need ideas for a low power rig.

[N2DTS]

Looking for 40 to 50 watts out without pushing things too much, with clean audio, D104 input, 40 meter xtal control.
A 4D32 screen modulated with a DX60 modulator does not sound or work good, so I need to do something else.
A 6146 with 6l6 modulators is about right, but 6l6's are low voltage tubes and the 6146 likes 700 volts or more.

I have a Viking 2 mod trans, a dx100 mod trans, and a 25 watt mulit match mod transformer.
I have 4D32's, 4x150's, 6146's, 6l6's, 6b4's, el34's, KT90 and KT100's, and other tubes.

Looking for something small, so would like to nix the driver transformer and run AB1 in the modulator.

[WQ9E]

Grab a Heathkit HP-23 (or most power supplies designed for vintage transceivers) and that will take care of your power supply needs.  Or take a look at the "economy" l power supplies as described by Lew McCoy in some of the 60s/70s era ARRL handbooks which use a full wave bridge rectifier and center tapped transformer to simultaneously provide HV and 1/2 HV for the high and low level stages from a single transformer.

[VE3LYX]

don

[N2DTS]

That is an idea, a dual voltage power supply, run the 6l6's at 350 to 450 volts and the 6146 at 700+ volts.
That might work well.

Grab a Heathkit HP-23 (or most power supplies designed for vintage transceivers) and that will take care of your power supply needs.  Or take a look at the "economy" l power supplies as described by Lew McCoy in some of the 60s/70s era ARRL handbooks which use a full wave bridge rectifier and center tapped transformer to simultaneously provide HV and 1/2 HV for the high and low level stages from a single transformer.

[KA2DZT]

I would use those KT-90s even to modulate a 50-60watt output rig.  Build two power supplies 650-700 volts and a 300 volt supply.  6L6s probably won't make enough audio.  807s can be run up at 700 volts but the 6L6s are only good to 450 volts which is more of a PITA to get with most common xfmrs.

The economy supply will also work but you'll need a big 700-800 vac ct xfmr.  A big old TV xfmr might work but most won't yield more than about 650 DC volts after a filter,  and will run cooler if you use a separate filament xfmr to lighten the VA load.  This type supply requires two filters, low voltage and high voltage lines.  I use one in my 6146 x 807s rig but the xfmr ran warm to hot.  It would have run much cooler had I used a separate filament xfmr.  I added a separate 650 VDC supply xfmr especially when I removed the 807s and put in 6550s.

What xfmrs do you have on hand??

Fred

[N2DTS]

I have a nice 1500 vct transformer, 750-0-750, 150 ma I think.
One 6146 is going to need about 35 watts of audio to drive it, so 50 should do.
I do not need to run things at 70 watts input tho, I could do 50 and get 35 or 40 watts out.
The DX100 mod iron looks like the best bet, the 25 watt Stancore multi match looks a bit light for the job.

I have 6ca7's, 6l5gc's, 1625's, 807's KT90's and KT100's, and some other hifi type tubes.
Its got to give 40 to 50 watts cleanly in AB1 I think, that is not easy to do with a low parts count.
Everything has to fit on a 17x10x3 chassis, power supply, tx, modulator, speech amp.

[KD6VXI]

I've got a single 6l6gc modulated by a single ended kt88.

Sounds great.   Use it on 10 am.   12 whole whopping watts of carrier.   Capable of fully overmodulating that carrier.   Both tubes run on 440 volts resting,  about 400 under full steam.

--Shane
KD6VXI

[KA2DZT]

I have a nice 1500 vct transformer, 750-0-750, 150 ma I think.
One 6146 is going to need about 35 watts of audio to drive it, so 50 should do.
I do not need to run things at 70 watts input tho, I could do 50 and get 35 or 40 watts out.
The DX100 mod iron looks like the best bet, the 25 watt Stancore multi match looks a bit light for the job.

I have 6ca7's, 6l5gc's, 1625's, 807's KT90's and KT100's, and some other hifi type tubes.
Its got to give 40 to 50 watts cleanly in AB1 I think, that is not easy to do with a low parts count.
Everything has to fit on a 17x10x3 chassis, power supply, tx, modulator, speech amp.

That chassis is going to be really tight for space.  I built mine 50 watter on a 17x12x3 and things were tight.  Mine has a built in VFO, yours is just going to be crystal control so you might have enough space.

That 1500vct 150ma. xfmr is just enough for the 6146 and not much else.  You will only get about 600-650 VDC after a filter.

If you lowered your power requirements a bit you might be able to build the whole rig on the one chassis with a single xfmr economy supply.  Problem is you need the right xfmr.

Besides the xfmr you mentioned do you have anything else in xfmrs??

Fred

[N2DTS]

They get big above that. I will check what the current is tonight.
Its icas so I think the transformer will do, I do not have to run things at full bore.
Got a dx100 HV power trans and others, but they get big, plus a big choke is needed.

Ok, maybe 20 to 30 watts out?

[N2DTS]

Got a diagram?
Can it be scaled up a bit?

I've got a single 6l6gc modulated by a single ended kt88.

Sounds great.   Use it on 10 am.   12 whole whopping watts of carrier.   Capable of fully overmodulating that carrier.   Both tubes run on 440 volts resting,  about 400 under full steam.

--Shane
KD6VXI

[KD6VXI]

Surely do have the schematic.   It's posted right next to all of yours... Lol

I was talking to a friend earlier.   Time to start making them on the pc.   Have a few scribbled on lined paper,  printer paper,  etc....   But need a better method of filing them.

The power supply and mod xformer came from a junked Cb,  a Tram d201.  That much I can say,  off the top of my head.   That's my limiting factor,  I believe.  

--Shane
KD6VXI

[VE3LYX]

Also have a 6l6, screen modulated by a 6bh7 1/2 driven by  traditional carb mic circuit. Other half of 6BH7 is a hartley VFO. I get about 6 to 7 watts out. No schematic, free build.
don

[N2DTS]

The transformer is rated at 175 ma.
100 ma for the RF stuff, and the modulator would be low duty cycle.
Get a 300 ma one and a 300 ma choke and I might as well build a dx100.
What is a ranger running? Don't tell me they have a huge power transformer in that rig...

[KA2DZT]

The transformer is rated at 175 ma.
100 ma for the RF stuff, and the modulator would be low duty cycle.
Get a 300 ma one and a 300 ma choke and I might as well build a dx100.
What is a ranger running? Don't tell me they have a huge power transformer in that rig...

No reason why it wouldn't work.  Not sure what the xfmr is but many can handle more current than their ratings.  Most are rated for continuous operation, ham xmtrs are on and off.  You'll still need something for the low voltage and filaments.

I have a similar xfmr here 1500vac ct @250ma but it's a 6in cube.  I tested it with a FWB and pulled over 200ma from it without any problems.

I just took apart a Johnson Challenger (was missing parts) and the power xfmr was a fairly big one for the rig it powered.

[kb3rdt]

How about a Johnson Ranger

[N2DTS]

The idea is to home brew something to keep me busy and out of trouble.
I did bid on a DX40 though, seems like a well made thing for a heathkit, just low power.

How about a Johnson Ranger

[VE3LYX]

Homebrew is a lot of fun. 50 watts seemed like QRO to me however I did build a 20M rig with a 6146B final screen modulated by a 12ax7 and another 12ax7 as VFO and buffer. I have a 829B 40m rig I am poking away at. Cathode modulated and a PT15 Supressor grid modulated simplified T1154 I have poked away at when things were slow. However some of the most fun I have had is building and using one tubers , pre 1929 rigs. Why they are so much fun I don't really know but they are. Even with 5 watts in I can work the east coast from mid Ontario. I find as long as I can work somewhere I am happy. The simpler circuits really fascinate me as they often work surprisingly well. It is all Lou's fault VE3AWA as he encouraged me to built my first tunable power oscillator and now I am addicted. Before Christmas I built three rigs. Each taking a saturday to build after gathering parts and finish and tune up on the following monday.  Had two of them successfully in the BK rally and one even has phone ability. Old rigs like rangers etc are fun(I have a pair of ARC5s and a DX60B) but homebrew is really a blast. Free builds are for me even more fun (not from a previous plan or schematic ) I admire your wish to homebrew one for yourself and wish you every success. The thrill of the first contact on something you conceived and built and trouble shot is a thrill that is hard to equal.
don

[N2DTS]

I enjoy the building, and everything I build is a mis/mash of designs and circuits from all over.
Since I have a bunch of high power rigs, medium power rigs, receivers, modulators, vfo/exciters, I wanted to try a self contained 20 to 40 watt xtal controlled AM rig that sounds very good, all in as small a size as I could get.
It could be combined with the prototype home brew receiver as a standalone station.

I find getting 20 to 40 watts out of something on AM takes lots of space they way I do things....

I like the two tube screen modulator setup for the fidelity and small size, but to get any power out I need a big tube or a lot of voltage.
A single 4x150 with 1800 volts on the plate would do 40 watts, but 1800 volts!!!

Plate modulation takes more transformers, more power supplies, screen voltage regulators or driver transformers, lots more tubes.

A ranger was amazing for the amount of stuff they packed into it, but its not as hard if you can pick any part you want or have parts made for you.

Small is hard.

[steve_qix]

With that low power (50 watts), you *could* build the ultimate tube final transmitter!

The modulator can be DC coupled with no transformers.  Perfect sound.

Use series modulation.  I did so for MANY years back in the day (1971-1972) with a 50 watt transmitter.

You could use a tube modulator, or you could an IGBT or MOSFET.  1600V IGBTs are not expensive at all.

So, run 400V on the final and a total power supply voltage of 1200V which will give you 200% positive peak capability!!

A 4D32 would make a pretty awesome final with an IGBT (or 2 of them - would have to do the math on the power dissipation) in the modulator.

If you want an efficiency improvement, you could go class H with the modulator which would bump the modulator efficiency up to around 70% at carrier.

Audio-wise, you probably can't get better than series modulation.  And it's simple to do, too!

[N2DTS]

What is the downside of series modulation?
High voltage on the cathode/filament or something?

[PA4WM]

Hi Brett,

Since you've been playing with screen modulation a lot lately,
why not use one or two 814 bottles.

According to the specs, they would do about 25/30 watts ccs, and 35 icas per tube... (1000v - 1250v)
The screen will take about 2mA, so you could easliy screen mod two tubes with the solid state modulator, or just with a single 6L6 as cathode follower.

Some say the 814 is worthless tube with a flimsy anode, but I think it is a great tube for low power purposses.
They shine nice... that is a plus.

[KD6VXI]

With that low power (50 watts), you *could* build the ultimate tube final transmitter!

The modulator can be DC coupled with no transformers.  Perfect sound.

Use series modulation.  I did so for MANY years back in the day (1971-1972) with a 50 watt transmitter.

You could use a tube modulator, or you could an IGBT or MOSFET.  1600V IGBTs are not expensive at all.

So, run 400V on the final and a total power supply voltage of 1200V which will give you 200% positive peak capability!!

A 4D32 would make a pretty awesome final with an IGBT (or 2 of them - would have to do the math on the power dissipation) in the modulator.

If you want an efficiency improvement, you could go class H with the modulator which would bump the modulator efficiency up to around 70% at carrier.

Audio-wise, you probably can't get better than series modulation.  And it's simple to do, too!

Oddly enough,  I'm doing just that,  Steve.

Have a pdm rig in the works.   Runs at 170 kHz,  and onto isolator coupled to the pdm pass mosfets.   This allowed any mosfet to be used as the switches,  at any voltage,  with the pwm running at normal voltages.

Had hoards made up in Asia.   Waiting on them to show up in the mail now.

Figure as designed,  should be good for 75 Watt carrier,  and 400 pep.  Want more,  parallel more mosfets on the output.   

The pwm is working,  just waiting on higher voltage fets and a transformer to test with.   Worked at 180 volts thus far...

--Shane
KD6VXI

[N2DTS]

Not interested at all in the solid state stuff.
That is a much different junk box that I do not have.
And I think I would much rather have a new flex into a big amp, it does so much more then a solid state AM transmitter if I was to go that way.

I am very much more interested in the method, not the end result.
I have piles of parts to use up, and do not want to add piles of solid state type stuff.
I have lots of tubes that want to do something, they are tired of just hanging around...

[VE3LYX]

I have series cathode modulator (homebrew) on my ARC5s. For me the downside is the HV used by the moduating tube. Simple enough to cure by adding the lost voltage across it to the power supply design except I dont have a bunch of transformers or a lot of gold to spend on it. Currently I net around 500 volts on the pair of 1625s and another 200 or so across the modulating tube. That isnt all bad as many ARC5ers only like 500 to 550v on their finals. In the same vein I made a single tube series cathode modulator for one of my TNT rigs, a 45 tube deal modulated by a 01A. Recently realizing my Bare Essentials 50l6 CW transmitter was also cathode keyed I tried the 01A modulator in the key jack of the 50L6 rig. I had to supply its own heater voltage but all HV is stolen from the rig being modulated. Much to my surprise it worked very well. The 1940 radio hand book (not ARRL) has a lot of info on this type of modulation plus several builds using it in some form or other. I found it very intriguing and know I have just scratched the surface. No mod transformer was the most interesting part. Some designs use a choke. I have tried it with and without. I have used 6J5s and 6l6s and everything in between so far. There are trade offs. Do you use a big enough tube to handle all the current or multiple small ones for less total resistance? Here I like carbon mic circuits because I like that dry crackly audio however not everyone does but that is simply a matter of building a speech amp to match your mic choice and has little if anything to to with the type of modulation since the same parameters exist for all types. Series modulation can also be applied to the plate with not a lot of design differences. That circuit is also in the 1940 handbook. I have also used the simple grid modulator as ARRL (1956 ARRL handbook) insists on calling it which is a variation of this idea and it works well too. It also requires no B voltage as it also steals from the rig when plugged in. Of course built in would be a no brainer. For me I find this very interesting. Simple uncomplicated designs the actually work quite well. In fact the only plate modulated rig I have in this shack out of around 15 transmitters is my benton harbor lunch box , the TWO er.
don

[steve_qix]

You can build a tube series modulated rig just as easily as a solid state one.

I think you would like the result!  Absolute high fidelity and DC coupled.  Doesn't get much better 

The plate efficiency of the modulator is low, but the overall efficiency is probably not much lower than a transformer coupled modulator when you consider everything.

When I built my first series modulated rig back in '71, I was absolutely shocked by how good it sounded.  It was totally unbelievable as compared to anything I had ever used before.