODAR AT5 - I don't know which came first!

('Ten Five = ten Watts, five valves).

Arthur Carpenter also designed a receiver, which more or less matched the transmitter as I recall.

[Aub]

Bazz,

There are still a few AM nets on Topband. You would probably be best to listen to some of the WebSDR sites around the country, which should give an indication of activity. Of course, there is quite a bit of AM activity on 80 metres, 3.615Mhz , every morning between about 0730 and 0900.

Cheers

Aub

[G4XWDJim]

I've got a few octal TT11 PA bottles which I'd be happy to give away if anyone would like to build one into a transmitter. Once this current restriction is over of course.

Jim

[Ian - G4JQT]

Not exactly what the OP is asking for, but I can recommend the Codar AT5 AM/CW transmitter. They pop up on ebay occasionally (often in a bit of a state) but they are very fixable. They are a refined design based on all those similar circuits appearing during the late 50s to mid 60s.

For those familiar with constructing valve stuff, they are quite reproducible - the only part that is not obvious is the modulation transformer which turns out to be just a 15VA transformer with a centre-tapped 240v primary.

I built one for the 60m AM channel.

Here's a tidied up AT5 manual from G3UPA: http://www.g3upa.com/httpdocs/PDFfiles/Codar-AT5.pdf

Ian

[saddlestone-man]

Mention of the 5763 made me wonder why it has an all-numeric code and not a 6xxx code?

Interestingly, looking at its datasheet, it has a 6.0V heater, rather than 6.3V. I presume everyone who used it in a topband transmitter connected it to the 6.3V wiring, along with all the other valves.

best regards ... Stef

[Aub]

Hi Stef. I hadn't realised about the 6.0v heater. I have a 5763 in my 80 metre transmitter, which is connected to the 6.3v heater supply with all the other valves.

Cheers

Aub

[Terry_VK5TM]

I had a 5763 in an AM 2m transmitter back in ..mumble mumble... and it was running on the 6.3V heater circuit.

In fact, a quick squizz through several magazines and on the internet all show it being run at 6.3V.

[Bazz4CQJ]

Originally Posted by Ian - G4JQT

Here's a tidied up AT5 manual from G3UPA: http://www.g3upa.com/httpdocs/PDFfiles/Codar-AT5.pdf

Ian

Thanks for posting that. The manual starts by saying that the AT5 used a new, Vackar-derived VFO and is "extremely stable".

Can anyone say whether the AT5 was more stable than the typical top-band rig of its era?

B

[Radio Wrangler]

The Vackar is still just an L-C based oscillator. It carefully limits the amount of coupling between the active device and the resonator, so its stability tends towards the stability of the inductor and the capacitors used... provided it is carefully designed. Other oscillators can also be carefully designed.

Oscillators in the better than usual class for stability have measures taken to compensate thermal change. A combination of notmal and NTC capacitors can be designed to produce a compensation effect. It's a matter of curve-fitting, so careful design can get it right at two points on the temperature scale, at least for the individual oscillator the compensation network was designed using.

The next step up in stability needs individual adjustment of temperature compensation. Oxley Developments made the 'Tempatrimmer' and cheaper 'Thermotrimmer' capacitors which gave fixed capacitance and variable temperature coefficient. These were expensive components, and the time taken to vary temperature and to individually adjust them is also going to be very expensive. The Codar stuff was, shall we say, marketed at the time at very attractive prices.

The Vackar has a reputation for stability, but it can be equalled, and it also can be spoiled. But it's entered the folklore as put in a Vackar circuit and you can say it's very stable.

To answer your question, you need someone with an AT5 who's had it in a thermal chamber and made measurements. I've not heard of anyone doing this.

If you're interested in the Vackar and want to know what makes a good one, Peter Martin G3PDM covered it well in the RSGB article for his HF receiver. This can be found in RSGB handbooks from the late sixties and through the seventies. I put a cut-down version in the ARRL handbook from 1995 onwards. It's good advice and still relevant to all VFOs.

I don't think you can still get tempatrimmers and thermatrimmers from Oxley, and they don't seem available on the auction sites. I put a little circuit for a varactor/thermistor variable tempco compensator in the ARRL handbook. It's oddity is that it's designed to be easy to adjust in a single temperature sweep, instead of repeated ones.

The AT5 doesn't have a reputation as a bad drifter, so it should be OK for its task.

There is also a less well known cause of drift. If a VFO is used on the same frequency as the output of a transmitter and a little RF leaks back into the oscillator, this can pull it in mild cases, and induce unstable oscillation in worse cases. People in the day were aware of this and some equipment went to lengths to either multiply up lower frequency VFOs, or use crystal mixer schemes all to avoid this effect.

So have a look at the AT5 circuit. If there isn't individually adjustable temperature compensation, its stability won't be amongst the best, so that shoots down the 'extremely' adjective.

David

[Bazz4CQJ]

Thanks for those comments David. My memory does not recall temperature compensation being mentioned very often in the numerous top-band Tx's published in PW in those days. However, when Codar decided to build such a compact rig inside a complete enclosure, it must have crossed their minds that it might well be an issue. The circuit diagram shows C5 to be some sort of tempco component, but at 10-40pF, it's certainly not the standard Oxley device which I think is only ~2pF? I have two stashed away waiting to be used.

I recall that in the early '70's is was quite easy to buy small ceramic capacitors with a wide range of capacitance values (low pF) and quite a range of tempco values. I suspect that apart from what anyone could find in their junkbox, that probably is not the case now.

B

[JohnBG8JMB]

AT5 mod transformer is NOT centtre tapped
It is 0-110-240v, the 0-110 is the modulator load, the 110-240V wdg goes to the TX

John G8JMB

[g4wim_tim]

If frequency stability is a problem / concern it might be worth considering a 'huff and puff' system - google "huff and puff oscillator stabilization" for some ideas.

Quite simple to implement and works well.

[g4wim_tim]

Originally Posted by Radio Wrangler

There is also a less well known cause of drift. If a VFO is used on the same frequency as the output of a transmitter and a little RF leaks back into the oscillator, this can pull it in mild cases, and induce unstable oscillation in worse cases. People in the day were aware of this and some equipment went to lengths to either multiply up lower frequency VFOs, or use crystal mixer schemes all to avoid this effect.

I had this problem on my AT5 based rig and it showed itself as a power hum on the 160M, I had to fit RF filtering on the VFO valve heaters to fix the problem.

[Ian - G4JQT]

I think the Codar stuff about the VFO is just advertising buff. As RW says, stability is mostly related to the type and temperature of the frequency-determining components. As far as I can tell (and I've had a few through my workshop) the AT5 is no better or or worse than many valve VFOs once it's warmed up.

Thanks for the transformer info John. Where did you find that? I've looked everywhere for detailed mod transformer info for that rig and the best I found was that the choke in the PSU and the mod transformers are the same - although the centre tap isn't used in the choke. A 15VA centre tap transformer works as well as the real thing - as far as I could measure.

I did play with the original mod transformer in my AT5 to check for symmetry (it's not labelled anywhere) while making my 60m xtal-controlled version, and found there was no noticeable difference in tone-modulated waveform if the outside connections were swapped.

[David G4EBT]

On the tidied up AT5 circuit by G3UPA at the link which Ian provided in post #29 above, the power supply circuit on page 7 shows Pin 8 of the plug as HT+2 and pin 7 as HT+1. However, the diagram below the circuit shows those transposed. IE: Pin 8 of the plug is HT+1 and Pin 7 is HT+2. The original Codar AT5 manual shows likewise:

http://www.vmarsmanuals.co.uk/new/at5.pdf

Maybe I'm misunderstanding something - my AT5 (bought second-hand for £10.00 in 1980), went to a new home back in 1990.

What I do remember is that from a safety perspective, Codar ought to have had the female socket on the flying lead from the PSU to the AT5, and the male plug on the AT5 rear chassis apron. As designed, if the PSU is switched on with the power output lead not plugged into the AT5 (or should it come out), there will be 270V HT+2 and 150V HT+1 on the exposed pins of the male plug from the PSU. But then it was different times and back then, if anyone had inadvertently touched the exposed male plug and got a shock, despite the unsafe design, the response would have been 'more fool you - you won't do that again'.

As to drift, many AT5 owners at the time stood the transmitter on top of the PSU to save desk space, so in addition to the heat generated in the AT5, the heat from the PSU added to that. I was never very keen on the flimsy slide switches used as the mains switch and the AM/CW switch. AT5s always sounded nice on air, as AM does. Looking back despite 160M being the lowest allocated frequency posing a challenge antenna wise, they were often used mobile with a loaded whip aerial bolted to the rear bumpers of cars, swaying in the breeze (when cars had bumpers). Room was found beneath the dashboard parcel shelf to house the AT5 & PSU. A Black and Decker drill made short work of doing that!

Another world.