160 /80 / 60 mtr AM TX project
 

As some of you may know I'm building a valve 160 / 80 / 60 mtr tx using a pair of 6146's in the final modulated by a pair of KT88's.

Aiming for about 70 watts carrier - similar to a Heathkit DX100 (probably could have bought one or even two for the cost of this build !).

Here are some pictures of the PSU chassis before final assembly.

Top left is the main HT / heater transformer.
Bottom left is a supplementary LT transformer and below that the HT choke and smoothing capacitors.
Top right is the 250V HT transformer for the exciter and to the left a couple of 85A2 to provide +85V for the VFO and -85V for negative bias.

As things progress I'll upload more pictures for comment etc.

Re: 160 /80 / 60 mtr AM TX project
 

That's excellent Tim. I used to cycle round to G3TFN in the sixties in Whitefield, Nr Bury. He'd built a a DX100 and ran it alongside an immaculate AR88. I seem to recall that he had modified the TX Output Stage to his own requirements.
I helped him with tuning up various aerial arrays [Lecher Lines?] but I don't know exactly what he did with the PA stage except that it was a bit exciting with the occasional valve flash over or colour change and frequently discussed on the air when he got it operational.
Happy days and hot milky coffee supplied by his wife!

Dave W

Re: 160 /80 / 60 mtr AM TX project
 

Beautiful workmanship.

I'm intrigued by your choice of 60m as well...it seems to be out of step with the other two. How did you decide that it would work, being unharmonic so to speak?

David

Re: 160 /80 / 60 mtr AM TX project
 

Hi David,

the VFO would normally tune 1.8 to 2MHz and be doubled for 80 mtrs.

To cover 60mtrs the VFO low end is dropped to 1.73MHz and tripled to 5.2MHz and the top end would be 6MHz.

At least that's the theory !

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

Thanks Tim,
That makes sense. Good luck with it. It looks so good it just deserves to work!!

David

Re: 160 /80 / 60 mtr AM TX project
 

A couple of pictures of the PSU - complete apart from the powder coated front panel.

It provides:

HT of 600V at 350mA
HT of 275V at 200mA
+85 V stabilized
- 85 V stabilized
12V at 1amp regulated for change over relays and power up relays
24V at 0.5A to power band switching relays
6.3V at 8A for heaters

Next job modulator on same size of chassis then VFO and RF amplifiers etc.

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

You probably don't need to regulate a supply for just relays, which could save youa bit of space.

For 28v band switching relays you may want to hit them with a pulse of current during switching and have the current back down for long term holding. It can take a useful bite out of that 12W of dissipation

David

Re: 160 /80 / 60 mtr AM TX project
 

Hi David,

the 12V feed is also for the microphone pre-amp in the mic - I plan on using an Icom SM-50 which I have, so a nice clean supply for that is needed.

Noted about the 24V relay consumption, but as you can perhaps see there's a dedicated 'blue' toroidal transformer which has more than enough capacity for both the 12V and 24V rails and also provides power for the 'start' relay rated at 16Amps 250V AC.

The 12 / 24V PSU is overkill current wise as the relays will get it no where near full load.

Thought this PSU was going to be much simpler but as I wanted it to be a seperate unit away from the main tx (due to its weight) this meant extra relays and control wires - not to mention some serious gauge wire for the LT side.

Given the UM2 for modulator, I expect the next modulator chassis itself will be nearly as heavy !

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

a few years ago i built a linear using four 872b's,put out about a kw,soon got put away when i saw the electricity meter when i keyed up!,not to mention my licence conditions!! m3vuv.

Re: 160 /80 / 60 mtr AM TX project
 

Bit of progress - PSU, Modulator and RF deck built and working, but needs more work to tidy up and front panel etc.

Re: 160 /80 / 60 mtr AM TX project
 

Lovely!!!

Re: 160 /80 / 60 mtr AM TX project
 

That looks a whole lot better than my early attempts at the modulator.

Malcolm

Re: 160 /80 / 60 mtr AM TX project
 

Very nice work Tim. I've just built a copy of an AT5 for 60m AM only. It was the third iteration before I was happy with the layout.

How do you achieve such a neat job? Is it very carefully drawn out on paper first, or do you make various prototypes first?

I've modified a DX40 to include 160 and 60m, but it's a lash-up really, and I want to tidy up and make respectable - just like yours!

Regards,

Ian

Re: 160 /80 / 60 mtr AM TX project
 

Pure class.

Re: 160 /80 / 60 mtr AM TX project
 

Hi Ian and others,

This project was conceived initially as a Codar AT5 copy but grew / evolved into this.

I always plan things out with cardboard cut outs first - can't be bothered with a full 3D CAD package, but seems to work out ok.

Just been doing some tests on the modulator and I think I have a problem in either the output valves or transformer as I can't achieve more than about 70% modulation before clipping sets in.

I notice the DC resistance on the UM2 transformer primaries are different, one side measures 71 ohms (pins 3 to 5) and the other side measures 92 ohms (pins 4 - 2). This might be just down to the way it's wound as under AC conditions the ratios seem the same.

Full winding 3 to 6 measures 114 ohm while the other side 4 to 1 measures 155 ohms .

None the less when running for real the 3 - 5 side (lower resistance side) seems to create a distorted sine wave on the anode whereas the other side seems ok.

Hope its not shorted turns !

Bias is balanced as is the drive signal.

I'll try switch valves over and / or different taps to see what happens - hope its nothing serious !

I wonder what the DC resistances should be for UM2 ? I've not been able to find them on line so far.

Any suggestions ?

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

The DC resistances are not the same, due to the inner windings having shorter lengths of wire per turn than the outer ones (smaller diameter). Also note that the taps are off-centre.

I have a drawing of a UM transformer that I measured many years ago. Unfortunately I didn't note whether it was a UM0, 1, 2 or 3 but the general principle and the winding ratios will be the same. From memory (and the high resistances) it would most likely be a UM1 or UM0. The DC resistances will presumably decrease as the size of the transformer increases (thicker wire for higher currents).

There were 4 windings, each with an off-centre tap. Two windings were each approx 4H inductance The other two were approx 6H. So the turns ratios were 1:1:1.22:1.22.

The 4H windings were numbered 6-5-3 and 1-2-4 from inner to outer. The 6H windings were numbered 10-8-7 and 9-11-12 from inner to outer.

DC resistances were measured between pin numbers as:
1 - 116R - 2 - 137R - 4 (overall 253R)
6 - 77R - 5 - 104R - 3 (overall 181R)
10 - 194R - 8 - 80R - 7 (overall 274R)
9 - 243R - 11 - 112R - 12 (overall 355R)


Inductances were measured between pin numbers as:
1 - 0.95H - 2 - 1.45H - 4 (overall 4H)
6 - 0.85H - 5 - 1.35H - 3 (overall 3.9H)
10 - 3.0H - 8 - 0.7H - 7 (overall 6H)
9 - 3.2H - 11 - 0.8H - 12 (overall 6H)

The resistance measurements were made with a digital meter and will have been quite accurate, and the inductances with an old bridge so maybe within 5% or so.

I assume you have found the datasheet for these transformers, widely available online and giving the terminal connections for various matching ratios.

I hope this is useful.

Re: 160 /80 / 60 mtr AM TX project
 

CAD...cardboard aided design. It's very nice work

Re: 160 /80 / 60 mtr AM TX project
 

Hi Dave,

thanks for that, ties up very closely with my measurements.
I've adjusted the RF PA taps on the mod tranny and can now achieve 86% positive peaks and 99% negative peaks - I suspect the positive peaks are limited by HT sag.

The RF PA 61646's draw 150mA at 527V HT for 60 watts of RF out - so an efficiency of about 75%.

The KT88's draw about 90mA each at full modulation power.

So everything is looking promising - I'll post some more pictures later.

Next problem is what to do about a 6146 screen voltage clamp - all is good when on tx and the screen sits at about 200V, but with no drive it rises to 400V and even with g1 at -85V the 6146's still draw 50mA when in stby.

So it's either a 6aq5 screen clamp (like the DX100) or I remove the screen volts while on rx. Will likely do the latter as I have a spare relay contact and don't have a 6aq5 to hand anyway !

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

Coming along nicely - 60 watts fully modulated on 160 and 80 mtrs - just waiting for a coil former to test 60 mtrs.

Front panel all drilled and fits - just needs powder coating.

In the fullness of time I may add a digital frequency read out in the space directly above the VFO knob, but for now I'm keeping it 'vintage'.

Meters are from China £3.50 each, left to right modulator anode currents, PA grid current (10mA FSD), PA screen current (30mA FSD) and PA anode current (300mA FSD)

Still lots to do both mechanically and electrically but getting there.

Many thanks to everyone for their help and guidance.

Some pictures of the front, top RF internals and modulator.

Regards Tim

Re: 160 /80 / 60 mtr AM TX project
 

Thanks to the help and generosity of members of this forum I now have the tx operating on 160, 80 and 60 mtrs.

It's producing a nice clean 50 watts of carrier and easily runs up to 100 % modulation.

The following pictures show the modulation envelope / spectrum at 75 and 99% modulation.

Harmonics of the 1kHz modulation are approx 24dB down at full power and modulation.

I had to juggle with the plate / screen modulation ratios to get a nice trapezoid shape and adjust the screen phase compensation.

The modulation frequency response is 340Hz to 3.5kHz at -3dB.

The other spectrum shows PSU ripple on the carrier down at about -36dB, not as low as I would like but seems to be due to ripple on the PA HT line.

Still needs a few mechanical items fixing up, but basically ready for air testing - wish me luck

Regards Tim