Questions on 160/80 metre AM transmitters.

[Aub]

Here's my first AM rig, Topband only. I built it in 1980, during my lunch breaks in the workshop at British Gas. The VFO is a 6C4 running at half frequency.

I used to power it from the power supply I'd kept from a dismantled Rigonda Symphony radiogram. The modulator uses a QQV03-10 in push pull. The modulation transformer is a HT transformer.

It used to get me all round Europe on CW ( when I was keen )

Hasn't been powered up for about 20 years. Don't all laugh at once!

Cheers

Aub

[Ian - G4JQT]

Very nice Aub. I wish I had facilities of bending metal better than just in a vice!

Here's my 60 metre copy of an AT5 for the AM channel on 5317 kHz:

[Aub]

That's very good Ian. Looks like the AT5 power supply next to it. I must make something for 60.

73

Aub

[Cruisin Marine]

A Nice couple of rig's there guys

[Aub]

There's another one of my old favourites in here:-

https://worldradiohistory.com/UK/Pra...PW-1966-11.pdf

Good old F.G. Rayer. He must have had a shed full of 160 and 80 AM rigs

This one has a beefier modulator.

Cheers all

Aub

[Bazz4CQJ]

Is that modulator a bit OTT? I've just mailed Ian re the bean can circuit. My recollection is that a pair of EL90's quite easily provided "full" modulation for the 03-10 and would probably do the same for a 5763 or 6BWB PA .

B

[Station X]

Originally Posted by Aub

There's another one of my old favourites in here:-

https://worldradiohistory.com/UK/Pra...PW-1966-11.pdf

Good old F.G. Rayer. He must have had a shed full of 160 and 80 AM rigs

This one has a beefier modulator.

Cheers all

Aub

I'm slowly obtaining the parts to build that. As usual it's the coils and variable caps that are difficult to source. Ed Dinning wound the transformers for me.

[Aub]

Yes, the EL90s should be more than enough for the QQV03-10 or the other valves. I suppose that modulator was a bit OTT but it's nice to have some grunt in reserve.

All the best

Aub

[Aub]

Originally Posted by Station X

I'm slowly obtaining the parts to build that. As usual it's the coils and variable caps that are difficult to source. Ed Dinning wound the transformers for me.

Hi Graham. Yes, old style coil formers are a problem aren't they. The fashion, over the last 40 years or so, to use toroids for everything hasn't helped. I'll see what variable caps I have in my collection. Is Ed winding you a proper mod. transformer, or is it a mains transformer as used in the article?

Cheers

Aub

[Station X]

Ed wound me a mains transformer for use as a modulation transformer.

I've sourced the 100 pF variable cap for the VFO. It's the 140pF 600V "Eddystone" variable cap I'm having problems finding.

I doubt I'll be able to build it on the small chassis Rayer specified. I try things first before bending up the chassis.

Probably best if I start a new thread on this rather than take this one off topic.

[Bazz4CQJ]

Originally Posted by Aub

I suppose that modulator was a bit OTT but it's nice to have some grunt in reserve.
All the best
Aub

That modulator has a lot of power output, but is the 12AX7 (ECC83) being asked to do a lot? The circuit I've used with the EL90's, uses an ECC83 in front of a another double triode, the first half being another amp stage and the second being the phase splitter for the output valves. I've certainly seen the single double triode in a number of circuits, but three stages is nice to have .

B

[AdrianH]

When I first thought about this project I just built a mod section using parts from an old TR1998/STR9X a VHF Aircraft TX/RXThat was driving a TT15 Valve for output, as in this circuit.

That uses two EC90's as output and driven by a EL90. HT 250 Volts and -11 Volts on the grids of the EC90's

Unfortunately the transformer labelled 2T2 was open circuit, the wires had corroded, but the output transformer 2T3 with thicker wire was still OK.

I thought I could still use the idea driving the two EC90's with an ECC83 in a phase splitter circuit and an EF91 pre-amp, basically like the typical Mullard audio amp circuit, but there must be something particular with the transformer driver as I could not get the required swing.

This was my circuit.


I note once again that the November 1966 PW posted by Aub, again uses a driver transformer, so I am guessing that it must take the grids positive and as such drive the outputs harder sinking more current through the output transformer and bigger swings etc. I would appreciate knowing if that is correct.

Also I am quite happy if others post ideas, projects within this thread as I am finding lots of interesting comments on low HF transmitters, perhaps there could be other stations to talk to if enough get built, dug out of cupboards etc.

Perhaps change the main Title to something suitable such as Questions on 160/80m transmitters ?

Adrian

[Station X]

Thread title changed at OP's request.

Should we restrict this to valved transmitters?

[AdrianH]

I am not too bothered, I guess we can keep this one valve, I know there were also Transistor circuits coming out in the 60's, so perhaps someone could start a transistor one if required.

Then there is still the Home made section for people's builds.

Adrian

[Aub]

Thinking about modulators again. In the Topband transmitter that I posted the pictures of in this thread, I used a push-pull modulator as the single EL84 I'd been using when I first built it, didn't seem to produce enough audio. Over the years, I've wondered if the insufficient mod was more due to core saturation of the half wave mains transformer I was using as a mod transformer, rather than the power itself. I changed the circuit to push pull, using a 250-0-250 mains transformer with a QQV03-10 deployed for audio use. This worked very well. I suppose the issue of core saturation went away due to being push-pull.

Aub

[G6Tanuki]

I guess if the magnetisations from the currents for the single EL84 and the PA valve flowing through the modulation transformer were additive it could have caused core saturation.

Reversing the phase of one winding would have the magnetisations cancel each other out and so reduce the risk of saturation. That's the cunning thing about the Codar-type autotransformer approach, which allows the use of a seemingly-very-small transformer.

[Dave757]

Originally Posted by Aub

There's another one of my old favourites in here:-

https://worldradiohistory.com/UK/Pra...PW-1966-11.pdf

Good old F.G. Rayer. He must have had a shed full of 160 and 80 AM rigs

Aub

Hi,

He certainly did! After his death a quantity of his sets were acquired
by my old friend Howard Turner(RIP) of Centre Electronics Birmingham,
and within a day or so of the purchase an irate relative called at the
shop and said they should not have been sold and demanded their return.

They were all returned except I believe for 1 set which had already been
sold on.

I wonder where they all are now?

Kind regards
Dave

[G3VKM_Roger]

Hi All,

Since the thread has run onto Top Band/80m TXs other than the AT5, I have attached some pix of a very nice TX I used to own. It came from the estate of G3NHU but was built by G3HRK and used mobile with a Command set RX.

The mechanical construction was first-rate, the set ran a 6AQ5 PA, modulated by a pair of the same and used an ex-SCR522 mod transformer and several other ex-military components. I passed it on to a VMARS member a couple of years ago.

Belated HNY to all.

73


Roger/G3VKM

[Aub]

Very interesting. Yes, where are they now? He also did designs for 4 metre and 2 metre AM rigs. I've got a 2 metre one somewhere, but I won't post it in this thread 😄

Aub

[Aub]

Originally Posted by G6Tanuki

That's the cunning thing about the Codar-type autotransformer approach, which allows the use of a seemingly-very-small transformer.

Yes indeed. Quite clever.

Aub