Plate modulating a triode connected Tetrode/Pentode

[KX5JT]

Is it possible and why would I not want to do it?  

As some of you know, I've been considering building a low power plate modulated bread-board transmitter.  I had been considering the 2e26 since I have acquired that vintage aircraft advisory transmitter which used the 2e26.  A closer look revealed I may not be able to acheive 25 watts carrier with a single 2e26.  I have several 807's and they seem to be able to deliver up to 25 watts with the proper plate voltage.  Since this will be my first attempt at a homebrew project, I was really wanting to keep things simpler.  I really want to use a triode.  It would keep it simpler for me.  

I'm going to use a 800 volt transformer with a bridge rectifier and filter caps.  That should give me around 1100 volts, but I'll have it on a variac to put be able to play with the B+.  Considering a pair of 6L6's for the modulator since Dennis, TFO was kind enough to include a few in a care package.  I suppose another power supply will be built for that, no problem I have a few smaller transformers.  

Can I triode connect and plate modulate the 807 ?  Does that make the output lessor, greater or only change the required drive?  

Be patient with me!  I'm really trying to come up with a simple transmitter project to learn but also one that can drive my linears to be able to put on the air for some fun.  The Henry can take 20 to 25 watts drive and happily give me back 300 to 350 watts.

If there is a good cheap triode capable of 20 to 25 watts carrier (plate modulated) tell me!   I was looking at the series modulated 807 design by WB9ECK http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm but those sweep tubes are hard to find and go for about 70 bucks apeice!

Anyway, I was just wanting to throw out some questions because I'm getting started on the B+ power supply part this weekend.  

73, John KX5JT

[Bacon, WA3WDR]

Triode connection will produce less output than tetrode connection can, because the screen will draw too much current if it is operated at normal plate voltages, and so the plate voltage will have to be reduced to screen voltage levels, or the current will have to be reduced greatly.  The tube will probably work better with the lower plate voltage, around 350V I guess.  Certainly a triode-connected 807 will be more linear in audio service, and have lower plate resistance, than a conventional tetrode-connected 807.  But, it will produce less output than the same tube in tetrode connection can. The nonlinearity can be addressed with screen grid tricks, and negative feedback can reduce the source resistance and the nonlinearity too.

With a little care, a tetrode modulated stage is much more linear than a triode modulated stage, and it is not very complicated, and you should easily be able to get  25 watts of carrier output from an 807 like that, so I recommend that you set your exciter up that way.  A plate voltage of 600 to 700 volts would be appropriate for that.

The Old Bacon, WA3WDR


(EDIT) Hmm, instead of tying the plate and screen together, you can tie the control and screen grids together, and that ought to be able to give you as much power output as the tetrode connection.  But as Frank says below, it will require a lot of grid drive.

[KX5JT]

Hey Slab,

I appreciate the response!  I'm likely going to go ahead and do it as a tetrode then.  I really want the ability to get 25 watts of carrier at 100% modulation.  I want to set it up for CCS so I can not ever have to worry about key down times. *grin*

Taken from http://www.qsl.net/kb7rgg/radio/807/807specs.html  (A GREAT 807 SITE)

807

Class C Modulated Amplifier (CCS)
Plate Voltage ................................. 475 Volts
Grid No. 2 Voltage ............................ 250 Volts
Grid No. 2 Dropping Resistor .................. 28K Ohms
Grid No. 1 Voltage ............................ -85 Volts
Peak Grid No. 1 Voltage ....................... 108 Volts
Grid No. 1 Current ............................ 4 Ma.
Plate Current (Zero Signal) ................... 83 Ma.
Grid No. 2 Current (Zero Signal) .............. 8 Ma.
Driving Power ................................. 0.4 Watts
Power Output (approx) ......................... 28 Watts

These specs look like just the ticket!   For the modulator, I have a few 6L6's and the Radio Handbook, 15th Edition has a nice modulator plan.  Using 6L6's in AB1 wtih 360v on the plates and 270 on the screens give 27 watts.  A nice match.   I'm going to have to learn how to get these two different voltages from a single power supply.  Then I'll need the right mod iron, but I'll cross that bridge later.  In fact my first goal is to get a xtal oscillator driving the 807 and that giving me my 20 something watts in class C.  I'm shooting for the AM window on 75 meters since I have xtals for 3.880, 3.885 and 3.890.  I've got tank circuit stuff from an old parts DX-40.  Single band xtal driven seems the simplest to start.  I wonder if my ART-13 mod iron would work here?  I guess I need to look at plate impedances and learn about that stuff too!

One step at a time!

Thanks Slab.  I love the encouragement from others.  YOU CAN DO IT! YOU CAN DO IT! WE CAN HELP!  

AMFONE ROCKS!  

73 John

[WD5JKO]

John,

    I like the idea your following. If you have trouble finding modulation iron then consider using a class A Heising modulator instead of push pull Ab1. The attached schematic shows what I've done with my Gonset G50. This rig easily puts out 25 watts, and will modulate 100% on voice peaks. The modulation transformer just needs to be a 20+ watt push pull audio output transformer used as a center tapped choke. The key is to balance the modulator class A plate current to that of the RF PA plate current. This will balance the magnetic fields in the transformer (no net DC) such that one of the major problems with Heising modulation is overcome.

   For the power level you need, this idea I present is viable. The class A audio with a little negative feedback is also very clean up to 100% mod on voice peaks.

   You could substitute an 807 for the 6146.

Here is more info about the G50 setup:

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

I have a 6146 RF PA tube screen grid clipper installed. That was just an experiment, and can be removed. It did however Help keeping the setup from overmodulating at the baseline. That said, the clipper threshold was based upon the cathode bias of the modulator. This bias increases with audio signal level which changed the clip point. Sustained sine wave modulation plots show the effect. All that said, unprocessed voice modulation was clean to 100%.

Just another idea,

Jim
WD5JKO

[WA1GFZ]

A pair of 6L6s will do a fine job modulating an 807. 807 at rf can be a bit tricky with VHF oscillations. You would be better off with the 2E26 or a pair in parallel.
807s make fine modulators. You can triode connect them but it will require a bit of audio power to drive them. Consider AB2 with 200 to 250 volts on the screen.

[The Slab Bacon]

John,
        That was the other "Bacon" not me. However................ Here is my take on the subject.

The 6146 is much more suited for RF service than the 807, with shorter base leads, it is much less finicky and easier to neutralize than the 807. The 807 is basically a 6L6 with a plate cap. The 6146 will also run along happily anywhere between about 300 to 800Vdc. At 450-500v it should make somewhere around 45w. Also they are cheap and plentiful, being findable in hamfest junlboxes used for a couple of bux. Run them as tetrodes with the screens well bypassed to ground and at lower frequencies, you may not even have to neutralize them.

As far as modders go, here is where you may have to do a little head scratching to make your decision. Just about any pair of tetrodes or pentodes will get it done for you. A pair of 2E26s, a pair of 807s, 6L6s, or even 6146s, etc. You must decide which service you want to run them in. (class "A", "B", "AB1", etc) You could even use sweep tubes or older "horizontal output" tubes like 6BG6s or 6DQ6s would work fine at the power level you are looking for.

Then consider how much driving power (audio) you have available. This will help determine the service you will want to run them in. If you have lots of audio to play with, then consider class B triode connected. If you have less driving power, then consider AB1 tetrode service.

As far as triode connections go: Tie the plate and screen together for class "A" service, this will give less output, also require less grid drive. Tie both grids together and feed as one will usually give something close to class "B" operation, giving more output, but requiring more grid drive. Bias requirements will vary with the tube selected.

For a makeshift mod transformer, you could use any beefy-assed push-pull audio outpoot transfoma. Feed your B+ into the center tap, hook the flying ends to the plates of the mod tubes, and tie one of the flying ends to the plate of the final for the modded B+ feed. I have used this setup on several small HB transmitters and it works quite well. Ot if you really feel ambitious you could try this:

http://www.amwindow.org/tech/htm/tvtomod.htm#top

There are many approaches to this, dont be afraid to think a little "outside of the box", that is where the fun is!! After all, I built a screen modulated 4-1000A HB rig.


                                              The Slab Bacon

[KX5JT]

Okay, sorry about the "Slab Bacon".  I see! Mr. Natural is OLD Bacon and Glowing TX Tube is the SLAB.  

Well, very good information from everyone here.  If I pick 6146 (which I do have several) for the final, would I be able to bring the level down to 20 watts simply by lowering the plate voltage or would that then put it out of whack?  That's the goal is 20 to 25 watts.  That's the sweet drive spot for my Henry.  Maybe a pair of 2e26's although I was hoping to keep it simple by using a single tube, I suppose all it takes is parallelling all the connections and that wouldn't be too difficult.  If I'm running this thing only on the 80 meter band am I still likely to have to deal with neutralizing an 807?

Jim your class A Heising modulation is certainly something I would consider.  I would like to achieve good fidelity, i.e. something a bit better than raw military communications with it's restricted band pass.  

As far as audio, I will likely feed line level to this proposed rig up since I have a small amount of audio stuff that I am happy with and it comes out on the far in at line level.  It might be nice to be able to choose between the "recording quality" audio and the D-104 however.

Looks like I'll have some studying ahead.  In the meantime I will start mounting transformers, diodes and filter caps.... I gotta get it rolling and stop dreaming.  

Love the input from those that have gone before!  Keep the ideas rolling and remember... it's my first project and I although I have a grip on BASIC electronics (I understand Ohms law and resonance and such but HATE math!)  I am learning the details as I go!

Hmm, going to Orange Hamfest tomorrow and who knows what kind of inspiration will occur there!  It's promising to be a nice AM operator turnout.  

BTW, Slab, I wonder if there are any TV's out there with suitable candidates for mod iron inside these days!  I imagine they are become farther and fewer between.

John

[W7TFO]

John,

Maybe try the old truck of the 'economy' hookup in your power supply to get those two voltages:

Use both the bridge and the center tap.  Check in your handbooks for the schematic, it is usually listed in every issue.  Essentially you get both 50% and 100% voltage, within the total limits of the transformers' ability.

A pair of 2E26's ought to give over 35 Watts in PP with the right bias & load Z.

dg

[WA1GFZ]

Start with 2E26 and just plop in 6146s when you want to increase your power. same pinouts.
Just leave a bit of extra space between sockets. You can start with 1 2E26 and build up from there.

[The Slab Bacon]

Start with 2E26 and just plop in 6146s when you want to increase your power. same pinouts.
Just leave a bit of extra space between sockets. You can start with 1 2E26 and build up from there.

2E26s are a terribly underused tube. Used ones are usually found at festers for $.50-$1.00 each. Think of it as 1/2 of a 6146. For that little tube, a pair in shove-yank can easily produce 50w+ of audio @500Vdc if you tickle them with a little grid current in AB2. You coud run them AB1 @400v and comfortably get 20-25w of audio.

One modded by a pair would be very happy at 450v running 25w out. Like Frank said leave a little extra room around the final and stab in a 6146 for a 3db QRO without any other circuit changes. That would make a very versatile little exciter.

                                                    the Slab Bacon

[KM1H]

Or use a 815 and have it all in one cute tube.

Carl
KM1H
On the generator for 2 days