1.8MHz A.M. bit of fun build.

[AdrianH]

I have been reading posts when ever I can and found a few that interested me. One was on the closure of the MW transmitters, another asking questions about small around the house MW transmitters and then a topic on Top Band Transmitter.

So it got me thinking about continuing my learning by building and a very simple TX may be easier for me that build a valve receiver. So I have started with a simple VFO, hope you do not mind the build method as I do not plan on using it as shown in the picture.

I was not sure if the circuit would work, in the circuit I am following, the suppressor grid was a separate connection unlike the 5840 Pentode which has it internally connected to cathode.

I spent while winding a coil on a home made former of paper with PVA.

I need to ask questions on the caps C2,C3 and C4, I would normally just try and get silver Mica for these but wonder if there is anything better around now for temperature drift?

The plan is a very low power, as in milli-Watts's Top Band A.M Transmitter that could well cover some of the medium wave if required around the home. Next stage would be a buffer and then a small amp stage and modulator.

Adrian

[G6Tanuki]

Silver-Mica is still a pretty good choice for the low-value capacitors in the grid/cathode circuit.

The alternative would be ceramic - the so-called "NPO" types - but avoid the modern 'High-K' variety which pack lots of capacitance into a truly tiny space but in doing so have a change-of-capacitance-with-temperature relationship you definitely don't want as part of a VFO's tuned-circuit!

[Herald1360]

Originally Posted by AdrianH

I spent while winding a coil on a home made former of paper with PVA.

I can't help but think that varnished bog roll tube would look much better

[AdrianH]

Originally Posted by Herald1360

I can't help but think that varnished bog roll tube would look much better

I could dip it in some Teak varnish if it makes it look better

Some silver mica caps and 100 Volt zener on order, and a variable cap from Ebay is on it's way.

Thinking of following it with a 6021 twin triode, but not sure if to use it as a cathode follower then amp stage or not?

Adrian

[g4wim_tim]

Hi Adrian,

if you're ever across this way once lock down is over, I have a few valves, coil formers and other bits and bob's left over from my valve TX build - you're welcome to them.

Who knows - might work you on 160M AM !

[G6Tanuki]

Originally Posted by AdrianH

Thinking of following it with a 6021 twin triode, but not sure if to use it as a cathode follower then amp stage or not?

What about using one half as the RF "power amp", and the other half as a modulator?

Common cathode-inductor at AF but decoupled to RF so you get 'cathode modulation'.

You'd need to neutralise the triode RF stage - use a centre-tapped anode-coil and a whiff of capacitance [twisted wires? a few inches of coax?] feeding back to the grid.

[Radio Wrangler]

Pretend it's a mini 3CX1500/8877 and go for grounded grid to avoid the need for neutralisation?

David

[AdrianH]

Sorry for the delay in responding, I have been a bit pre-occupied in getting my old HP scope to work and taking a few measurements from the makeshift VFO.

I have also been trying to put my ideas down in Kicad, so I can then work through them.

Initial thoughts are from the anode of the VFO into a potential divider to drop the level, I seemed to have something like 40 Volts pp here but with lots of loading on the osc. Then into a cathode follower, do not need gain just lots of buffering, looking for approx 8 Volts pp into the grid of the 5840 U2A. The 5840 Cathode to be at 4 Volts with 1mA flowing through the valve and therefore the grid at -4 Volts with respect to Cathode.

The hope is to drive the 5840 in class B and if I followed the charts correctly then the 4 Volts positive peak should drive the valve up to something like 12mA (max at 16mA for the valve).

The second half 6021 triode to be a audio amp and split the current through R12 between the Anode current and g2 of the 5840 Pentode. I am trying to see if i can modulate the Pentode with g3, it will probably not be good but worth a try?

It has taken me a while to get this down and see I have a few more comments!

Must type and copy faster.


Adrian

[AdrianH]

Originally Posted by g4wim_tim

Hi Adrian,

if you're ever across this way once lock down is over, I have a few valves, coil formers and other bits and bob's left over from my valve TX build - you're welcome to them.

Who knows - might work you on 160M AM !

Ha Ha, not with this thing ever. But possibly with the FT-902, the problem as ever here is the lack of antennas, or somewhere to hide a 160m vertical?

Adrian

[g4wim_tim]

Hi Adrian

I have a design for a 160M auto tuning screw driver type antenna, published in Rad Com a while back.

Works quite well and is relatively low profile - could even be disguised as a flag pole.
Needs a bit of machining for the motor and tuning element but could be made to cover 160 thru 40 mtrs with reasonable efficiency for its size.

[Andrew B]

Originally Posted by Herald1360

I can't help but think that varnished bog roll tube would look much better

I use Shellac, put lots of coats on a bog roll tube, makes it stiff and it looks like Paxoline., push the boat out a bit and wind it with gold plated wire.

[AdrianH]

I have tidied up the project on strip board and got to the stage of adding the 6021 valve. All I can say is that I am glad they are more robust then semiconductors, leaving shorts on the track where they are not meant to be does not help get a circuit to work.

The VFO is quite stable providing I keep my hands from the coil.
I have included a picture of the present state with an empty toilet roll for comparison, it should keep the fanciers happy
Corrected a few errors on the circuit.
One thing I am not sure about is the output from the Pentode, I do not have the variable caps for output matching so any ideas on how to match the anode to a small antenna or match into a 50Ohm load for the scope.

Adrian

[Herald1360]

At the fleapower involved won't a couple of ex domestic receiver air spaced caps do? Maybe pull out a few vanes on the tune side one to increase its breakdown voltage rating- they're normally around 500pF which is higher than your circuit shows so you could afford the drop in capacitance.

[G6Tanuki]

I'd do away with the potential-divider resistors [R3/4] between the VFO and the cathode-follower. They will only reduce the drive to the cathode-follower and - in turn - reduce the drive to the 5840 'PA' valve - which for best efficiency you really want to be running in "Class-C", for which you need lots of grid-voltage swing.

As mentioned, a couple of old scrap-broadcast-receiver tuning capacitors will work just fine as C17 and C18.

Be aware however that at 1.8MHz "a few feet of wire" as an antenna has a really-low radiation-resistance; getting a good power-transfer from the output-valve to a short bit of wire will be a challenge.

Personally, I'd replace the "L4" choke with a parallel-tuned circuit, then experiment with tapping the antenna-wire at various points along the coil to find the 'sweet spot'.

That's what was done in the likes of the MK128 1950s low-power military set.

[AdrianH]

Thanks for your comments Chris and G6Tanuki (sorry do not see a name.)

Unfortunately the variable caps is a problem for me, being new in this area I do not have the bits boxes built up to supply these. I am sure something will come to mind.

Adrian

[Herald1360]

An ask in the parts wanted section would likely turn up something. I'll have a furkle and see what I've got lurking.......

[AdrianH]

Thanks Chris, not exactly sure what I am going to be looking for at the moment, I have some 5-65pF trimmer capacitors yellow bodied but think they are only low voltage, the style that came in yellow, green and grey etc.

This little project if it actually works will only be for very local am to my radios, just a bit of fun and in theory can operate it on Top band. I doubt the output will make 100mW, which had me thinking of the old telephone handsets that used to operate on that frequency.

The valve has a reported Ra of 260K Ohms and how to get that to radiate into a small whip aerial is something I can not figure. Am trying to stay un-tuned output so guess a tapped inductor or pick up coil may be the best way. I will first try to figure if I can apply modulation from the screen grid and how well it does?

Definitely suck it and see time.


Adrian

[TrevorG3VLF]

The Ra is the variation of current with voltage. It is very high with pentodes i.e. the current does not change much when the HT voltage is changed,

To get maximum power out of the valve you need a large change in current and voltage and this is normally determined by the load line. A rough idea of the optimum load is voltage/maximum current. This will be much lower than Ra except for maybe a triode.

A pi match is normally used to go from the high load resistance to the very low aerial resistance. Make sure there is reliable HT isolation for safety sake.

[AdrianH]

Hello Trevor;

Forgive me for putting down a few thoughts on the Pentode, it is my way of seeing if I understand the operation of it, if wrong please correct me.

The second grid G2 acts like the anode of a standard Triode, so is the main regulator of current in the valve. Electrons have enough velocity to mainly pass through g2, a sort of overshoot and then become atracted to the typically, higher potential of the anode, g3 being there to stop electrons going back to g2 when the anode volts are lower then g2. The screen grid g2 would also act as a capacity shunt, if suitably decoupled to reduce the anode to grid g1 capacitance. And as you said the current does not change much with higher anode voltage.

It was why I am trying to modulate the volts on g2 to effect amplitude modulation, up to now with out success.

I have also been stumped with another problem. Without the osc running, voltages on the cathode follower are correct. I have 67.5 Volts on the cathode, 65 Volts on the grid. But, if I let the osc run I am getting a rectified effect, the cathode voltage rises to around 110 Volts and I start to get a clipped and distorted waveform out of the stage. I need to understand why, it could be the pentode stage on the output of the cathode follower causing it, I could be driving grid current and rectification leading to DC on the cathode bypass capacitor, just not sure what is going on. This was before my Hp182 scope decided to pack in again.

Adrian

[g4wim_tim]

Hi Adrian,

here's my tuppence for what it's worth.

As mentioned above delete the potential divider between osc and triode for sure then use the triode as a convenional buffer amp - ie common cathode with RFC for anode load.

I'd also reduce the PA grid leak down to about 22k and measure grid drive current of the PA as it wants to be driven into class C.

The screen modulation will work but needs to be set to the correct non modulated voltage to stand any chance.

73 Tim