Cathode Modulation

[W4DNR]

Hello Guys,

I asked Steve to move my questions about Cathode Modulation  to a new thread instead of hijacking the "Screen Modulation Again"  thread.

A little about me :

I'm a Broadcast Engineer in Huntsville Alabama ,,.

Retired from full-time TV engineering supervision three years ago after 44 years.

I still contract with several AM and FM radio stations to keep my fingers in the hobby   ( I've never had a job... just a hobby of playing with other people's transmitters ).

Over the years, I've probably rescued several AM transmitters from the scrap pile and my garage has a couple of mostly intact  transmitters and a lot of Iron .... modulation transformers , HV transformers , missing a Mod Reactor ....

I am considering a build with several 833 tubes that I have for a 160 and 80 meter AM project.

I could bring the big iron in and maybe find a suitable mod reactor, but the subject of cathode modulation came up at the local ham club a few weeks ago, and I thought that maybe some of you have had experience with cathode modulation in some shape or fashion.

Anyway, I would appreciate any advice on how to cathode modulate a pair of 833 tubes.


Don W4DNR


 ( moved posts below )

Just Curious !!

Screen modulation...

Cathode modulation ...

Several broadcast transmitters use Cathode Modulation,  but I don't recall
any that use Screen Modulation.

Is there a reason that Screen Modulation is popular on the Ham Bands ?

With high current transistors and FETs  in the tens of amps, it
looks like modulating the cathode of a .5 amp tube would be a
piece of cake ...    dealing with voltages well below screen voltages.

Seems like pulling a cathode closer to ground would be safer than trying to modulate several hundred volts

Thanks

Don W4DNR

[DMOD]

Several broadcast transmitters use Cathode Modulation,  but I don't recall
any that use Screen Modulation.

Is there a reason that Screen Modulation is popular on the Ham Bands ?

If a screen modulation circuit is designed correctly, it results in very good quality audio of over 100% modulation with no need for modulation transformers or modulation reactors.

Below is a circuit of the Continental 50kW tube transmitter that uses a 5kW Screen Modulated exciter. Continental designed stand-alone screen modulated transmitters from 1kW up to 10kW.

Early television transmitters used screen modulation to place synchronization information on the composite NTSC waveform.

So it was used in broadcast transmitters.

Cathode modulation is only used in low power Broadcast or Ham transmitters.

Cunnngham developed low power (30 and 50 Watt) cathode modulated AM transmitters for AM Broadcast stations that had to drop power at night.

Seems like pulling a cathode closer to ground would be safer than trying to modulate several hundred volts

Not really. Screen modulation requires modulating a medium voltage screen voltage (lower than plate voltages) with very low currents, resulting in low power modulating components.


Phil - AC0OB

[W4DNR]

Thanks for the great info, Phil.

I didn't say that there wasn't any broadcast transmitters using screen modulation.... just that I didn't recall or work on any.  

Gates/Harris cathode modulated their 5KW MW5s
( possibly others in that power range ) and for sure the
MW50 Broadcast 50,000 watt transmitter.

Screen modulation sounds great except that all my big tubes are triodes.
833s and 3-500Zs.    

Has anyone ever cathode modulated a 833 or 3-500Z ?

If so, are there any schematics of a cathode modulator for triodes ?

Thanks

Don W4DNR

[DMOD]

Gates/Harris cathode modulated their 5KW MW5s
( possibly others in that power range ) and for sure the
MW50 Broadcast 50,000 watt transmitter.

Screen modulation sounds great except that all my big tubes are triodes.
833s and 3-500Zs.    

Has anyone ever cathode modulated a 833 or 3-500Z ?

If so, are there any schematics of a cathode modulator for triodes ?

Thanks

Don W4DNR

Gates Harris used Pulse Width Modulation (PWM) and later used Digital switching of individual PA modules in their HP transmitters.


When I think of cathode modulation, I think in terms of an analog signal modulating the cathode directly using either a tube, transistor or a transformer, as in the Cunningham 30-50 shown above or the schematics shown below.

Phil - AC0OB

[W4DNR]

Gates Harris used Pulse Width Modulation (PWM) and later used Digital switching of individual PA modules in their HP transmitters.


When I think of cathode modulation, I think in terms of an analog signal modulating the cathode directly using either a tube, transistor or a transformer
Phil - AC0OB

Agreed !!

But even in the PWM transmitters like the SX-1 and the GATES ONE,
the modulation going to the RF stage was pure analog after passing through the PWM filter.  The RF amps ( FETs ) were pulled to ground by the analog audio ... not the PWM..

I have the parts to do either PWM or brute force class A  analog modulating.
I'm not worried about efficiency  for the small amount of time I would be transmitting.  
I just wondered if anyone has tried cathode modulation on a triode and
what the results were.

An SX-1 PWM modulator and PWM filter ( I have them ) that is rated at 10 amps and over 200 volts  should be able to cathode modulate a pair of 833s, but you never know.     That's why I'm coming to the experts.

Don W4DNR  

[DMOD]

An SX-1 PWM modulator and PWM filter ( I have them ) that is rated at 10 amps and over 200 volts  should be able to cathode modulate a pair of 833s, but you never know.

It sounds doable to me.  

Maybe you could start a new thread on this topic.

Phil - AC0OB

[w4bfs]

cathode modulation is a mixture of plate and grid modulation in a triode ... the mix is variable from 0 to 100% of either and directly affects the operating efficiency from 35% (100% grid mod) to 75% (100% plate mod) .... modulating power required is proportional to what % is plate modulation

Jones did yeoman work on this in the 1940's and is available in ARRL handbooks of the 40's, 50's and early 60's

[ab1mn]

Hi Don (and all),

This reminds me a bit of when I was in high school and used "cathode" modulation on a surplus transmitter with a pair of 810s in the final. I think the modulator consisted of 3 or 4 6L6s in parallel connected to the center tap of the 810 filament transformer. Because there was no cathode in the 810s, I guess you would technically call this "Filament" modulation, but people always seemed to give me a funny look/response when I called it that.

I seem to recall a number of articles on Cathode modulation, and even one that you could plug into the key jack on your Cathode keyed CW rig. I have been tempted to look into this as a potential modulator project for my old Heathkit AT-1 transmitter.

Cheers,

Bob (ex K7DYB, now AB1MN)

[KD6VXI]

In a directly heated tube,  the filament is the cathode.

--Shane
KD6VXI

[Carl WA1KPD]

I seem to recall a number of articles on Cathode modulation, and even one that you could plug into the key jack on your Cathode keyed CW rig. I have been tempted to look into this as a potential modulator project for my old Heathkit AT-1 transmitter.

Cheers,

Bob (ex K7DYB, now AB1MN)

Bob,
I am building cathode modulation into a Globe Champ (?)- The 2 x 807 CW rig. I got the article from an old QST and can send it to you. Very easy to build.
Carl
WA1KPD

[W7TFO]

Screen mod=cheapass hamstuff, IMO.  Almost as goofy as controlled carrier .

PM me for a great CM schematic, easy to source the parts.

73DG

[DMOD]

Screen mod=cheapass hamstuff, IMO.  Almost as goofy as controlled carrier .

73DG

The screen modulation in many ham transmitters was implemented cheaply and gave it a bad reputation.

If implemented correctly, SG modulation sounds very good, as in the low distortion, high power Continental Broadcast Transmitters.

Even cathode modulation can sound good if the circuitry is designed properly.

The low power, directly modulated cathode modulated transmitters of Cunningham also sounded very good.

PWM is a very efficient way of modulating the cathode but carries with it a bit more complexity.

Phil - AC0OB

[WD8BIL]

Screen mod=cheapass hamstuff, IMO.  Almost as goofy as controlled carrier

WA1QIX has a superb sounding DX60 screen modulated rig.

And, if I have to say so myself, my modified Drake T4X sounds as good as any plate mod'ed rig.

Screen mod can be (and has been) done quite well !

[K1JJ]

WA1QIX has a superb sounding DX60 screen modulated rig.
And, if I have to say so myself, my modified Drake T4X sounds as good as any plate mod'ed rig.
Screen mod can be (and has been) done quite well !

Agreed!  Both specially modified rigs sound great and sweep like a dream.   They can probably be scaled up to larger tubes too.
The downside is efficiency and the critical need for getting the plate loading tuned perfectly when QSYing.  If loaded for better efficiency, the waveform goes to heck and flat tops, etc.  Keep 'em tuned well and they sound FB!

PDM is also cool. You obviously have the talent to do it, Don.

Personally, I would do what Trump suggests:  "If you're going to think anyway, think big!"  Put in the extra effort and figure out how to plate modulate something homebrew.  Plate modulation is such a class act, more efficient, and loading/tuning is not critical at all. In fact we can adjust our output power with loading.

I just love getting on with my 4-1000A plated modulated rig and  saying, (bragging) "running a plate modulated mawl here, OM"   :-)
Half the fun is telling your buddies about the rig and half is actually running it - it's like riding a Harley vs: a  scooter, caw mawn.

I am exaggerating, of course, but you get the idea.  For guys with less access to iron, a broadcash mod transformer and reactor can be found by putting out a "wanted" ham ad and the rest of the parts are usually easy to find in this day of "down-sizing".

T

[steve_qix]

I have built vacuum tube pulse width modulated transmitters akin to the Harris MW5 (well, only running at a kw).  They were indeed modulated at the cathode side of the RF amplifier, but this is not cathode modulation    This is high level modulation identical in EVERY way to what most hams call plate modulation.  The plate-cathode voltage is changing and the grid-cathode voltage is NOT!  The grid circuit floats along with the cathode so the grid-cathode voltage remains stable.  This is an incredibly important concept.

True "cathode" modulation actually varies the cathode-grid voltage, and is, in fact, largely grid modulation.

So, if we're talking high level modulation applied to the cathode-grid side of an RF amplifier, that's great - it works very well, and requires a substantial modulator just as one would need if the high level modulation were applied to the plate side of the RF amplifier.

Grid modulation, by the way, gets a bad rap.  I have built superb sounding grid modulated RF amplifiers!  Grid modulation is capable of the highest fidelity, and is very easy to accomplish.  The one unwavering rule one must follow when designing and building a grid modulator: The modulator MUST be a low impedance device because the grid impedance varies widely over the modulation cycle.  Follow this rule; make it DC or near-DC coupled; adjust things correctly and you'll have a find sounding transmitter indeed.  Inefficient, but good sounding.  Use a FET source follower or tube cathode follower.  Don't use a transformer 

If you have a lot of spare watts to dissipate, and if excessive energy usage is not an issue, grid modulation is certainly the easiest way to get good audio!

[W4DNR]

Thank you Steve,

I think the marshmallow is on the stick.....

Now comes the hard part of building the fire.

Yes, grid modulation by any other name ...

And yes, I was thinking about that MW-5 except for exchanging the mod tube for a solid state device.

Link couple a bit of RF between the cathodes and the grids .

Could be fun.



Don W4DNR

[w4bfs]

True "cathode" modulation actually varies the cathode-grid voltage, and is, in fact, largely grid modulation.

So, if we're talking high level modulation applied to the cathode-grid side of an RF amplifier, that's great - it works very well, and requires a substantial modulator just as one would need if the high level modulation were applied to the plate side of the RF amplifier.

Grid modulation, by the way, gets a bad rap.  I have built superb sounding grid modulated RF amplifiers!  Grid modulation is capable of the highest fidelity, and is very easy to accomplish.  The one unwavering rule one must follow when designing and building a grid modulator: The modulator MUST be a low impedance device because the grid impedance varies widely over the modulation cycle.  Follow this rule; make it DC or near-DC coupled; adjust things correctly and you'll have a find sounding transmitter indeed.  Inefficient, but good sounding.  Use a FET source follower or tube cathode follower.  Don't use a transformer 

If you have a lot of spare watts to dissipate, and if excessive energy usage is not an issue, grid modulation is certainly the easiest way to get good audio!

yes ...well stated ... there is a little known aspect of grid modulation which speaks to what Steve said ... if you use a high transconductance tetrode or pentode pa tube, you can develop sufficient power out without driving the tube into grid current, either audio or rf.   it is still operating class c so the mixture can produce high quality grid mod am with a simple (12ax7) modulator tube doing the job .... see electric radio articles on the grid currentless wonder ... I want to build one using a 4cx1500 .... imagine, a 3 tube legal limit am tx

[DMOD]

As soon as I finish my Digital VFO, I will fire up SHAME (Solid State Hollow State AM Exciter) for a smoke test.

Phil - AC0OB

Jed will be observing the smoke test.

[steve_qix]

Thank you Steve,

I think the marshmallow is on the stick.....

Now comes the hard part of building the fire.

Yes, grid modulation by any other name ...

And yes, I was thinking about that MW-5 except for exchanging the mod tube for a solid state device.

Link couple a bit of RF between the cathodes and the grids .

Could be fun.

Don W4DNR

It can be done !   You run an interesting crossroad.... you are using tubes in their weakest mode (high current) VS solid state devices in their weakest mode (high voltage).

I have built rigs like this in the past, and have used various methods to deal with the high voltage (high voltage for solid state, that is!).  In one such implementation, I used two 1600V IGBTs in two pulse width modulator modules in series looking into a common PWM filter.  This is very complex to do because at least one modulator floats above ground at the power supply voltage, which in my case was 2000V.  The RF tubes were 6DQ5s (6 in parallel).

Another implementation I tried (successfully) was to build essentially what amounts to a switching power supply that could put out 2000V on audio peaks.  This was a transformer coupled design, with push pull MOSFETs in the modulator operating at around 160VDC, ultimately stepped up to around 2000V, rectified and filtered.  This was in a Valiant.  Again, very complex problem - more than I realized - from the transformer design to energy recovery and transmission from the secondary side of the transformer to the primary side (and the power supply).  In all PWMs, you have to recover the a energy caught by the damper diodes, and feed this back to the power supply.

There are 4000V IGBTs available, but they are expensive.  But, such a device could safely be used with 3000V, which would allow for a carrier DC of between 1000 and 1200V (the 3000V is the peak voltage at full positive modulation).  Again, not incredibly high voltage as far as tubes are concerned, but it would work - particularly with tubes like 811s or other relatively low voltage tubes operating in parallel to get some good power.

[K1JJ]

Cathode modulated:   Here are my trials and tribulations and finally the successful implementation of a PDM  FOUR  4D32 finals  X  FOUR 6LF6 modulators  tube PDM rig.

The "Dual Quads" PDM rig.

http://amfone.net/Amforum/index.php?topic=35596.0

(Pages 4 and 5 have some nice photos)

I had poor luck with the IGBTs as modulators - went through quite a few.  In the end, the tube version will do 500-600 watts carrier and 2500 pep output on AM. 2 KV on the plates. Sweeps beautifully. I really love this rig.

T

[DMOD]

A good basic, (series) CM schematic was given by Bob Stout here on AMFONE some time ago:

Phil - AC0OB

Culver presented one of the first IRE (later to become the IEEE) papers on series cathode modulation:

[steve_qix]

Sweeps beautifully. I really love this rig.

Yeah, but you're never on the air !!!        C'mon back on!

[W7TFO]

Not my best drawing, but here is a simple setup that works pretty well.

73DG

[N0WEK]

Not my best drawing, but here is a simple setup that works pretty well.

73DG

Nice!

Were you running a single 833 or two of them? What kind of plate voltage?

[W7TFO]

1 ea 833A @ 2650V.  Loaded up to 500mA, usually a little less.

The vertical line B+ going North from the 8uF cap needs to go one horiz line more to get the screen + correct.

I shouldn't draw on cold meds....

73DG