The AM Forum

Well, it's time to end the old thread about building a pissweaker 25w rig.  The end result was a pair of 813s series modulating a single 813. Works great and all finished.
http://amfone.net/Amforum/index.php?topic=46165.0

Time to move on and build something else...


After thinking and talking it over with a few guys, I decided to build a big AM PDM TUBE rig. It is a challenge that scares me a little, so this is good.  I was toying with the idea of doing something like Rick/KHK's big series modulated rig, but I already have one now, though small, but wanted to swing the other way for a high efficiency rig.

Haven't decided on the tube type yet, but it will be at least a pair of 3-500Zs PDM'd by a pair of 4-400As.

Check out the first steps in the pics below.  Shown is the PDM generator, an old 2006 QIX board I have used for both MOSFET and PDM tube rigs in the past. It didn't work when I first fired it up, but after a few hours I have it putting out a nice stable squarewave with good modulation from 1 Hz to about 10 KHz where the artifact LP R/C  filter rolls it off. It also has a very nice low level negative peak limiter (actually clipper) which I feel is one the the best implementations out there. It limits those negative peaks well as an insurance policy. Just a touch at -95% is a good setting.. I use a 10-turn pot to set it.

I added a pot so I can adjust the switching freq from 30 KHz to over 300 KHz.  But 70 KHz is a popular switching freq used by the tube BC PDM pros, so that's where I set it as shown on the freq counter.  

Also, an important part of the rig is the big integration PDM output filter using L-C-L-C-L-C.   The inductors pictured are homemade; wound on 5 gallon plastic cans - and have worked with my old 4-1000A rig PDM'd by a 4X1. They are about 34 mH and the caps are around .001 @ 10 KV.  The filter values will have to be tweaked based on the final plate impedance.

Just cleaned up a very strapping 35" high X  19" wide X 31" deep cabinet and sprayed it Baby Blue. It will have a side window viewing and easy ACCESS of about 2' X 2' using Plexiglas OR sheet aluminum, for testing or permanent use.  It will have three levels inside, one for the PDM deck and one for the PDM filter and PS, fil xfmrs, etc. and the RF deck on top.  At least that's the plan for now.  For safety and stability, I am working hard to fit EVERYTHING into this one cabinet.  This and the tube choice is my next design challenge.


The finals will be in regular class C, maybe 80% efficiency at best.  The PDM tube modulator will probably hit 88% efficiency or even better, based on experience.  I have a pre-driver chip in the PDM generator putting out 10 V p-p with good current. A 11N90 MOSFET or two in the cathode of the PDM modulator tubes will drive it well.  No need for the GFZ driver board this time and no audio transformers.

Have built a few of these rigs in the past using quad 4D32s X 6LF6s, 4-1000As and other big tubes. It is an advanced project and can be dangerous due to the floating supplies, meters, etc.  But the results are really amazing when you key it up and wonder where all the heat has gone... ;)  It will generate audio "shark fins"  on the scope just like the big MOSFET class E rigs. I would compare it very closely to my recently built series modulator rig's performance...  FB triangles and square waves.

In no rush, so it will be several months before completion I would guess.

A journey starts with the first step -  and here we go.

T

Steve did a nice design job on this PDM kit board.  (The 10 turn pot on the front is for precise negative peak limiting adjustments on the fly.)

More:

The homebrew filter coils are really big... wound on 5 gallon plastic paint cans.  I tried ferrite cores on the 4D32 rig, but these air coils were superior.  Figure the coils take the place of the mod iron hassle, more or less.

The cabinet weighs about 90 pounds, on casters. It was a bulky bitch getting up on the HV supply enclosure.  It needs to be re-sprayed in areas I see.


The tubes... three 3-500zs PDM'd by two 4-400As?  Still thinking about it.   A bigger lineup wud require a new HV supply.

More:

Running out of work space.  But there is a rolling table (out of view) that I use for the main chassis work.


There's the original PissWeaker's certificate from 1996.  During the first net call up in March, 1996, there were 25 check-ins - all having proudly built a homebrew 10 watt or less AM transmitter. Many of the guys went on to build their first big rigs.

So, how many millihenry are those :)  And what do you use for caps in the low pass?

You get me all good and ready to build a series mod rig and then this.

Though I'm leaning a single 4-1000 pdm a trio of 500Z tubes (if I can find them in the shop, I think I have 5 NOS left).

So many tubes, so little time.  BUT, I'm interested in this.

--Shane
KD6VXI

Hola Shane,

Yes, it wud be nice to have some company on this project. Hope you build one.  Well, at least a series modulated rig can be easily converted over to a PDM rig. They are so close in layout aside from the switching part.  So you didn't waste too much time I hope... ;)

A tube PDM rig is really a thrilling rig to run.  I remember one day TimTron was over here and I was running a big PDM AM rig at the time. He looked longingly while it was transmitting with the 1000 yard stare in his eyes. He said he liked it and was impressed how much power it put out with so little heat. This was back in the early 2000s I think.  There's just no big homebrew AM PDM rigs on the air... well there's one I know of using 4-400As.

Yep, a trio of 3-500Z class C triodes to eliminate the floating screen supply would be cool  PDM'd  by a 4X1.   I am thinking about that right now.   The 3-500Z is well suited for grid current and class C according to the datasheets.

The filter coils are between 25 to 34 mH. The filter values were based on a design by Frank/ GFZ years ago. But first figger out the plate impedance.  The caps I used were those big micas (Santago?) we talked about in a previous thread.    .001 at 15-20KV is a working value, but again, the filter design comes last.

Don't forget the high speed damper diodes.   Also, an L/C parallel  trap on the filter output to attenuate the PDM switching freq down below ~ -70 DB.  These sidebands can appear at +- 70 KHz up and down the band until the trap is tuned and the last remnants of switching is sucked out.

And, unless you go thru some elaborate work to make the negative peak crossover analog, expect to dial the internal NPL up to limit negative peaks to -95% or so.  But it will come in time. Be patient, refine -  and the rig will fly very well eventually.

I'll draw up a schematic in the near future and post it here.

T

BTW, Shane - I wonder what you are using for a 3-500Z fil xfmr?  That's about 44 amps at 5 volts CT with 10 KV insulation.  I wud have to find one or probably wind one.  The 4X1 modulator can use a standard 7.5V @ 21 amp CT fil xfmr.

Also, minimum noise is important.  I remember with Steve's PDM 450TL rig he used a single 4X1 PDM tube and just blew some air past the pins.  The 450TLs needed little air so the overall rig was quiet.

I have also looked at using two or three 833As in the final since they need no forced air.   The bottom line is because there is not a lot of heat, the noise can be reduced with the right tubes.

For safety and stability, I gotta fit EVERYTHING into one 35" high  X   19" wide  X   31" deep cabinet. This is the next design challenge as well as choosing the tubes.
 (Including fitting in the PDM filter and big coils)    

T

Well, this will be one of those things I follow and then get to start in a month or two.  Talks are progressing on a new job, so I may be relocating.  Hopefully with a full gatage / radio shop this time!

My existing pwm setup has a npl built in, pretty much copied from Steve's design.  That then runs into a POS peak stretcher I demo'ed here that allows up to 420ish pct pos peaks, continuously variable from symmetrical to 4x non symmetrical.  Of course anything over 120 to 130 doesn't sound good on most radios, but it's cool wo watch a 1 Watt carrier modulate ip to 16 watts without any baseline!

For filament xformers, the CBers have them in droves.  Dahl stocked on the shelves up to 90A xformers for 6 tube amps. Since a lot of the CB ops are going to ceramic now, these are sometimes parted out. I've also used a pair of sb220 xformers on a previous 3 tube 500Z amp. Just tie everything together in phase and your good to go. As an interesting aside, that 3 tube amp was run off an inverter in the car originally.  With an inverter I put, a si gle sb220 fils xformer was enough to light the 3 tubes.  And did so for maybe 15 plus years???

Switching supplies are also used by some guys.  You have to pick the correct ones, though.  They have to be able to ramp up voltage and not just instantly shut down due to looking at the near shorted cold fils.  I've not used them, but others have.  VE7RF might be able to shed more light on that if he pops into the thread.

Gotta run, time for a job walk. 

--Shane
KD6VXI

I like to use plasma therm filament transformers meant to run a single 5cx1500. They light a trio of 3-500’s well.

Where are these sold?   And Shane, same question.

You both realize that the RF final (3-500Zs) will need a filament xfmr that is insulated for 10 KV between its AC primary, secondary and frame, right?     The 4X1 PDM modulator can be a regular fil xfmr, however.

You might want to get with W7TFO. I helped him kill a Harris pwm rig a few years ago.
The plasma therm stuff came from buying rf gens at the right price and parting them out. Yet you are right. I wouldn’t trust 10kv on those.

Yep, I already sent Dennis an email earlier today thinking he may have something.

It's not too hard to wind your own HV insulated fil xfmr. Just get a Variac core and strip off the parts. Remaining is a core with a 120VAC winding.  Just wrap Kapton on the core or use heavy HV wire to make the 30 turns or whatever to make a CT fil winding.  I have one wound for 7.5V that I could take off some turns to get 5V.  The 44 amps is quite a lot when using three 3-500Zs so will take some heavy wire.   The more I think about it, three 3-500Zs in class C neutralized will make a nice RF final.  The 3-500Z is small and only moderate voltage is needed and it will put out the soup.  I am also thinking about two 4-400As as the modulator rather than the 4X1 so to stay with the general voltage class of 3-500Z vs: 4-400As.

Frank/GFZ suggested I could wind the PDM coils as layered with 4", 3", 2", 1"  PVC forms and supported in place inside each other with threaded rod on each end.  This makes a compact coil. I will do this if I cannot find a commercial BC filter.  The Q is higher with wider air spacing compared to a coil layer-wound with close tape spacing.

He also suggested the placement of components in the cabinet:  Three levels - the PDM modulator at the bottom and grounded, the PDM filter in the middle and the RF final at the top.  The fil,  screen, HV iron and general infrastructure can be  where it is needed with shortest leads.   The bottom will be sparse where the HV xfmr could sit in the rear.  I like it... safe and compact all in one cabinet.


BTW, here's an older article by Steve/ QIX about PDM and AM transmitters. It's a good refresher that I read every few years:

http://amfone.net/21stAM/pdm_article.html


T

I had forgotten about the insulated windings for fils.

I do have an amplifier here that does have the necessary filament xformer but I don't want to take anything from it as it's a one off 4-1000 a guy built in the 60s.  There was a magazine article written about it and he also showed it at a convention of some sorts.  It is believed he was trying to take some Collins HV business, he was already firmly entrenched in the FAA as an xmitter / amplifier supplier.

Hammond still winds Dahl stuff.  I'll have to look at my Dahl catalog and see if there are any with the required insulation  and then look for those.

Facebook is where most of those guys hang, Tom.  There are also Cb only forums, but you'll puke if you go read some of their crap 🤔😂

--Shane
KD6VXI

Shane and Pat:

What are you guys using for blowers that are relatively quiet but effective?  I thought I saw someone post a Rotron? or maybe something else that was about $100 new.  I wanted something that could be reduced in speed with an AC  Variac or even DC.  I think VE7RF or W1TTT posted a link to one about a month ago, but cannot find it.

T

EBM Pabst

That is the reference quiet blower company.

Or find one in the spec book that provides what you need with the lowest wheel rpm.

I did that with the GS35b amp I'm shipping out to Hawaii today and it's not silent but it's pretty damn quiet.  Quieter than the AL1500 on the Ben h now.


--Shane
KD6VXI

I second Rotron, ebm, pabst.
Dayton make some full ball bearing blowers that aren’t bad but don’t seem to be as smooth as the others.
I source mine through eBay most of the time.



Sam

As requested (in the prior thread linked to this one), here is a summary of my plans for quietly cooling a 3CX2500F3 series modulator tube....

My goal is to design and construct a cooling chamber that reduces the noise to a low enough level that the rig can be near the operating desk and microphone, without compromising the quality of modulation.

The 3CX2500F3 requires a good deal of air flow, and it has been said that the tube radiator fin assembly can cause some quite loud squealing noises if the airflow is sufficient during high dissipation.  It appears necessary to address three different causes of cooling noise - first, the noise caused by the fin assembly as air flows through it, second, the flow of air moving into and out of the the transmitter enclosure, and third, the motor and squirrel cage vibration telegraphed by its mounting to the chassis and cabinet.

In order to provide the required airflow, without exceeding that minimum, I plan to use a variable-speed blower with a DC motor.   My experience with AC motors has not been satisfactory with regard to speed control by a Variac.  In most cases, induction motors, that synchronize with the power line frequency, run warm to hot when slowed down with a Variac, and this is, of course, to be expected.  I chose to try an automotive heat/air conditioner blower unit designed for use with 12 to 15 volts at maximum speed, which can be speed controlled by reducing the voltage either by series resistance, regulator, or PWM power source. This option will allow either manual or automated speed control, in response to heat cycles between receive and transmit periods.
  
The series modulator tube will be dissipating around one kilowatt during periods of unmodulated carrier.  There is no need to run the blower at full speed during receive periods, however it should provide substantial airflow for a period of time after each transmission before slowing down to maintain a safe seal temperature while continuing to remove heat dissipated by the tube filament.

In order to minimize telegraphed vibration, the plan is to take the motor and squirrel cage assembly, and create an enclosure and air flow plenum by cutting and laminating several sheets of one-inch thick styrofoam.  This assembly will then be enclosed in two-inch thick cellular foam rubber, before mounting in the rack cabinet.  

To reduce the intake and exhaust airflow noise, a labyrinth airflow path is planned.  The entire transmitter will be enclosed in a six-foot high rack cabinet, 22 inches wide, and 33 inches deep.  The floor of the cabinet is 80% open, and the cabinet frame is made up of cast and extruded aluminum sections, with removable sheet aluminum skins for the sides.  A sheet of 3/4 inch thick plywood will be used as a base for the plate transformer and other HV power supply components.  Around these components, 3/4 inch holes will be drilled to allow air to enter from beneath the plywood, through a pleated air conditioner filter.  By providing many moderate sized holes around the components, a large volume of air may enter, flowing at a lower speed than the intake or exhaust ports of the blower assembly.
 
The sides of the cabinet, and the front and rear, where possible, will also be lined with two-inch thick cellular foam rubber sheets.
 
Air will flow into the enclosed blower assembly, and then on to a plenum enclosing the 3CX2500F3 modulator, and the 4-400A driver tube.  The exhaust side of the plenum will also be perforated with 1/2 inch to 3/4 inch holes over a large area.  
Two or three layers of fiberglass sheet insulation will be placed above the tube plenum, in a manner creating a labyrinth path whereby the air flow much change direction several times, thus muffling the noise from the motion of the air, as well as the higher frequency noise caused by the tube heat radiator fin assembly.  The airflow will be sufficient that the cellular foam rubber and styrofoam are not exposed to extreme temperatures.

The final amplifier, consisting of push-pull Eimac 304-TL tubes, will reside above the foam labyrinth.  Air will flow around the chassis sides and rear, up to the meter section of the rack.  Behind the meters, and under the top of the rack will reside the high voltage bleeder resistors.  The top of the enclosure has a sheet aluminum skin, which is perforated along each side, from front to rear, allowing sufficient air to escape.  

The intake air filter design, as well as the top cover perforations, were employed by the cooling system when this rack was occupied by six card cages of computer logic, consuming energy from a 5 volt supply at over 200 amperes.  In operation, the computer was extremely quiet due to the air flow design and several small fan assemblies, so it is expected that similar noise reduction can be achieved with this platform.

Further development of this series modulated transmitter will not proceed until the noise abatement development indicates the entire rig will be acceptable residing near the operating desk.

Rick...   You mentioned that the bottom of the rack will be about 80% open for air circulation.  If you have or anticipate having small critters like kittens or puppies around, it would be good to use some sort of mesh to exclude them.  I had a 4-1000A in a rack on casters years ago, with a hole in the bottom.  I was in the radio room and could hear my Maine coon kitten, Bullet, making a racket and occasionally meowing.  It took a while for me to figure it out, but he had weaseled his way under the rack, up through the hole and was exploring the circuitry.  Luckily, the power supply and breaker was in a separate enclosure and his environs were cold, so he was safe.  After extracting him, I covered the hole with cane metal.
73 de Norm W1ITT

Norm, thank you for the safety suggestion.  The 80 percent open area is covered by the plywood transformer support panel, with 3/4 inch holes, large enough for small critters.  (I was attempting to clarify that I did not need to cut the bottom of the enclosure to allow cooling air to enter from the bottom.)  Under this 80 percent open area is the pleated air conditioner filter, and under that is a sheet of perforated aluminum with the entire area covered with 1/4 inch holes, so I think I have rather safely "covered" the large bottom opening.

I certainly hope this project ends up sounding like a light warm summer breeze, and not a Katrina disaster.  Time will tell!

Rick:

Interesting on using 2" foam to cover the inside of the rig.  Something like insulation in a car.  I will digest your posting above and see what I can do.

I am still thinking and toying with the idea of making the big rig a screen modulated 4CX-3000 tetrode using the Continental 317B broadcast transmitter circuit of screen modulation.  If I do, I will need to have some quiet air on the tube and use some of your ideas.

Here is a "simplified" schematic of Continental's screen scheme. They used this a lot in their earlier rigs and claim the screen efficiency approached plate modulation once all the modulator tube losses,  fils, iron and everything else were accounted for. Not to mention the fidelity and less cost of screen mod. They are also using a high efficiency linear after the screen mod tube.

I may be using their OB2, 807 and replacing the 4-65s with a single 813 as the screen modulator feeding the 4X3.  Frank is also looking into designing a solid state screen modulator.

QUESTION:  Does anyone have the detailed schematic of this Continental 317B screen modulator?  This "simplified" version off Google does not show parts values or voltages. It wud be a big help.

I'll bet there are a number of guys who would be interested in building a big screen modulated rig once all the design (and even a PC board) is ironed out.

Thanks.

T

The Continental 316B has circuitry very similar to what you are looking for:

https://www.steampoweredradio.com/pdf/continental%20electronics/Continental%20Electronics%20316B%20AM%20Transmitter%20Circa%201959.pdf

It is a big manual, you need to dig for the schematics, but they are there in full detail.

Three 4CX5000s modulated by a trio of 4-65As.  The OB2 and 807 bias regulation circuit is clearly defined in the detailed schematic in the above manual.  I believe the 813 would be a good substitute for the 4-65s, as it can tolerate a much higher screen voltage.  The 813 does have a somewhat lower maximum plate voltage rating, but we all know that limit is not cast in concrete!  The simplified circuit showed the 4-65A drivers "triode connected", but the detailed schematic of this transmitter indicates a standard tetrode configuration.

I also found the book for the Continental 317C:

https://worldradiohistory.com/Archive-Catalogs/Continental/Continental-317C-1-50-kw-AM-Transmitter-1975.pdf

This one uses a 4-400A class-A audio driver for the parallel 3CX3000A1 screen modulators.  The 4-400A input is all discrete transistor circuitry, almost exactly what I want to do with my series modulated 3CX2500F3.  The class-A 4-400A works into a 40K series plate load resistor at around 5 kV, capacitor coupled to the 3CX3000 screen modulator, working as a cathode follower, exactly what I envision doing to place the modulator between the positive plate supply and the final amplifier.

A wealth of good information can be gleaned by studying these schematics!

By the way, this thread has been "PROMOTED" from QSO all the way up to TECHNICAL FORUM!   WAHOO!

Years ago, when I built my pwm, I bought some 2kv fets for it.  I had planned in using them to scream modulate a 4cx5k (and then a 4cx7500 someone dropped in my lap).  Never got around to it and moving has most of the stuff still in boxes.

**sigh**

--Shane
KD6VXI

Regarding noise control --

You don't want to use "foam" to absorb sound, you want to use acoustical materials that are flame retardant, high temperature capable, and non-friable.  Stuff like rockwool panels are not expensive, won't burn, won't melt, won't out-gas, and so forth. Some other things:  a bunch of little holes are noisier than one big one. High velocity is noisier than low velocity. Sharp edges are noisier than rounded. Think smooth curves, radiused corners/edges, low speed, high volume, passing along absorptive materials.  Finally, sound will travel through the rack walls themselves wonderfully.  Consider lining it all first with a sound barrier type product (look up Dynamat), then put panels on the large areas to absorb sound.

My day job is pro audio; when I read stuff like building a box of styrofoam with a fan in it, my eyes can only see the aftermath of the Station fire in Rhode Island 17 years ago...  Please don't make a literal flamethrower out of your "flamethrower" QRO rigs.....

Ed

Ed, thank you so much for the thoughtful constructive criticism.  I do have many rock wool panels that were used in a small recording studio, and they will be perfect for the enclosure and duct work.  I was not aware that the foam would magnify the acoustical noise, that is also very helpful information.  I was aware of the smooth curves vs sharp corners, and that is part of the plan.  High frequency sound does not like to find its way around corners, hence the labyrinth path.  Slow air flow a large volume is much quieter than fast air flow, using multiple parallel paths. 

Hi Tom, good project !!

Important question: Were any of your previous tube PWM transmitters capable of modulating a triangle wave, without distortion, to 99% negative (or even 97% negative), and 150% positive?

I ask this question because, as far as I can tell, there is no analog compensation in your design (you didn't mention it).  There is a very good reason why Harris and other manufacturers use analog compensation in their tube PWM transmitters - tubes are BAD switches - AND - the PWM filter cannot be fully emptied as the current approaches 0 (high negative modulation), causing the modulating waveform to "flatten out".

So, with analog compensation, the modulator is essentially operating in analog mode for the last 15% of modulation of so (from 85% to 100% negative).

-AND-, since the tube is a bad switch, analog compensation drives the tube harder as more current is required (positive modulation).  The design referenced elsewhere in this thread (my PWM article) has analog compensation, and it is very important if you want a low distortion transmitter that will actually modulate a triangle wave from 99% neg to 150% pos.   If the rig will modulate a triangle, it will modulate anything else, too !

Footnote:  Why isn't analog compensation part of the solid state rig designs (class E rigs)?  Two reasons.  1) We have the class E "gift".  MOSFETs, when driven into saturation at RF frequencies, will pull current with 0V across the device (0 volts on the drain), and will pull the DC drain voltage negative.  This is perfect for discharging the PWM filter completely at 100% negative modulation.  Tubes don't do this.    2) MOSFETs are nearly perfect switches, so no compensation is needed to linearize them.

If this is a new design, analog compensation should really be part of it.  It's not that hard to implement.  ;)

Anyway, thought I'd mention it.

Ed: Very interesting on using acoustic, fire-proof soundproofing material.  I'll be looking into that.  This is why it pays to drag our feet and come up with several designs before proceeding.

Rick: The quietest BIG blower system I ever had was when I mounted a big 220V blower just outside the shack pulling in outside air. It was in a waterproof enclosure. Man, was it quiet in the shack, being just outlet air noise and no motor, bearings, etc.  The problem was the moisture and cold air in the winter that made things rough in the shack. But it was the quietest amplifier I ever ran, especially considering the amount of air it could quietly pass.  To eliminate the intake air noise, bearings and motor vibration, using fiberglass ducting with smooth insides worked well.

So maybe I can do it with a muffler type routing like you described.  And thanks for the Continental manuals. The full blown version screen modulator is more involved than I thought. Lots of power resistors and tubes.....  ;)

Steve:  No, I never installed any analog compensation on the tube PDM rigs.  (4X1s  or quad 4D32s rigs)  I was about to but when I saw the complexity of the circuit you sent me, I held off. Or maybe it was something that Harris had designed or could it be I was cathode switching and you were grid switching the modulator?  I seem to remember the Harris version looking like a rat's nest.  But my un-compensated rig did pass nice triangles up to about 93% negative and I just masked the other 7% by setting the negative limiter tight. I thought the rig worked well up to 150%, but don't remember how far it would go without the comp.

But tell ya what:  If you have an analog comp circuit that has a good chance of working well, send it over and I will give it a try. I planned to cathode switch the modulator(s) with an 11N90 as before. I think you were talking grid driving it, so that may be an issue?

BTW, what do you think would be the best lineup of tubes in the 4-400A / 3-500Z class?   Would three 3-500Zs in the RF final PDM'd by a pair of 4-400As be best -  OR  three 4-400As in the final and two 3-500Zs in the modulator, or some other combination of these tubes?  Would a single 4-1000A as a modulator for three 3-500Zs be a good choice?    I will be starting at about 4.5 KV (low) at first and may have the option to bring it up to 7KV later on.

I'm still thinking of designs and talking to my "radio-lawyers" about the various choices for a big rig.  I like the tube PDM project and the possibility of a big screen modulated rig. Also, a big linear with heavy bias on AM could be a possibility.  I think my true heart lies with a tube PDM rig, being the most difficult, least blower noise  -  and most heat efficient for both winter and summer use.


T

Interesting !  If you do go the PWM route, analog compensation is absolutely necessary in order for the modulator to operate in a linear fashion.

Graphically, what happens is this:

(http://www.classeradio.com/pdm_figf.gif)

With small pulses (such as over about 85% negative modulation), the rise time of the waveform lengthens, and gets worse and worse as the pulse gets smaller (closer to 100% negative modulation).  This is due to energy stored in the filter, which is not fully depleted at low current.  The result is that the modulating waveform flattens out, this flattening increasing as 100% negative modulation is attempted, and then the modulator output suddenly goes to 0 (100% negative modulation).

At the other end (150% positive modulation), the modulator tube, being a tube, has a somewhat nonlinear transfer characteristic, particularly at higher currents.  The result of this is a non-linear modulation characteristic at high levels of positive modulation.

So, how much of a perfectionist do you want to be ?  You can certainly get away with no analog compensation, but there will be distortion.  This is amateur radio.  Most people probably won't be able to identify what they are hearing, and it will sound "fine", so to speak.

An interesting example of this is Rob W1AEX.  He always sounds really good.  However, one day I noticed a distinct improvement in his audio.  It was striking, and Rob always sounded "fine" (good, actually).  Now he sounded Excellent !  He got a new SDR, and it was better than his previous SDR.  But, without the comparison, I didn't think anything of it.  Rob sounded good.  Now he sounds even better !

I would go the perfectionist route  ;)  I figure if something is going to take that much time, and cost money, etc., might as well go all the way and make things as perfect as possible.

That said, with cathode switching of the modulator tube, the idea would be to construct the modulator driver such that you vary how hard the switching MOSFET turns on, depending on the modulating signal.  Set the cathode MOSFET such that, at carrier, the cathode MOSFET is only turning on about 30% or 40%.  Set the bias of the modulator so the tube turns on hard at this level (getting close to full grid dissipation).   Make sure you have enough swing at the cathode such that the modulator turns off completely when the cathode MOSFET is off.

Conceptually, this sounds easy.  In practice, with cathode switching, it's an interesting problem.  It is easier with grid drive of the modulator, because you just put a source follow in series with the driver MOSFET to vary the voltage fed to the driver MOSFET under modulation.  I'll think about a similar solution for cathode switching.

OK Steve -

Yep, it's tough to sleep knowing a rig can be improved further... ;)

I have no problem going with grid drive for the PDM modulator since we are in the design phase.   It sounds easier to control and tweak.

My 6LF6 PDM modulators (cathode driven, no analog comp) required heavy grid and screen currents to best modulate the 4D32s. It was to the point of exceeding the data sheet numbers to find the sweet spots.

If you can a draw a grid drive circuit with analog drive using an 11N90, I will try it out and test it.

BTW, do you have a preference for choosing the lineup of 3-500Zs and 4-400As for PDM or RF finals?  It seems that tetrodes are used a lot as PDM modulators. I see you used a single 4X1 tetrode as a modulator and 450TL triodes as RF finals, thus my thinking this way.

Thanks.

T

Hi Tom,

Here is potential, "conceptual" switching modulator circuit using parallel 572B's. I have not simulated the circuit in-total as this is just an idea for consideration.

Phil - AC0OB

Very good, Phil!   I appreciate the  circuit since there is nothing else I have to work with at this point.   It is a start.  Is this an analog compensation circuit? ( I don't think so)

I'm curious about the 75V p-p that gets dropped to 15 V p-p thru the 9:1 xfmr...    

I may decide to build the PDM rig with an empty grid drive module -  and by the time I'm finished there will be a working analog compensation circuit available. I am convinced about the need to linearize the modulator this time around.


T

I had an error on the schematic for the pulse transformer as it is a 3:1 turns ratio, not what was previously shown. (9:1 impedance ratio, Primary-to-Secondary).

Gate Drive Transformers provide electrical pulses and are primarily used as pulse transformers to drive the gates in switch-mode power systems. These pulses control timing in the circuit by turning semiconductors such as MOSFETs and IGBTs on and off.

Gate Drives, which are often toroidal, are also used for impedance matching and voltage isolation.

Here is what they look like, about 1.25" X 1.25"
https://www.schaffner.com/product-storage/datasheets/it-series-single/

The goal was to provide voltage isolation while switching on and off the MOSFET.

To reduce any distortion we need to find a switching transistor with the highest speed switching specs, lowest inductance for the filters, and the lowest filter capacitance to allow a fast a rise time as possible.

Someone here at one time had some software for designing these filter circuits for ClassE and as I recall it had an optimization subroutine for speeding up the rise times of the waveforms.


Phil

Here is another idea since we require a 50V supply so why not use it to drive the pulse transformer?

Your PDM generator (Vs = 15V) would feed a 15V Gate driver which pulls up M2's gate to saturate M2. M2's Source pulls up to +45V generating a 100 mA current pulse through the pulse transformer which generates a 15V pulse train on its secondary. The secondary's 15V positive pulses in turn saturates M1 to pull the grids of the 572B's up from -200V up to about 45V to saturate the 572B's.

Since the 572B's are in parallel, the total plate resistance is 1/2 that of a single tube, meaning that the cathode of the 3-500Z will be brought closer to ground than if only one tube were switching it.

K6IC reminded me of Nigel's post regarding filter designs here on AMPhone:

http://amfone.net/Amforum/index.php?topic=38161.0

There may be others as well.

Phil - AC0OB

Kinda late to the blower vibration/telegraphing, but this mount was made from materials and worked very well for me. I hear the air right now but there is no vibration transmitted to the rack It's a dual 6FT rack 35" deep with door-type sides. It seems to have eliminated the mechanically transmitted 60Hz type hum and I can't feel or hear any vibration. It's a 1/2 HP 3450RPM dayton ball bearing motor . The foam rubber is kinda squishy and jiggly. Closed cell, not the kind of foam that eventually rots and leaves a sticky residue -that would be too weak to support the blower. Maybe this is too complicated but I never made a mount before. Don't know what to call the foam rubber exactly but its compliance is likely responsible for isolating well.

Patrick, thank you for sharing the details of your blower mount.  Great that you got rid of the annoying hum and vibration.

I was thinking of doing something similar, and I have decided to make the blower housing and plenum from fireproof fiberglass, enclosed in a metal chassis, shielding the entire modulator tube assembly.

Right now I have the lower power series modulator spread out on the bench, pine-board style, with 810s and 8000s, looking at plate curves and distortion measurements.

I have found through experience that using a transformer to drive the gate(s) of the driver MOSFET (high side drive) only works if it is GUARANTEED that the duty cycle will not get too close to 100%.  Once 100% duty cycle is reached, nothing passes through the transformer.

I use transformer coupling for some PWM circuits, but only those where the duty cycle is carefully controlled to never exceed 90% or thereabouts.

The proposed PWM generator does not do this.

However, there are other ways to drive a source follower (high side drive) that are DC coupled, and a 100% duty cycle signal (full on) can be passed safely.

I am not criticizing the circuit - just pointing out something that not everyone may have noticed.

This circuit might be a starting point. 

(http://www.classeradio.com/pdm_figg.gif)

The concept is sound ( I have built this exact circuit it is works well ).  Analog compensation is included, and the high side driver is DC coupled.  Notice the transformer is used to facilitate an "active pullup" for the gate of the output MOSFET.  Since a resistor is used as a pull-up, it is necessary to "help things along" to ensure a fast rise time at the gate of the output MOSFET.

It is not necessary to build it exactly.  For instance, the MOSFETs can be FQA11N90s, and other op amps can be used.

Tnx for the info, Steve.   There is a lot there compared to a single 11N90 in the cathode.  I will study it some more.


Different subject:  AIR BLOWERS - Check out this one.  Would it push enuff air thru a 3CX-3000A7?   The noise level is only 32 DBA  which is the sound level of a quiet room. It also has a muffler option.  It has a 6" inlet and outlet.  It will put out 402 CFM which is 3 times the needed air for a 100 CFM 3cx-3000A7 tube.  It says it can handle restricted airflow environments, but how about tube fins? I dunno.  8 speeds controlled with PWM.

I need to reduce the air noise in my linear or it cannot be used on AM.  SSB is OK cuz it can be masked thru a noise gate, etc.

Any thoughts on this for $109?  The reviews are great, but they are not using it to cool a tube, rather for ventilation.

https://www.acinfinity.com/hvac-home-ventilation/inline-duct-fan-systems/cloudline-s6-quiet-inline-duct-fan-system-with-speed-controller-6-inch/?gclid=EAIaIQobChMIjbXZ4pTo7QIVxcqGCh1EqwB3EAQYASABEgJ30fD_BwE#product-description


Here is a 4" version for $89.     Airflow: 205 CFM | Noise: 28 dBA | Bearings: Dual Ball:

https://www.acinfinity.com/hvac-home-ventilation/inline-duct-fan-systems/cloudline-s4-quiet-inline-duct-fan-system-with-speed-controller-4-inch/

Any thoughts?

T

Tom...  In regards to that air mover, I suspect that "restricted airflow" means different things to an HVAC guy than it would to someone who is  pushing high velocity air cooling a finned tube.   Fluid dynamics change with velocity.  The Dayton line of blowers all have impellers that are designed to shovel air into a back pressure situation.  And of course they shovel noise to a certain extent.  For noise abatement, I like the Dayton slow rolling blowers...  1500 rpm or so...such as their 2C647D.  The noise is less and of a better spectral mix, not as tiring, and it carries the sound of power and confidence needed in so many on-air situations.
I'd also be wary of the PWM motor controller in that duct mover.  It may well require filtering so as not to trash your receive environment.
Perhaps coolies down cellar, pulling a rope through a hole in the floor to activate a bellows to move air through the transmitter would be sufficient.
73 de Norm  W1ITT

Thanks for the dual info, Norm.  

Yes, the PWM controlled blower cud make noise and the blower will probably sag with a restrictive load.

I use a blower now (similar to the Dayton 2C647D with new bearings) that uses a Variac and can run very slowly, but still, on AM it is audibly offensive -  though OK on SSB.

The best air system I've ever used on this rig is a blower located on the outside of the house, pushing in air. It is amazingly quiet that way. I shud do it again. The big amp really needs to be serving in AM service in addition to SSB.


T

long-winded on blowers:


I looked at that consumer duct type one, and nothing makes me think it will cool a 3CX3000.
https://www.acinfinity.com/hvac-home-ventilation/inline-duct-fan-systems/cloudline-s6-quiet-inline-duct-fan-system-with-speed-controller-6-inch/?gclid=EAIaIQobChMIjbXZ4pTo7QIVxcqGCh1EqwB3EAQYASABEgJ30fD_BwE#product-description

Marketing babble instead of a pressure-volume graph:
"mixed flow design using a stator blade and dual hydrodynamic wind circles"

gimme more of them hydrodynamic wind circles.

That kind of fan (according to it's internals picture), a type of vane-axial fan, is designed to be quiet and efficient when moving high volume air with low back pressure like <1". At some point, when the pressure rises, the 205 CFM dwindles to very little, and long before the pressure drop through the duct and 3CX3000 is surmounted.

The larger version of 405CFM is probably good for exhausting the whole rack while a high speed forward-curved-blade blower cools the 3CX3000 inside. Low speed backward-curved-blade centrifugal fans also work well for high volume low pressure and are also quiet.

EBM Papst was mentioned earlier and I downloaded their catalogs. Lots of learning in there. Then went looking at different fan designs.
(note these are all 'fans' but centrifugal ones are popularly called blowers.
https://www.rs-online.com/designspark/fan-types-why-choose-a-forward-curved-centrifugal-fan
https://www.rs-online.com/designspark/fan-types-why-choose-a-backward-curved-centrufugal-fan
https://www.rs-online.com/designspark/fan-types-why-choose-an-axial-fan
don't confuse tubeaxial with vaneaxial, they perform differently. They are not like centrifugal fans though.
https://www.tcf.com/wp-content/uploads/2018/06/Fan-Performance-Characteristics-of-Axial-Fans-FE-2300.pdf
and
http://www.dynamic-air.com/fan-selection-guide.pdf


A great fan for the 3CX3000 is a centrifugal fan (blower) with forward curved blades and RPM about 3450 which is suitable for a reasonable size wheel. That will make the most air flow through the radiator at the needed pressure with the least power and noise.

The forward-curved blade, high RPM, and wheel diameter is what makes the pressure. The wheel width and horsepower makes the volume.

What this whole noise situation is up against is that a blower with a high enough speed and large enough wheel (and fraction of horsepower) to make the pressure at the desired volume of 3cx3000 is going to be noisy. Noise reduction techniques must therefore be used to abate unwanted noise.

Also keep in mind the blower pressure must overcome the resistance of any ducting because the only thing that matters is CFM through the tube.

The table below from Eimac is simply a measurement across the tube, probably in the standard socket.

Therefore a manometer in the tube socket plenum can be used with the chart below to tell if the blower is doing enough. I have a bunch of nice 0-5" Magnehelic gauges pulled from STMicro when they started ripping the guts out of the building. These are wonderful and a lot less messy than a water manometer.
I just used one last week to discover the negative inlet pressure and positive outlet pressure across a large unknown blower used with a kitchen fume hood in my house. LOL Now I know what $450 Dayton blower to order and get it right the first time.

Eimac: The 3CX3000 @4KW dissipation:
sea level: 127CFM @ 1.40" H2O base to anode flow
10,000 FT: 185CFM @ 2.55" H2O base to anode flow
------------------------------------------------
sea level: 178CFM @ 2.50" H2O anode to base flow
10,000 FT: 260CFM @ 4.50" H2O anode to base flow

Whew! I'm winded after all of that hot air!

For efficient air movement, I believe that the method using six hydrocoptal marzal vanes works well, assuming you can get them a a decent price.
de Norm W1ITT

You could part-out a retro-encabulator to get a half dozen, but I have not seen them on the bay for quite some time.  Shipping might be a bit expensive.  I wonder if any members here have attempted to reverse engineer the device?

Specifications here:  https://youtu.be/RXJKdh1KZ0w

Rick said:Those are the swerving hydrocoptal marzal vanes that I use with my retro encabulator! I keep them right next to my interossitor!

But I accidently discovered that if you cross-couple a flux capacitor to a static inductor with alternate reversed phase polarities, the resulting superior conductor renders the marzal vanes totally unnecessary. Flux capacitors are plentiful, but just try and find a static inductor!

Well that's good because you'd need a deplenerator to operate marzal vanes in the manner proposed, and they are hard to find.

Tnx for the elaborate blower info, Pat.  

I decided to take two approaches for the big linear amp. First I ordered a Dayton 2C647D blower for $63.  It's a 1500 RPM @ 120VAC motor on a Variac. (Haven't received it yet)

Norm, your first post convinced me:
"For efficient air movement, I believe that the method using six hydrocoptal marzal vanes works well, assuming you can get them a a decent price."

Second, I mounted the existing blower on the outside house wall in the cold sitting on a steel frame. The 4" smooth fiberglass duct is about 36" long -  from the blower outlet to the amplifier tube, so not bad.  The dB noise level in the shack has dropped about -15 to -20DB. It's pretty quiet now - except for the small whine of the old blower bearings.  The new Dayton shud fix that with all white noise I hope.


This linear amp is the only one in the shack that has an external anode -  so needs brisk  air flow. All the other rigs in the shack are glass finals with small internal blowers or muffin fans and I can usually keep them Variac'd down and quiet due to the heat radiation and convection cooling.

With the cold air, I can reduce the speed down to 1/2 of normal and still keep the tube cool, so helps noise even further. I use a digital temp sensor above the final tube with a warning alert. During the late fall to spring the rig will run cool and at low noise. I don't run it during the summertime anyway.

I plan to get a 4" shutoff valve to keep the cold and moisture out when not in use.


* Still designing up the PDM rig. Hard decisions.  I got a generous offer (an offer I can't refuse) for some old BC transmitter PWM coils which will help immensely to shorten the usual days spent winding coils. I'll talk more about this later.  I've been taking the time to integrate the shack and catch up on the little modifications needed after building a few rigs. Once done I can focus more on the PDM rig build.

T

I received three really BEAUTIFUL FB commercial PDM coils from Pat / KD5OEI / Opcom.   He sent them to me for no charge. Much appreciated Pat!

There is a lot of stuff that gets given free to others on this BB.

I've always wanted a set of BC PDM coils to build around. These are small and compact and out of a Gates 5KW PDM transmitter.  The ones I've wound in the past were huge and I've had one arc over. These coils are the size of a beer can. Compare them to my older 5 gallon pail coils!

Looking at these coils, the first coil had a flashover in the past and I had to remove one of the pi-sections. It measures about 16 mH total now and is the input coil without ferrite.   The second coil with a ferrite adjustable core measures 26 mH and the third coil with a ferrite core measures 43 uH.  They are all the same size and same number of turns, so those ferrite cores really make it a widely adjustable filter. Once I figger out the tubes to be used and plate impedance, these coils can be dialed in with the right capacitors.  Usually it's a L-C-L-C-L-C  lowpass filter at 70 KHz with a rolloff of about 10 KHz.. It will have to be modeled out based on the tubes and the value of filter caps I have in the junk box.

Interesting that the two ferrite core coils have a 100 pF cap across the coils.

I coated some of the damaged turns with nail polish. That arc over affected the adjacent coils somewhat. Shud be OK. Otherwise I could always add some more external L if needed.  This shud make a FB PDM rig if I can get by the analog compensation obstacle.

This is like getting modulation iron for an analog rig, but it's "digital" instead.   A good bath and they will look vely pletty.

T

More:

Hi Tom
Yes, let's get out the soldering irons and why not breadboard?  When I did the Pine Board Project back in 2019, never realized how many folks - girls and boys- would 'dig in'.  Hundreds followed after it hit the cover of QST .  The beautiful diagrams of W4IQN (retired GM graphic artist) and having Antique Electronics add the parts list in their catalog.    It ended up using the full coil  on 160, tapped for 75 and 40.  Bandswitched those  along with the three crystals.  Even added a receiver RF out and mute. 

I designed the mic preamp with two band equalization and the audio was always reported excellent.  The final piece was an 807 final to get a good 40 watts out.  All of the many great diagrams are on our site. www.heilsound/pineboard. about 50 pages, diagrams and pictures.

Fun project and will look forward to your new project, Tom.  Always enjoy your work.

Dr. Bob Heil, K9EID

www.heilsound.com/pineboard

Hi Bob,

Cool on the pine board projects.

Next, you might build a tube VFO for your xtal transmitter.  

T

Thanks for yoru note... yep- let's do that!,,,... fun project I do believe.   Presently I have Blessed to have Wayne WA4FTY 'borrow' my Haegerty digital boards for a month or so and when I got the boards back - this is what I received.   What a builder !  Check his WRZ page where he built the only Harvey Wells receiver and matching 4-400 final. 

Stay in touch.  Love to see your 40 watt tube transmitter.

73
Dr. Bob Heil, K9EID

A few thoughts.

I would advise strongly against using any plastic foam inside a transmitter.
It's a severe toxic hazard in the event of a thermal event (aka fire).

In addition it is a poor material for sound absorption, and poor for blocking
sound transmission.

Also, it degrades with temperature and exposure to oxygen and UV.

To drop noise in a duct there are a variety of methods - they tend to be documented
in acoustics books and the more common ones in HVAC texts and papers.

The simplest idea is to create what works out to be a low pass filter. The simplest
one is in the form of a duct that opens briefly to a larger volume, and then continues
out to a smaller volume. Turns in a duct that are NOT lined with absorptive material that is
effective at the frequencies of interest, and effectively so, will not attenuate
much of anything.

What is an effective sound absorbing material?
Most are marginal.
Foam is one that is reasonably poor.
Every material that is commonly available works best in fairly thick applications.
Thinner applications work mostly at HF, and even then not so well.

One of the best turns out to be wool felt. If that can not be found then
cotton felt is not bad. There is also nowadays sold synthetic and combination
synthetic & cotton felts. Some of it has been fire retardant treated for domestic
and commercial installations.

Just like car mufflers, a tuned Helmholtz type filter system can be effective, and made
mostly from metal or similar. This is done all the time in HVAC installs to get rid of
fan noise, especially where high velocities are needed. Should be possible to apply
for us.

Rockwool and fiberglass are actually poor acoustically, and not effective
at lower frequencies until they are used in fairly thick applications.

There is "solid" material sold for use in domestic and commercial buildings similar
to the ceiling tiles but thicker, in 1 to 4" thick sheets. It's either fiberglass or
rockwool with some sort of binder. In some situations these might be useful.

So, stay away from styrofoam or Urea based "foams", latex "foam rubber" has
a short half-life so not a good one either.

Preventing sound transmission through surfaces is similar but a bit different.
The people who do sound studios and speaker enclosures have studied this in
depth.

Two main methods:
- decoupling of two surfaces
- constrained layer

The constrained layer idea is a way to turn vibrational energy into heat within
a composite surface. Essentially it's a high pass - low pass - high pass - low pass,
etc. So, a soft layer, stiff layer, soft layer, stiff layer... etc.

Sometimes space permitting one can incorporate one layer of granular material, like
fine sand as a layer. The sand which vibrates turns the vibration into heat. So that
would be between layers of something like metal - silicone rubber - metal - silicone rubber -
metal - sand - metal, silicone rubber - metal. Or wood/rubber/wood etc...

The decoupling of two surfaces is the classic method - aka "shock mounts".
Even if it holds up a wall or the entire floor or ceiling. For example.

We'd engineer the operation of a tube, or the design of a tank circuit, it's a good
idea to apply the same to sound control and reduction...

                           _-_-bear

The absorption of and resistance to transmission of sound is often shown in graphs
of attenuation vs. frequency WRT thickness...fyi.

EDIT: forgot to mention many foams are mildly electrostatically charged, and even those
that are not tend to pick up airborne dirt and grease quickly, and are not easily cleaned.
This creates an additional concern.

Interesting info, Bear.

Another solution to fan noise:   I just completed my outside installation of a Dayton 2C647D blower for the big linear amp and did measurements .  From peak power to fan noise, (S/N on ssb) looking at the SDR spectrum analyzer, I am seeing -45 dB delta. I am using the exact same mic and audio chain as on AM.  This is using simple flexible neoprene 4" ducting, about 3' long from the wall outside the house to the tube chassis inlet.

I've found that at least -30 dB of signal to noise is needed to avoid the "machine shop" sound. -40 to -45 dB  is superb. At -45 dB there is barely any sign of air noise, especially on the air when we mix in room ambience that can include residual hum, scope fans and other sounds that can be classified as a "quiet room."

When I go outside and listen to the sound of the blower; the slight mechanical whine and the sound of the air inlet, I estimate it is at least 80% of the noise energy being generated by the blower. The other 20% is inside the shack.   The air noise and mechanical sounds must be dissipated somewhere, and in this case most of it is outside.  

As for cold air, the temp starts at say, 30F outside and after an old buzzard transmission peaks out to about 110F above the tube.  That is warm to the touch, but not anywhere like starting at 70F.  So, for noise and temperature, this method is a reasonable solution for me. It is quiet and gives a temperature edge from fall to spring.  In addition, the cold air lets me run the blower at about 1/2 normal speed. This helps to cut noise tremendously. So there is a confluence of advantages that all add up to a silent sound when using a remote blower mounted outside the house during the cooler months.

As for tube seals, I think the delta change between 30F to 110F   vs:   70F to 150F  will affect the tube seals about the same. I've axed a few guys smarter than me about this and they either did not know or didn't think that starting a tube at 30F was a problem. I ran this external anode tube for about 2 years this way back ten years ago with no problems.

This also brings new meaning to "filament cold start."   I use a Variac to bring the fils up slowly in addition to a delay timer that doesn't let the tube key up with HV until seven minutes of fil heating have passed.

The good news is with the filament heated up and running - and the outside air at 30F, the air above the tube hovers around 55F.  So the runup from 55F to 110F is not too extreme.

T

Call me nutz but could you make a small audio amplifier, (I'm talking pocket transistor radio size), with some sort of common emitter circuit whereby you invert the noise created by the duct work and cancel it? Chust thinkin!

Yep, that would probably work.   A microphone facing the blower area and then added to a mixer with the real signal 180 degrees out would do it.  They do this trick on luxury jets and cars I understand and who knows where else.  I'm surprised there isn't an MFJ box made specifically for this purpose.

Maybe the difficulty is adjusting and maintaining proper audio phase, (and variable amplitude needs across the specturm) but in this digital "pure signal" whirl with a computer screen interface it should be do-able to a fine art.

T

Do you mean the house HVAC type or the aircraft type?

Hi Pat,

Did I say fiberglass?  It's actually 450 degrees F neoprene silicon at $16/ ft ducting. Orange.   It has internal solid wire spiral but has a smooth inside.  I got the high temp cuz at one time I was pulling air out of the top of the tube.

 https://www.pegasusautoracing.com/productselection.asp?Product=3621&utm_source=google&utm_medium=cpc&utm_campaign=3621&gclid=EAIaIQobChMIqPXKubqB7gIVB7zACh0oWwfoEAQYBSABEgLA8PD_BwE

Nice stuff.

T

That's what I mean by 'aircraft' stuff. It's the best.

Help! QUESTION:

I have a PS filter choke that is causing a really loud 60 Hz acoustical hum. It is so loud that it gets into the mic audio.   The supply is a 3 KV / 6 KV DC output with parallel or series primary xfmr connections.   The 100 pound transformers feed a SS diode bridge rectifier.  (6A 1KV diodes)

The choke is a big 140 pound 15H BC type that goes from the diode bridge anode connection to ground, the same ground as the filter caps as negative lead filtering. I have about 133 uF at 10KV caps. About 400K of bleeders.

I tried a different choke and it also buzzes.  When I short out the choke or disconnect it from the circuit the hum goes away.   I checked the diode rectifiers and they are OK.

The supply works well at 3KV with no choke, but when the choke see 6KV OR I draw some serious current at 3KV, the acoustical hum begins.  Even idling at 6 KV the choke hums badly.

I haven't tried the choke in the positive lead yet.   It wud take some work to do and I figgered there is really no difference when in the negative lead.

I tried one transformer alone and there is no difference which eliminates circulating current between the transformers.

There is a one-second step start with 10 ohms in the primary.

Using a X100 scope probe there is barely any AC ripple on the output cap. Clean. Smooth DC.  But across the choke I am seeing the slope of ripple and a damped oscillation. Maybe this is normal for a choke, I dunno.

Would a standard   C-L-C filter in the hot lead put less stress on the choke, thus less 60 Hz noise?

What could cause this problem?  Any tests?    

T

Tom, when my dad had his radio repair shop in the early 1940s, when someone brought in a radio and asked why it hums, he typically said "It probably does not know the words".  Perhaps that is the problem with the two chokes you have tested?

But on a more serious note, I would assume the chokes have a loose structure, possibly loose core material, or the bobbin on the core may be loose, allowing the vibration.  Not knowing the makeup of your chokes, I cannot suggest whether any of the hardware can be tightened to reduce the noise level.  I agree that the problem would probably persist, whether or not the choke is in the positive or negative supply leg.  I would also assume that the filter choke would output much less hum or noise if you use capacitor input (C-L-C) instead of choke input (L-C) filtering.

The choke is just doing it's job, but perhaps it does not need to be employed in your power supply.  One advantage of having a choke input supply is that it may allow the plate transformer(s) to run cooler by reducing the peak current, and spreading the capacitor charge current over a longer portion of the AC waveform.  Without the choke, the current flows through the transformer secondary in large current peaks for the short duration of time that the transformer output voltage is greater than the charge on the storage capacitor.  This high-current peak causes more heating of the transformer windings, due to the I-squared R losses in the winding resistance.  Assuming the transformers are sized conservatively for the load placed upon them, this may not be an issue at all.  With step-start, you should be good to go, and the only remaining advantage of keeping the choke in the circuit is ballast in the event of a tornado or earthquake.

Since you plan on employing the PDM approach, additional hum reduction may be achieved by the "feed forward" feature of the PDM generator chip.  But with the size of your filter capacitor bank, and the dual plate transformers, I cannot imagine this would be necessary.  If you run the higher voltage (you mentioned having two options) your PDM series tube will run more efficiently (assuming it can handle the peak voltage) because the "on" time will be of less duration for the same amount of output voltage.

Bottom line, you may not be able to make the chokes run silent without disconnecting them from the power supply circuit.  If they do not add any value to the rig, why bother?

Couple thoughts...

Can you post a scope photo across the choke?

What happens if you disconnect 2/3 of the capacitors?

Is it possible with that much capacitance that the peak charging current is well past the choke’s current capability? Not knowing what it came from, I’m speculating, but I’m going to guess the capacitor in its original usage was on the order of 10-15% of what you’ve got....

Ed

Rick and Ed,

Thanks much for the comments.

Making progress:   After a some tests I've determined that the power transformers themselves are causing the hum. When I put 240 on each transformer  (or primaries in parallel) the acoustical hum is huge. This is with no load on the transformer, just idling with the sec free.   Both are the same.

When I put 120 V  on the primaries (pri in series) the hum is almost gone.  It acts like the primaries are saturating with circulating current when 240V  is applied.   I'm setting them up now as series-series config and hoping that will solve it. Once they are quiet I can add the bridge, choke and caps.    I plan to add a Variac to allow adjustment for the various rigs.  I prefer not to by controlling it with series/par taps, but in this case I am stuck with series.

BTW, I found 10 shorted diodes on one leg and 3 shorted on another leg of the bridge rectifier stack.  Old battle scars I guess.


T

What is the input voltage rating of those transformers?  Do you have to go all the way to half voltage to cure the hum?  Maybe 240 is too much, but they may quiet down at 220?  A small buck transformer and parallel primaries would work ok?  A Variac test might be very revealing as to the saturation point.  Or, perhaps the transformers hum because they don't know the words either?