Screen Modulation for a 4X1?

[WA1HZK]

After hearing WA1SSJ's signal with his screen modulated Heath Kit I am beginning to think this screen modulation scheme may have some future amusment value. He had a beautiful sounding signal last night and was easily making 100% audio that looked like it came out of a plate modulated Hi-Fi Rig! Has anyone played around with maybe a 4-1000 and screen modulation? I immediatly ran off a copy of the HK DX-60 and then all of the posted mods for those radios and this circuit looks promising for a sorta big rig. I have gone the route of building high power plate modulated equipment and am planning to play around with a ceramic triode soon in linear mode but that transmitter would still have no soul! I still want to put together a rig that can maybe be built for a few hundred bucks and make some power. I figure a full boat plate modulated project runs in excess of a couple grand now-a-days and I figure we need something that has a fair performance and does not need big mod iron that generally puts the project out of reach for most guys. I'm watching Brent (Tina) build a PDM rig and that is one complex sucker! For us old buzzard tube guys how about a 4X1 that makes good power and is buildable by more of us with tube junk boxes? A final that could use a Ranger, etc as the VFO and maybe the audio driver? Anybody can go linear but even a 4X1 in linear mode is no screamer and better to heat the cellar than make RF. Any bright ideas? I have a 4-1000 linear sitting doing nothing and would be happy to rewire it for a better use and try out some suggestions.
Keith
WA1HZK

[Steve - WB3HUZ]

Talk to Frank,  KB3AHE. He's been running a homebrew screen modulated 4x1 for the last 3-5 years.

Pix here.

http://www.amwindow.org/pix/htm/kb3ahe/kb3ahe1.htm

[W8ER]

and it sounds pretty good!

Keith .. I like the idea of the screen modulated 4cx800's.  ($100 for a pair .. brand new .. Schvetlannias) Half been tempted myself.


--Larry W8ER

[The Slab Bacon]

I've been running a screen modded 4x1 for almost 5 years now, You've heard it and never noticed it. Necessity was the mother of invention on that project. I needed a big bore transmitter and couldnt find a big mod tranny at the time. It wotks very well and has been like an appliance since it was built. It loafs along at "legal limit" and is just about indestructible. The operating parameters are a bit critical, but not hard to maintain. If you would like more info contact me on the air and I will be glad to let you know. It is too much to type, and more quickly handled by the spoken word.
                                                    de KB3AHE

[WA1HZK]

I'll be looking for you. If you can possibly shoot me a shetch of the details of the modulator schematic, I would like to start there. I will start construction and have the intent of publishing the results here to get several guys going in that same direction. Sorta like an alternative to Class E discussions among us "Glass-Fet" guys. Fax the schematic to 603-887-2117 or better yet, e-mail it to keith@criticalradio.com to get started. I cannot wait to see it. I know you probably had to do more then supply a 700 volt modulated signal to the class C screen grid? You have our attention sir.
Keith
WA1HZK

[w3jn]

Better yet, Keith, if you get yer raggedy ass down to Timonium you can attend Frank's pharty and see the beast in the flesh.  You can look at Frank's xmitter too!  :mrgreen:

73 John

[DMOD]

I've been running a screen modded 4x1 for almost 5 years now, You've heard it and never noticed it. Necessity was the mother of invention on that project. I needed a big bore transmitter and couldnt find a big mod tranny at the time. It wotks very well and has been like an appliance since it was built. It loafs along at "legal limit" and is just about indestructible. The operating parameters are a bit critical, but not hard to maintain. If you would like more info contact me on the air and I will be glad to let you know. It is too much to type, and more quickly handled by the spoken word.
                                                    de KB3AHE

I just acquired a  (NOS?) EIMAC 4-1000 recently and was wanting to avoid a lot of iron as well and am considering going with screen modulation for an AM homebrew.

If you guys have some schematics of Screen Mod rigs it would be greatly appreciated.

Thanks

Phil - AC0OB

[w4bfs]

Keith ... the screen mod rigs are a proven practicum and work well ....I am investigating a possible method of producing high level am with high efficiency and not requiring iron transformers and using old buzzard technology ... I need to hook up with the Tron and DUQ and other gurus  to bounce this off them for possible merit .... 73 ...John

[The Slab Bacon]

Geezzee, how many times do i have to post it? 

Other than crapping out 2 very old plate transfomas, it has been crankin for 9 years now.

Here is the mod scheeme. the low level audio and rf exciter stages have been omitted from the schematic for simplification.

speaking of which, does anyone out there within a 2 hour or so drive from Baltimore have a 6900-7200vct potted piece of plate iron they want to sell?? must be at least a 50 pounder or heavier and would prefer a 120v or dual voltage promary.

                                                                The Slab Bacon

[N3DRB The Derb]

I'd like to see someone do a tube with handles AHE style. 

[KD6VXI]

I'd like to see someone do a tube with handles AHE style. 

I've heard of a plate modulated 4CX20,000.  I HAVEN'T seen it, but have heard the supposed transmitter in action.

The clown runs it into a 2.5 wl long longboom yagi.  The antenna itself will show up on Google Earth.

The guy originally started with a triode 10, then made that a modulator, and uses a 20 as the final.

This is <<heresey>> at this point, as I haven't seen his actual station, but..........

--Shane

[DMOD]

Thanks TSB, much appreciated.

Phil - AC0OB

[W1GFH]

...a 2.5 wl long longboom yagi.  The antenna itself will show up on Google Earth.

I've seen documentation of outrageous HF contest (ARRL Sweepstakes, Radiosport, etc.) station antenna farms that'd likely show up on Google Earth. The Japanese are especially good at that kind of stuff, putting out hugely strapping signals running legal limit RF power. It's all in the antenna.

[N3DRB The Derb]

no, I meant screen mod a tube with handles.

[KD6VXI]

no, I meant screen mod a tube with handles.

I planned on screen mod'ding a 4CX1000 a friend in .uk gave me, complete with socket....

Now the socket is being reworked to house a 3CX1000 in my Harris RF-103 to get that beast back on the air.  Finding a 3-1000 was proving fairly difficult..  Instead, it's getting a 3CX1000 in it.  I need to find another set of the ceramic (BeO maybe) insulators that go on each leg (screw) of the socket, though.  To do a grounded grid socket out of it requires a couple mo parts......  Project on hold until then.


--Shane

[KD6VXI]

...a 2.5 wl long longboom yagi.  The antenna itself will show up on Google Earth.

I've seen documentation of outrageous HF contest (ARRL Sweepstakes, Radiosport, etc.) station antenna farms that'd likely show up on Google Earth. The Japanese are especially good at that kind of stuff, putting out hugely strapping signals running legal limit RF power. It's all in the antenna.

Here's what happens when you decide to run big QRO, 3 X triodes with handles, and have your gamma cap fail.  After this, conversion to a direct fed 50 ohm yagi was done.  No Mo failures since.  Nicer pattern across a wider freq. range as well.

--Shane
 

[Knightt150]

  I have been running screen modulation on 3 rigs for 4years now. When I inject a 1Kc tone into the mike jack the 100 percent modulated AM waveform on the scope looks text book perfect. A lot of people gripe when listing to screen mod that there receiver avc bounces (the noise floor ) comes and gose. When listing to SSB the same happens (just set avc to fast or slow).Your rig will be more green also (saves on AC) because when you are not talking the the power out is less only when you talk dose the little meter on the back of the house spin faster.   

John W9BFO 

[KD6VXI]

Your rig will be more green also (saves on AC) because when you are not talking the the power out is less only when you talk dose the little meter on the back of the house spin faster.   

HEEEELLLOOOOO....

Muh KW metah is readin mo than muh dosy, kuh-moan!

--Shane

[K1JJ]

Another alternative for a 4-1000A on AM is to run in in GG linear, but biased into class C.  Add enough diodes in the filament center tap and it will be well below cut off.  Then drive it with a normal AM carrier (20 watts) and the carrier puts the tube back into conduction for clean operation.  This will work only on AM. On ssb, the amplifier must be biased back into class B or higher to conduct the low level audio at the crossover point. (like blower noise)

With 3500V on the plate, I can get 500 watts of AM carrier out of a single 4X1 with an easy 125% positive or more. I think this efficiency is EQUIVALENT to running the tube class C, screen modulated as some guys do. After all, what's the difference if it's grid or screen modulated when it's running class C? They're both low level modulation schemes.

The big advantage of running GG linear, class C on AM is the ease of tuning it up for a perfect signal.... just tune for maximum output, then load it a little heavier for good measure, just like an ssb linear. It's not that easy with a screen modulated amplifier, which is more critical and a scope is mandatory to load heavily for clean peaks. Also, the drive power is added to the output in normal GG fashion. In addition, a GG amplifier is about the most stable amplifier in the whirl. A grid driven screen modulated amp will probably need neutralization, at least on 20M and higher, unless heavily swamped at the input.

I also like to run a precision balanced modulator as the driver - the audio is literally flat and perfect from DC to 60kc. (NE-602)

I think the allure  of a screen modulated class C amplifer is simply in being able to say that. However, saying, "class C GG cathode modulated linear amplifier" sounds more like a ricebox/linear combo, cuz that's really what it is...

Quadzilla runs under this class C, GG linear amp principle - and I love it! 

T