micro rig for 40 meters...

[N2DTS]

After being amazed at the signal reports running my Argonaut 5 on AM at 5 watts carrier on 40 meters, I think I want to build a small rig for that band.

I figure a pair of 2E26 tubes would do 20 watts carrier nicely, I have a cute 25 watt multi match mod transformer, or a mod transformer out of a Viking 2, I have some small caps for the pie net, small plate chokes, etc.

What to use for modulators?
Some small zero bias triodes would be easy, but even 809's are too big.
A pair of 6B4's might not do enough power, I have plenty of 6L6's which might do in AB1, plus some other tubes like 6ca7's and so on.
What would make a good (clean) modulator?

I like to have at least as much clean audio drive as RF power output, so say 25 watts of audio, which is what the little mod transformer is good for.

What tubes/circuit to use?

[WQ9E]

You might be able to do something with the 815, it is a cute and compact twin pentode.  RCA rated it for 54 watts output as a class AB2 modulator in ICAS service.  It sticks up about 3.75 inches above the chassis.

[WD5JKO]

Lots of options...

My vote would be to use the Gonset G-77 modulator topology...pair of 6BQ6's with G2 drive (G1 grounded). This will be zero bias, and NO screen supply needed. You will need perhaps 1 watt of peak drive though..before saying NO, look at that schematic on Bama. Other very similar tubes are the 6AU5, and 6AV5..essentially the same as a 6BQ6 but without the plate cap. These also have the plate on pin 5, with pins 4, 6 not there in the base...this avoids the sparkie destruction from pin 3 to 2 at the socket like with a 6L6 or EL34, 6550, etc.

Recall I sent you an email with the harmonic distortion spectra, and IMD test where the harmonics were 40 db down, and IMD products 40+ db down? That was from my Gonset G-76...a scaled up in power version of the G-77.

Jim
Wd5JKO

[Steve - K4HX]

6L6 modulated by a 6N7.

You may remember Mike, N3HJQ (?). He almost always ran very low power (5 watts or less) and usually had a decent signal on 40 meters. With a good antenna, you can be as loud as the typical 100 watt station when only running 5-10 watts.

[N2DTS]

Some good idea's so far.
I do not think I have any 815's, I do have a centron 6336 or something like that, looks like a big dual triode, but I know nothing about it and do not have a spare.

The 6BQ6 is an idea, and I have spares from the G76's I used to have.

I figure I could go two ways, line level input, phase inverter, AB1, or something class b or AB2 needing driving power, driven by an 8 ohm to 5K transformer.

I like the idea of a line level input and not using a driver transformer.

Of course I could use a chip based power amplifier, say 10 watts into a transformer if I wanted it all on board, but that seems like cheating, and I do not think it would be as clean as an AB1 setup....

[N2DTS]

Ok, looking through all the diagrams and tube manuals, I think its going to be a pair of 2E26 tubes modulated by a pair of 6L6's at about 400 to 500 volts.

It should QRO up to 35 or 40 watts carrier.

VFO, driver, rf amp, modulator (line input) on one chassis, maybe with the power supply built in.

[WU2D]

Why the pair of 2E26's? do you have a 3: or 2:1 transformer? I like 6L6s modulating a single a 6146.

[W8VG]

Brett:

Good luck--am sure whatever you come up with will work well.  I'm having a blast on both 40 and 75 with my PW rig......runs about 6.5-7.0 watts carrier.  Used two small 5X7 inch chassis and separate ~300V supply--mostly from stuff I had laying around.  Figured that I can eventually use modulator with some other small tube transmitters like my 6L6 CW rig.    

Inspiration came from small UTC VM-0 I found NIB at the Manassas Hamfest when I lived on other coast.  Has a date stamp of "Sept 23rd, 1946" on it in red lettering.  Picked it up and lugged it home, thinking "someday, I'm gonna build a rig around it" and started drilling and blasting last winter.    

RF Deck is basically 6CL6 Osc driving 5763 final.  Both are spares/pulls from boatanchors I've collected over years.  The modulator is two 6AQ5W's in PP driven by small 1 watt mono board amp (from Rainbow kits).  Tried both a Rat Shack output xmfr and small Antek toroid xmfr wired backwards as audio driver tranney--get better audio reports with the Antek but is probably overkill.  Audio from amp fed into both LV windings in parallel, the two 115 windings in series feeding grids in PP.  Fidelity is pretty good--getting good audio reports.  

Is band-switching.  On 40m, RF deck started to take off on it's own so had to improve shielding under the chassis and ended-up neutralizing 5763 using small johnson air-trimmer.  Neutralized perfectly and is stable now.  Can get pretty close to 10 watts out with my heathkit IM-17 bench supply but for the shack I threw together separate LV/filament supply that runs a bit lower--about 270-280v depending on line voltage.  So I get about 6.5 on 40 and about 7 on 75 into Oak Hills QRP WM-1 wattmeter.  Modulates fully and has plenty of head-room.  

Got Xtals for 3870, 3885, and 7293 and am having a blast when condx are good.  Gotten solid Q5 reports from Tucson up to N. California with it.  First time I put it on 40m (7293) band seemed dead but Dave K6QL in San Diego come back told me I was hitting him 20 over with it. So I'm also hooked.  Even worked up the nerve to put it on 3870 after dark QSOing with some of the taller ships but prefer to use it during the day--especially on 40 for QSO's here in the So Cal area.

Good luck--have fun building!  

Best 73's
Geo W8VG
Fullerton CA  

[w1vtp]

Why the pair of 2E26's? do you have a 3: or 2:1 transformer? I like 6L6s modulating a single a 6146.

I was thinking self-biased 6L6's.  Pair of 2E26's?  Push pull?  I like that idea but Mike has a good idea using PP 6L6's ---> 6146  or (hmmm) a 6L6 driven by a 6AG7 modded by a pair of 6L6's.  Fun to dream.  Should work well on 40 meters

Al

PS: Geo.  What about a pic of the inside of that nice looking setup?  I'm curious

[W3GMS]

Brett:
Good luck--am sure whatever you come up with will work well.  I'm having a blast on both 40 and 75 with my PW rig......runs about 6.5-7.0 watts carrier.  Used two small 5X7 inch chassis and separate ~300V supply--mostly from stuff I had laying around.  Figured that I can eventually use modulator with some other small tube transmitters like my 6L6 CW rig.    
Inspiration came from small UTC VM-0 I found NIB at the Manassas Hamfest when I lived on other coast.  Has a date stamp of "Sept 23rd, 1946" on it in red lettering.  Picked it up and lugged it home, thinking "someday, I'm gonna build a rig around it" and started drilling and blasting last winter.    
RF Deck is basically 6CL6 Osc driving 5763 final.  Both are spares/pulls from boatanchors I've collected over years.  The modulator is two 6AQ5W's in PP driven by small 1 watt mono board amp (from Rainbow kits).  Tried both a Rat Shack output xmfr and small Antek toroid xmfr wired backwards as audio driver tranney--get better audio reports with the Antek but is probably overkill.  Audio from amp fed into both LV windings in parallel, the two 115 windings in series feeding grids in PP.  Fidelity is pretty good--getting good audio reports.  
Is band-switching.  On 40m, RF deck started to take off on it's own so had to improve shielding under the chassis and ended-up neutralizing 5763 using small johnson air-trimmer.  Neutralized perfectly and is stable now.  Can get pretty close to 10 watts out with my heathkit IM-17 bench supply but for the shack I threw together separate LV/filament supply that runs a bit lower--about 270-280v depending on line voltage.  So I get about 6.5 on 40 and about 7 on 75 into Oak Hills QRP WM-1 wattmeter.  Modulates fully and has plenty of head-room.  
Got Xtals for 3870, 3885, and 7293 and am having a blast when condx are good.  Gotten solid Q5 reports from Tucson up to N. California with it.  First time I put it on 40m (7293) band seemed dead but Dave K6QL in San Diego come back told me I was hitting him 20 over with it. So I'm also hooked.  Even worked up the nerve to put it on 3870 after dark QSOing with some of the taller ships but prefer to use it during the day--especially on 40 for QSO's here in the So Cal area.
Good luck--have fun building!  
Best 73's
Geo W8VG
Fullerton CA  

Geo,
That gear is way to nice to be operating from on the floor!  Raise it up a bit!!
73,
Joe, GMS
 

[N2DTS]

Yes, that rig looks very nice!
Why the 2x 2E26?
Because if one is good, two are better, but the tube choice is driven by the fact the 2E26 is happy at 300 to 500 volts, and a 6146 likes higher voltages.
The rig will run around 375 or 400 volts I think.

I do not like the idea of using an audio tube for RF with its plate out the octal socket next to the grid.
If I modulate the rig, the B+ could get up past 1000 volts.
Plus, a plate cap works well for going to the B+ and pie net stuff above the chassis, the grid stuff can stay below.

No shortage of 2E26's here, or 6146's, but no need for the big high voltage 6146.

I am dwelling on the grid drive stuff, I need some power to provide a good grid leak voltage for class C plate modulation. A 5763 should do real well.
I need to come up with a design, vfo (6c4), buffer (6au6), 5763 (driver), 2x2E26.
In my home brew vfo/exciter, I ended up not using the driver tube, it caused all sorts of problems and I drive the grid of the 6146 with the buffer and get plenty of output, but its not modulated.
Maybe I can switch the 6au6 for a 5763 and get enough drive.

I plan on running the vfo on frequency and keying it, so no digital freq display, but I have a nice old buzzard small Millen dial.
I figure I could put the TX in a cabinet with one of the home brew receivers and have a large low power rig in a box.

 

[W3RSW]

If you go with a pair of 2e26's or a single 6146 mod by 6L6's then you might as well just fire up the Ranger.  Not exactly micro anymore.  - A little Mission creep here?  
Original Rangers ran 550 B+ too.

If you want a nice little rig then a 5763 modulated by a pair of 6aq5's or if you must blow the micro idea but still use your 25 watt xformer., Then a pair of 5763's mod by a pair of 6cl6's might be nice.  Would look nice on a cake pan and two or three band-switching-lite  tricks would keep the component count low

[N2DTS]

Well, I do not have a ranger, and 20 watts seems like a good power level for 40 meters.
And I like to run things at CCS ratings with plenty of audio headroom...

[WD5JKO]

There is always the 6DZ7 for a modulator. This is a dual 6BQ5 in an octal base, and glass the size of a 6L6gc. One of these can make over 20 watts push pull, class Ab1. Use that to modulate a single 6BQ6 RF tube.

Jim
Wd5JKO

[W8VG]

Why the pair of 2E26's? do you have a 3: or 2:1 transformer? I like 6L6s modulating a single a 6146.

Al

PS: Geo.  What about a pic of the inside of that nice looking setup?  I'm curious

Thanks Al.  Don't have any pics from under the hood but did find a pic of the layout I shot with my phone during the maiden voyage last spring.  Check it out.  Pretty simple--used chassis bottoms for panels and the power supply is "home depot" construction using another Antek tranney and old LV choke salvaged from a rusted up DX-100 moons ago.  Panel for that is just some home depot sheet metal cut with my scroll saw using a guide--turned out okay--pretty straight.  Couple light coats of primer and krylon machine grey--baked in XYL's oven while she was back east visiting friends <hihi>

Geo,
That gear is way to nice to be operating from on the floor!  Raise it up a bit!!
73,
Joe, GMS

Joe,  Tks--am toying with idea of some shelves over the table--above my Valiant and A4 but is still in the "getting round to it" stage.  Back in Stafford, I had whole unfinished side of the basement but here, I'm restricted to a very small den right off the kitchen.  Many of my boatanchors are still out in garage unfortunately.  And now the XYL has "line of sight" so I don't get away with much either <hihi>.  Am hoping to get upgraded at the next qth to either a basement or empty garage <hihi>. 

Best 73's
Geo W8VG

[N2DTS]

That looks good!
I also build with what I have around, or parts out of old rigs and things I used to have, and sometimes out of past home brew projects that did not work out.

[W6TOM]

  I had good luck on 40 meters operating my GRC-9 off the DY-88 dynamotor/vibrator supply, only 5 watts of carrier.

[N2DTS]

That is a lot of radio for 5 watts!

[N2DTS]

I pulled the vfo/exciter out of the rack and tested it standalone, and i get about 40 watts out of it.

http://n2dts.smugmug.com/Ham-radio/i-ZKsgzjr/A

So maybe the next step will be to build a 6l6 modulator for it and see how it does.

Modulator chassis chess...

http://n2dts.smugmug.com/Ham-radio/i-ZtSdM8q/A


I seemed to do ok today on 40 meters with 5 watts out of the Argonaut 5, 20 watts should be fun.

For the 6l6 modulator, I suppose I can use a zener diode for bias in the cathode circuit, right?

I am looking for the cleanest signal, and I think you get the most power with the least distortion using fixed bias, right?

The RCA tube manual says a pair of 6L6's in AB1 runs 360 volts on the plate, 270 on the screen, -22.5 volts bias, 18 watts out, 2% distortion.

The 6L6GC does 450 volts, 55 watts out, at 1.8% distortion.

Is there any difference between cathode bias with a zener and grid bias with a bias supply?

[W3RSW]

Only the drop in plate voltage swing equal to the Zener bias hence some power.
Be sure to audio bypass the Zener with a electrolytic of decent value.

Nice looking stuff .  I especially liked seeing your prebuilt layout with all the "Knight kit" components arrayed as if they'd just come out of the box sixty years ago.

[N2DTS]

I have been looking at diagrams for hifi amps till my head is spinning.

I think I might be better off making it a delux setup with adjustable bias and some sort of regulated adjustable screen supply, so I can use a range of tubes/voltages/power output.

Like I did for the vfo/exciter power supply, I can use a big wire wound pot and a string of VR tubes to get almost any regulated voltage I might need.
Plus, glowing VR tubes look cool.

Also, besides meters, I think I will toy with adding a magic eye tube for modulation level like the Eico 730 modulator setup.
I love magic eye tubes, and have a bunch...

[N2DTS]

OK, I got the other projects done, and am starting on the modulator/power supply for the micro rig.

Looking at hi fi amps, I guess the lowest distortion and maximum power output comes from running regulated screen supplies and fixed but adjustable bias, right?

Getting fixed adjustable bias is going to be a pain, the trasnformer I have will give about 360 volts, 6.3, and there is a 5 volt winding.
If I want 0-30 volts bias, I guess I need a seperate transformer to give the bias voltage, or is there some other way to get it?

I have a 40 volt transformer, but the chassis is getting very crowded with all the parts...

So I have some nice 6l6gc's (RCA), and will drive it with a 6SN7 phase inverter.


Since I am using a mod transformer, I do not have screen taps for the 6L6 screens, so the next best thing is regulated screen voltage, right?
Some designs use a resistor to the plate, but I suspect that is not as clean?

Plus, I might want to use/try different tube setups at different voltages...

It seems, if you want to build something right, it ends up being large, even at lower power levels...

[W3RSW]

Yeah, funny how that works, especially if you want decent safety and scrote factor compared to bean counter limited , commercial rigs.  All to often decent circuits are stuffed into a cosmetically good looking form factor cabinet without sufficient cooling. - think, Ranger.

Then someone like Elecraft comes along with tons of room inside a very well made cab. ---Unless you stuff it with every accessory known to man.  Their 3D fasteners that add rigidity and strength to structure are fabulous.

[N2DTS]

Yes, I love the Elecraft stuff, built two K2,s, the 100 watt amp and tuna, and a K1.
Its a shame they do not make a good AM rig.

I think the winner in jamming marginal stuff into a small box goes to WRL with the globe chump.
 

[N2DTS]

I am getting noplace fast on this project.
I was playing chassis chess with the parts I have on hand, trying to figure out how to build ac modulator and power supply on one or two chassis, when the thought hit me: how do I key the high voltage to the modulator tubes is the power supply uses one transformer for HV and filiments?
I do not want to key the high voltage, relays do not like that.

So I wondered how the Eico (730?) modulator do it?
They seem to key the center tap on the transformer.
I never tried that before.

I then thought maybe I could get the voltage from the VFO-exciter power supply, and measured it, 750 volts!
No wonder I get 40 watts out of the 6146.
That supply has a full wave bridge setup, choke input, with the center tap providing lower voltage (about 300 volts).

So I tried grounding the center tap and got something like 250 volts, which gives me about 5 watts out of the 6146, too low.

This is getting very complicated...