10-20 Watt Plate Modulation

[KX5JT]

Okay, so this might be a real Johnny Novice question.  Nevertheless, this is the place it will be answered properly so here goes.

Which RF TRIODES that fall in the 10-20 Watt range are good for plate modulation? 
Would it be easier to use tetrodes or pentodes? 

I'm thinking small plate modulated exciter for a linear amplifier to bring up to good power levels.   Any thoughts?

John KX5JT

[KX5JT]

I've seen some hints at using the 2e26 Pentode to acheive "plate modulation" in the range I am seeking but the thought of a working with pentodes before I really try my hand at a triode seems daunting... so much more... involved it seems.  Bah, maybe I should just dive in?  Of course I have no idea where I'd find appropriate mod iron. :/

[k4kyv]

The 801  comes to mind.  It is rated at about 15 watts of plate dissipation, basically an upgraded rf-friendly version of the old 210.  There is another one (841?) that is a high-mu version of the 801, and would make a good class-B modulator tube.

I don't know how available those tubes are to-day, nor if audiophools have managed to drive up the price and suck them all up.

[w4bfs]

a popular mod these days is to use a 2E26 in place of a 6146 in AF 67 or Ranger to drive a smoke bucket ... I believe Joe 'jcp has done this to his and it sounds good ... at this power level there are MANY ways to gen high quality am .... consider Stu's approach to mod iron (torroidal power transformers) because they are in current production and seem to be good spec .... one of the most important of life's ironies is that in order to learn, mistakes must be made and then corrected .... enjoy the process ....73

[KX5JT]

a popular mod these days is to use a 2E26 in place of a 6146 in AF 67 or Ranger to drive a smoke bucket ... I believe Joe 'jcp has done this to his and it sounds good ... at this power level there are MANY ways to gen high quality am .... consider Stu's approach to mod iron (torroidal power transformers) because they are in current production and seem to be good spec .... one of the most important of life's ironies is that in order to learn, mistakes must be made and then corrected .... enjoy the process ....73

What really got me interested is that NOS 2E26 tubes can be had for like 5 bucks!  I certainly like that!

Don, I'll check around on the 801 tubes and see if they are even around and how much they go for.  Thanks for the tip!

[N2DTS]

Would the triodes be in push pull?
Easy to do at low power levels.
A pair of 809's, T55's or other small triodes would work, you dont have to strap them, run them at low voltage/power.

For little tetrodes, you can likely run a 2e26 or even a 6146 at 400 volts and get a little power out, and have lots of headroom, and have the tube last a long time.

Brett

[KX5JT]

Plate modulation in push-pull?  Is that common?  How is that biased?  Class B on each tube but 180 deg out of phase?  Wouldn't that result in something "linear"?  Doesn't modulation require a non-linear mixing device? 

I know I'm pretty green but how else am I gonna learn?

[N2DTS]

You should pick up some old Bill Orr handbooks.
One of my favorite rigs was a pair of 812a's (811a's work just as well) in push pull, modulated by a pair of 811a's.

You plate modulate them just like anything else, the RF is the only thing in push pull.
You cant really neutralize triodes in any other configuration without special circuits, but in push pull they neutralize easy and very well with two little caps.

For triodes, I would do push pull, for screen grid tubes its pie net.

Brett

[KX5JT]

The 801  comes to mind.  It is rated at about 15 watts of plate dissipation, basically an upgraded rf-friendly version of the old 210.  There is another one (841?) that is a high-mu version of the 801, and would make a good class-B modulator tube.

I don't know how available those tubes are to-day, nor if audiophools have managed to drive up the price and suck them all up.

http://www.vacuumtubes.com/801.html

That's a lot of dollars per watt!!!

[KX5JT]

You should pick up some old Bill Orr handbooks.
One of my favorite rigs was a pair of 812a's (811a's work just as well) in push pull, modulated by a pair of 811a's.

You plate modulate them just like anything else, the RF is the only thing in push pull.
You cant really neutralize triodes in any other configuration without special circuits, but in push pull they neutralize easy and very well with two little caps.

For triodes, I would do push pull, for screen grid tubes its pie net.

Brett

I had Bill Orr's latest edition of The Radio Handbook about 10 years ago before I got serious about AM.  It didn't survive the "Big D".  (Much of my possessions didn't survive the "Big D".)

Out of print now and used copies are fetching hundreds!!  Well older editions are reasonable.  Maybe the 19th Edition for under 20 bucks might be useful!

[N2DTS]

In push pull, with triodes, grid leak bias, plus some protective bias, unless you pick 811a's which work at 1500 volts without any fixed bias.
The way push pull works is both tubes are in class c, there is a coil for each tube with a link between them where the drive goes in.
A quick and clean way to do it is to drive the grids with a ricebox, at the link, you can use plug in coils, little ones for the grid, big ones for the plates, on the plates, its the same setup as the grids, each plate goes to its own coil, with the output link between them, for output loading, the link moves in and out of the coils.
So you have a plate tuning cap, and a swinging link for loading.

You can do screen grid tubes in push pull also, but need the screen supply and protection circuits, and so on.
With triodes, its simple.

Brett

 

[kg8lb]

Aren't the old 1626s found in ARC-5 XMTRS close to that power range ?  How about triode connected 6AV5 tubes ?

[KB2WIG]

The folloong may be of interest..

"
The Radio Handbook, William Orr (editor), 15th edition 1959, 810 (!) pages

This is sort of an ARRL handbook on steroids.  Very complete, covering basic theory though practical construction of mostly ham radio equipment, though much applies to any vintage electronic project.  Has some transistor and semiconductor info but 90% vacuum tube, even a circuit for a tube-based VHF walke-talkie!  There are even sections on "high-fidelity techniques" and "electronic computers"!  Even includes a math section and info on how to set up your workshop (I like the "workshop-in-a-closet"!)

                                                                                                       "

http://www.pmillett.com/tecnical_books_online.htm



klc

[N2DTS]

Yes, I have the red one, and the yellow one.
I love the power supply section, great info.
These books are filled with info about all the stuff we do with AM gear.

Brett

[W7TFO]

The "Radio Handbook" known better today as the 'Orr' handbook, is the direct descendant of the Jones handbooks that started in 1935 and has been published continuously but not annually .

It has nothing in common with the ARRL handbook, which started earlier in 1926 and are published annually.

Over the years a few other names got ham handbooks published, i.e. Fred Collins, eTc.

Overall, the best ones were by the year, some from Jones, some from the ARRL.

There are boatloads of other radio books, but those are technical texts and not specifically for ham radio.  Terman, Morecroft, &  Ballentine are in my library as well.  They cover just about every aspect of radio science, but can be daunting reading as there are few projects, just lots of diagrams and theory.

Dennis

[Opcom]

The 8025 is a small directly heated triode that will run zero bias at about 300VDC. 30W dissipation with natural cooling. works to 500-600MHz. 6.3V fil. with center tap for minimal inductance (not for DC grounding). will pass 80mA, will take 1000V. 35 watts output in class C at 500MHz. orange-red plate at CCS, bright orange red at ICAS. The 8012 is exactly the same but has flying filament leads and different pins on the sides. It is very linear as well. Nice looking little tube and guaranteed to glow real good. I have a case of them nos and would part with a few if there was interest. I realize it is a bit larger at 30W than requested. datasheets with drawings and curves (8012) are avail in a search on duncanamps.com

[kg8lb]

 A pair of very inexpensive 6N7 will do about 10 watts .

[Art]

Errr, how about a pair of IRF510s, a 50W audio amplifier with a public address 300W AF output transformer to series modulate them (no insulated mounting from the chassis required), and a light bulb? 20W of plate, drain modulated, gozouta. Put a perforated cover on the top so the roof can be decorated with light dots. . . . sorry..  got carried away

The 2E26 is a good idea and the power supplies are not difficult at all. You can get grid voltage from the B+ . . . and grid block key the final. Not that keying is a real issue but you might want to use the transmitter for CW. A 6CL6 oscillator is not too hard to whip up to feed the 2E26. But why use an itty bitty toob? 'a pair of 4-400 modulated by a pair and you don't need the lineyar. Same effort, just bigger components. . . you can use your rice box to supply low level RF . . . . now we are talking real light dots on the ceiling . . .

Failing all this a modified DX-60 and that lineyar can sound sweet at about 150W carrier . . .

w0ba

[KX5JT]

.................

The 2E26 is a good idea and the power supplies are not difficult at all. You can get grid voltage from the B+ . . . and grid block key the final. Not that keying is a real issue but you might want to use the transmitter for CW. A 6CL6 oscillator is not too hard to whip up to feed the 2E26. But why use an itty bitty toob? 'a pair of 4-400 modulated by a pair and you don't need the lineyar. Same effort, just bigger components. . . you can use your rice box to supply low level RF . . . . now we are talking real light dots on the ceiling . . .

................

Failing all this a modified DX-60 and that lineyar can sound sweet at about 150W carrier . . .

w0ba

I was thinking about doing small plate modulated project as a learning project because I bid on and won dirt cheap an old WWII era 10 watt 2E26 based "Aircraft Advisory transmitter" which probably has most of the components I need, including plate and mod transformers.  So that will give me the main components, assuming their good.  *shrug*  I'm hoping it'll be a confidence builder before attempting a big rig.

I already have a nice sounding modified dx-60/linear but I didn't build/design those!

John KX5JT

[kg8lb]

 Sounds like a fun project. Could turn out to be a nice transmitter.

[G3UUR]

John,
       You'll find any triodes will require more RF drive than your 2E26 for good, linear, high-level AM. The 2E26 can easily be driven by a 6AH6 crystal oscillator and modulated by a 6L6 at the 20 Watt input level. The Hammond 125J push-pull output transformer makes a good substitute modulation at this power level, and although rated at only 8 watts for push-pull applications it can be wired so that the magnetic fields from the DC feed to the PA and modulator tubes almost cancel and more AF power can be run through it without saturating the core. Just feed the +350V HT into the red lead, connect the 6L6 plate to the brown lead, join the blue and black leads together, and the green lead supplies modulated 350V to the 2E26 plate and screen.

A 12AX7 twin-stage speech amplifier provides ample audio to drive a 6L6 to 10 watts output from a crystal mic.

More food for thought!

Dave.


   

[WD5JKO]

Dave,

     I've done something very similar with a low power AM transmitter. This one is a modified Gonset G50 that can provide about 25 watts carrier output. I find that the push pull transformer as you described a very good alternative and upgrade over standard Heising modulation. I did not use the voice coil winding to "boost" the audio further to the RF B+ because the G50 is a transceiver, and I needed the winding to drive a speaker during receive.

    With my circuit I can achieve 100% modulation on peaks, but not sustained due to the modulator cathode bias that rises with increasing since wave audio drive.
 
    I find that for low power AM, 25 watts RF out and down, that a class A Heising modulator using a P-P output transformer (acting like a CT choke) provides extremely clean and linear modulation that would be more of a challenge to achieve with conventional P-P plate modulator. The big difference is the primary to secondary leakage inductance issue with a standard modulation transformer is eliminated.

Here is a link to what I've done, start with the schematic_p1, p2.

http://pages.prodigy.net/jcandela/G-50/

Note: you will see some distortion at 80% modulation in the neg trough because I am diode clamping the 6146 RF PA screen grid so it will not swing below the modulator cathode bias level (about 30v). Without this diode clamp the carrier level goes to zero easily when using voice modulation.

Jim
WD5JKO

[Opcom]

I think we ignored the 815 if we are going for beam tubes.. dual section unit, 20W dissipation, can do about 40W out CCS. Common & cheap type also.

[KX5JT]

Well we are looking for something that can comfortably drive a linyear.  10 to 15 watts does a good job.  I'll have to see what I have in that aircraft advisory transmitter when it arrives.

[WU2D]

I used to have 10M rig in this class. An 829 both sections in parallel modulated by an 829 P-P using that 2:1 $4.95 "100W" transformer from Fair Radio. That was a cool 30 Watt Class rig which used those 75 cent tubes (at the time - they must be up to 3 bucks now).

For a cheap small triode, the 6S4 is hard to beat with a 7.5 Watt plate and its medium Mu. I would think that 4 of those would make a high quality 10 Watt AM rig with a high "cute" factor.

Mike WU2D