Curiosity comes knocking: Plate Modulation VS. Screen Modulation

Discussion in 'Amplitude Modulation' started by N6YW, Jul 6, 2016.

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  1. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    This is sure to elicit some fine conversation as it's seems to be controversial and technically challenging for some. Plate Modulation is the proverbial king of the hill as it seems, but there is a lot to be said for Screen
    Modulation as it does sound very sweet when built right and I have friends who produce some very fine
    sounding audio from their "Girly Modulated" rigs. But why? And, why is Plate Modulation seemingly preferred
    considering the cost and weight? I would like to get to the "meat on the bone" of the subject by inviting some
    of our erstwhile experts to break this down in a manner that we can understand and contemplate.
    Most of us understand that cost was a factor with designs of Screen Modulated transmitters, like the Heathkit DX-60, Hallicrafters HT-40 etc. Many hams back in the day were able to get on the air without too much financial burden because of them, thank you very much! But then you had the Johnson Viking Ranger that not
    only sported Plate Modulation, but a built in VFO and operational ability on 160 meters!

    All of that aside, it occurs to me that if Screen Modulation is a desirable asset in transmitter design in shaving
    cost and weight, why don't we see examples in BC transmitters? Pulse Width Modulation designs come to mind.
    Let's take a look at the pros & cons of each in order to see where we end up. This should be a fun one not to mention educational.

    I will start of by stating that I own both types of transmitters and while I love the Plate Modulated transmitters
    and use them almost exclusively, my little radically modified Hallicrafters HT-40 produces a very stout and
    commanding signal from a small box, by virtue of a beefed up power supply, redesigned audio, driver and mod stages, and implementation of a bigger sweep tube.
    Frank K3YAZ who lives here in Tucson built the one I own and it truly is a best "Bang for the Buck in the Box" than any AM transmitter I have owned. The thing sounds awesome considering what it is and it keeps pace
    with my Ranger and even produces 10 additional watts of carrier, controlled carrier that is and that in of itself
    is worthy of an entire thread of discussion.

    So, let's get on it and see where this goes. It should be an excellent thread.
    Thanks in advance for commenting.
    73 de Billy N6YW
  2. K5UJ

    K5UJ Ham Member QRZ Page

    Not a lot of time as I'm finishing up my lunch hour but the rigs like DX60 are control carrier modulated i.e. carrier power varies with modulation %. Don't know much about screen modulation except tetrodes being both plate and screen modulated to make it to +100% positive. Not sure if grid modulated rigs can make it over 100% positive. They can sound good though when operated correctly but it is easy to not operate them correctly and then they sound bad with distortion. AM broadcast is currently experimenting with controlled carrier modulation but they call it something else. The idea is to save money on electricity. Not all commercial broadcast tube rigs were plate modulated, but the industry pretty much used class C plate modulated finals because they seemed to be the most stable and easy to tune up and operate for long periods of time.
    N6YW likes this.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Controlled carrier is just one form of screen modulation. My first "phone" transmitter was a WRL Globe Chief 90A with a WRL SM-90 screen modulator. That arrangement produced like 80% modulation on a good day and more like 65% to 70% modulation most of the time. The quality, of the modulation, was very good but the carrier definitely was not filled with audio.

    Glen, K9STH
    N6YW likes this.
  4. N2DTS

    N2DTS Ham Member QRZ Page

    I do both, and really like screen modulation.
    It has no limitations in bandwidth, phase shift is minimal, and mic or line level audio is all it takes.
    I have built 2 solid state modulators designed by w2imx, very low parts count, does anything from a 2E26 to a 4-1000.
    I have also built two glorified dx60 screen modulators modified by wa1qix that work and sound great.

    Not all tubes screen modulate well is what I have found.
    4-125/250/400/1000 do very well, as do 6146's and 4x150a/4xc250b type tubes, 813's did not.

    I use that dx60 modulator to modulate a pair of 4x150 tubes to 200 watts carrier and 1000 watts pep.
    A screen modulated final needs only to be built to support the plate voltage, not 2 or 4 times like plate modulation, so the RF deck can be voltage wimpy.

    Two downsides of screen modulation, you need a lot of plate dissipation to get much power output, and tuning can be critical.
    You get used to the tuning, I load till I get 4x the carrier power on the watt meter and no carrier shift under modulation.
    I have a single 6146 that does 25 watts carrier and 100 watts pep and I have an RF deck that has three 4-400's in it that I run at about 350 watts carrier.

    The pair of 813's plate modulated by a pair of 4x150a's will do 700 watts carrier without working hard.

    If you do not have good iron, screen modulation can work very well, you can pump a clean 10Hz through without blowing anything up...
    Lots of fun to play with.
    N6YW and K3XR like this.
  5. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    The simple advantage of HLPM is you get to use all of the tubes' ability to produce power, as the contribution from the modulator is added (and then some!).

    To wit, a pair of 4-400's will easily make a kW out (not just input) @ 125% positive peaks as well.

    It is more expensive, more complex, and more prone to failure than screen mod.

    Flamethrower vs. bar-b-que starter?

    However, the better analogy is "There is no substitute for cubic inches"....

    KE5OFJ, KM1H and N6YW like this.
  6. N2DTS

    N2DTS Ham Member QRZ Page

    Its harder to get a really clean signal with plate modulation.
    You have all the distortion in the high level modulator, then you stick it through a modulation transformer that puts all kinds of phase shift into the mix, and the frequency response roller coaster, then, after all that you modulate the plate and maybe or maybe not the screen of the final tube, none of which is linear.

    With screen modulation, you can take clean line level audio and pass it through one active device (tube) and modulate the screen.
    Or run it through a class A solid state device.

    125% modulation is no problem, I get a higher peak power out on the screen modulators because of the lack of phase shift.

    Screen modulation is about the same as using an linear amplifier, 1000 watts of plate dissipation is good for about 300 watts carrier.
    Plus, you can go 10 Hz to 20 KHz flat response, no iron in the audio path.

    There is no best way to do AM, plate modulation, an sdr into an amp, screen modulation, or class E.
    Since we are supposed to be limited in power, a pair of 4-400's gives a nice 300 watt carrier level and can hit 1500 watts.
  7. W7TFO

    W7TFO Platinum Subscriber Platinum Subscriber QRZ Page

    In a triode no one can hear your screen...

    LU8EQ, AF7XT, W3ATV and 6 others like this.
  8. N6YW

    N6YW Ham Member Volunteer Moderator QRZ Page

    My pair of 4-400A's squat right on a KW and sail past 4 KW but most of the time at half power everyone hears me just fine
    and it sounds wonderful too. The knuckle draggers in Texas Slop Bucket land take notice too which is always nice.
    However, our local AM gang employs all kinds of fun boxes into the mix these days and my absolute favorite sounding station
    is John WD7F when using his "Two Hole" DX-60B into his TL-922 amplifier. In all honesty, next to K6JEK's home brew class E
    monster, I don't think I have ever heard a rig sound any better. It's just an outstanding transmitter as heavily modified by
    Frank K3YAZ who did my HT-40.
    The thing that is so fun about these little boxes is they are manageable, easy to work on and don't take up a lot of room.
    I think of it in terms of a Lotus Europa VS. AC Cobra. Both are fast as hell but the Lotus is nimble in the corners but the straight
    line will always belong to the Cobra. Both have great looks and can fit just about anyone, and plain fun.
    The analogies are endless but a bucket full of ponies usually wins.
    I love both.
  9. WB2CAU

    WB2CAU Ham Member QRZ Page

    Unfortunately screen modulation has mostly been associated with "controlled carrier screen modulation" which always sounded bad to me. And I never thought much of it until I heard "budget" transmitters modified to eliminate the controlled carrier effect. True screen modulation (without carrier control) is indistinguishable from high level plate modulation if set up correctly. It even has the potential of sounding superior since it can eliminate the oft-times weak link in quality... the modulation transformer. The downside is final amplifier plate efficiency.
  10. K6CLS

    K6CLS Ham Member QRZ Page

    Fascinating discussion, thanks gents.

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