How much peak power can a class C 6146B put out?

[N2DTS]

Seems like a fun project.
Trying things until you find something that works real well is the fun part.

I do not really follow it, but people have built PDM rigs using tubes, have they not?


I really dislike rigs that have blowers or fans, at least if you can hear them.
There is nothing like a silent rig if its close to the operating position.

I hear guys running amps with big blowers in the background, and it bothers ME hundreds of miles away!

[steve_qix]

Very, very few people have [successfully] built a tube PWM rig.

It's so different from building a transformer coupled modulator, and the concepts are vastly different.

The thing is, with transformer coupled modulators, there are degrees of working "correctly".  It can be pretty poor, and still work or if someone is very clever, has the right components and does everything perfectly, the rig will sound superb.  And then there's everything in between.

Not generally so with PWM.  It's either right or it's horrible - at least most of the time.  They're not THAT hard to get right, but the rules for making it so are hard and fast.  Tubes are harder than solid state in the PWM world for a number of reasons:  the high voltages involved; tubes are poor switches and need compensation; the impedances are higher so the filter design is more critical; heater voltage is required, necessitating a special transformer; etc. etc.

But, if you can get over the technical hurtles, tube PWM rigs work well.  I've built a number of them over the years and they sound every bit as good as the best out there.

I wrote up a fairly detailed account of the last tube PWM rig I built including a lot of theory about how things work, and work-arounds and corrections for various problems that most builders will encounter.  If anyone is interested, I'll post the link.

Regards,  Steve

[W1AEX]

Tom,

Back when the 4D32 was a rare tube I replaced it in my Viking One with 3 x 6146B's and switched to an outboard 811A modulator that has its own plate supply. The Viking plate supply and the modulator plate supply are connected to the same variac so I can roll them up and down to set the power. My 35 year old 3 x 6146B's running @750 volts can do ~135 watts carrier output without complaining and the 811's will modulate them close to 100% to produce a bit more than 500 watts PEP. Rolling the variac up so that the viking's plate supply is above 800 volts it's easy to reach over 150 watts carrier but I don't push it that hard.

Looking forward to hearing the tube PDM rig!

Rob W1AEX

[K1JJ]

Yep, they can be a challenge, alright.

In the past I've built three big tube PDM transmitters. All involved 4-1000As.  They eventually worked FB, but they were especially dangerous due to the floating supplies, filament transformers, 10KV power supply, floating input RF circuits at HV, etc..  To work on it required extraordinary care. After being away from it for a while, going back into it for a repair was a big risk.  So I eventually tore each one down and went back to conventional plate modulation, which is what I run now for the 4X1 rig.

A big tube PDM rig also needs a few specialized parts, like a custom wound PDM filter that can get very big if wound on an air core. We need to wind a custom fil transformer to handle full plate voltage. The damper diodes can be finicky - I eventually went to tube rectifiers for one big rig. It goes on and on.

The 6146B rig runs a more "reasonable" 1600 volts, which I can test without excessive fear, but still dangerous.   I suppose I would eventually find the problem with this particular tube PDM modulator, but again, I probably wouldn't be satisfied without linearization, compensation and all the other stuff needed for tube modulators. The time involved could be better spent elsewhere with optimizing an IGBT modulator.  A hybrid - one half SS and one half tube is the best of both whirls. I still want tubes for the ease of QSY on any band and the fun of tubes.  But 100% solid state? -  I've had several class E SS rigs and became bored with them... but certainly nice rigs in operation!

In the meantime I found a beautiful heatsink in the cellar that is 5X5X5" with 16 fins.... about 320 sq" that will fit into the 6LF6 space perfectly for the new IGBT. It will have a copper spreader and plenty of air, so should be a good start.


Now I need to come up wid a new name.  It's no longer a dual quads rig.   Maybe a Hybrid name of some kind....

More as the parts arrive...

T
 

[KC2ZFA]

I wrote up a fairly detailed account of the last tube PWM rig I built including a lot of theory about how things work, and work-arounds and corrections for various problems that most builders will encounter.  If anyone is interested, I'll post the link.

I would love to see this !

[steve_qix]

Ok, here's a link to the article.

Keep in mind, this article is *old* - I wrote it back in 1986 or thereabouts.  There are parts available today that were not available back then.  But the basic theory is exactly the same.  So, if anyone wants to actually build a tube PWM transmitter based on this article, please contact me first about newer technologies.

The whole PWM generator and driver will be MUCH MUCH easier than the one shown in my article because I had to make everything - triangle wave generator, controls, comparator circuits, etc.  Now, all of this is available in ONE CHIP !!

Otherwise, the article is as true today as ever.

The article is here: http://www.classeradio.com/pdm_article.html

There is 1 transmitter on the air today that is based on these circuits.  Larry NE1S has a kW PWM rig.  I still have my complete PWM generator and driver - everything right up to the grid of the 4-1000 PWM modulator tube.  It didn't seem right to dismantle this PWM generator and driver - I used it for so many years.

Regards,  Steve

[N2DTS]

That really seems like doing it the hard way.

[steve_qix]

That really seems like doing it the hard way.

Well, that article is very old, and now there are much better components available, and much of the circuitry can be literally reduced to one component!  The whole PWM generator including the anti-aliasing filter fits handily 1 small PC board.

Other than that, what part(s) of the circuit do you think can be simplified?  This is not rhetorical - I'm very interested.

Regards,  Steve

[K1JJ]

I'll bet he means compared to standard plate modulation with a mod xfmr, tube PDM is a harder and more involved way to do the same thang.


This is a point when we compare a plate modulated rig that has been carefully optimized with negative feedback and running the modulators in AB1 to a well designed and working tube PDM rig.  The PDM rig, if set up correctly, should be a little cleaner on the test instruments, but probably not enough to notice to the average ham ear.

T

[N2DTS]

I meant from the standpoint of using a nice mod transformer and some zero bias triodes.

A bunch of chips, wave generators, filters, damping diodes, etc, plus a tube/tubes and the high voltage.

[steve_qix]

For sure, PWM is really, really far from any transformer coupled modulator - quite a different animal entirely.  And, it is somewhat more challenging to implement with tubes than a tube transformer coupled modulator for sure.  In solid state, it's not so difficult for many reasons.

On the plus side, the power supply will be smaller, the transmitter will be lighter and it will cost somewhat less to build it (because you don't need to buy a high quality mod transformer and reactor).  It it's done right, the audio results will be quite good, and of course it's more efficient.

[w8khk]

Well, that article is very old, and now there are much better components available, and much of the circuitry can be literally reduced to one component!  The whole PWM generator including the anti-aliasing filter fits handily 1 small PC board.

Steve and Brett, I have been following this thread with great interest.  I have always been more comfortable building the traditional AM rig with plate modulation, and have avoided PDM due to the cost and complexity of getting involved with the PDM generator and the filter circuits.  But a medium power PDM rig using tubes for the switch and the RF section seems very interesting to me now.  I was somewhat disappointed when Tom pulled the tube switch and changed to the solid state switch, as I wanted to see a successful completion of tubes for the power stages and learn more about it from his experiences.  But I understand his reasoning, and continue to follow his progress. 

That said, I am very interested in pursuing a PDM rig using tubes for the RF and the switch.  I have a few questions, and apologize in advance for the apparent thread hijack. 

I have several RF decks that I might consider using.  Is it possible, or should I say, practical, to run the RF deck with the cathodes referenced to ground and the switch in the B+ side of the RF deck?  If not, I will build an RF deck that will be compatible with the cathode floating at HV to allow the switch between RF stage cathode and ground.

When you talk of a simple, small board where most of the PDM functionality is implemented in one chip, is that the same board that drives solid state mosfet switches, or a completely different board?   If it is the same board, is it easy to interface it to a tube switch, either triode or tetrode?  Is there any advantage in using either a triode or tetrode for the switch?  I have an adequate supply of NOS 6146s, 4D32s, 4-65s, 4-250s, 4-400s, 4x-250s, etc.  As an initial experiment, I might even like to try the 6AS7, 6080, or 6336 for a low power rig, just to get some experience.  Designing, building, and tuning the PDM filter sounds like fun, after reading the document you supplied in the link.

Brett, looking at the photos you linked was quite an inspiration and has influenced me very positively to get back into more home brewing projects.  Your receivers are a class act, very nice construction.  I especially like the way you mounted the IF transformers on sockets so that you can swap and troubleshoot.  I will do the same thing on mine.  Your modular approach to transmitter building, with variable supplies and extensive metering is the way I like to build.  Your attention to detal and organization produced equipment that you can be proud of.  I would like to see more photos of the 4x250 based modules, either modulator or RF stages.  Looking at your shack, it is mostly home brew, very little in the way of manufactured rigs.  Way cool!

[WD5JKO]

Darn, swept the Dual Quads rig.  I'm having a bad time with severe high frequency audio distortion above about 1700Hz.  The audio is perfect at 1 Hz, 100Hz, 1000 Hz.   Frank thinks it's the PDM filter design, so trying some different values.  

Blew out the MOSFET driver.  And I thought I had this in the bag.     Plate modulation with iron is so much easier... [sigh]

T

  Tom,

   If the audio response  problem continues after using the SS PDM, look at the 6146 self modulated screen circuit. Best I can follow in your earlier post is you have a 25K screen dropping resistor, and four .001 uf capacitors in parallel bypassing the screens to the cathode cage. The reactance of .oo4uf at 5 Khz is about 8K. Perhaps the self modulated screen concept was getting phase delayed, and at reduced level as you swept above 1700 Hz?

Jim
Wd5JKO

[N2DTS]

I like building, at least simple stuff, the class E and pwm stuff seems a lot more complex and needs more fine tuning.

Since a lot of the pictures are old, they show some non home brew gear, but that stuff is long gone and the only piece of ham gear I have is a 32V3 that has no cabinet or audio driver transformer (gave them away), so its worthless, but makes a good exciter.
Modern rigs float in and out, none of it gets used much if at all, except the sdr receivers.

The idea of using octal sockets for the IF transformers (and spacing things out) was not a good idea, but helped with the prototype. I had to change the IF amp tubes to something with less gain as building it that way made things unstable with high gain tubes.
I only had PC board mount IF transformers, hard to chassis mount, but thay have a very broad response, being intended for car radios (AM).

The home brew receivers out perform anything else I have had (for AM).
Much less noise then anything else I have ever tried, even the sdr receivers.
 
There are pictures of the 4x150 and 4D32 rigs, top and bottom, plus the 4x150 when it only had one tube.
I plan on redoing both with better grid circuits and more space on the 4x150 deck, and I might add a 3rd tube....

I use a cordless hand drill, a dremel, files, a few hole punches, and spray paint from cans.
Its also odd that nothing I built had a design, I took a bit of this, a bit of that, and tried different things.
If it worked, I used it, if not, I re did it.
That method would be very expensive with class E or the PWM type stuff, since I do not have those sorts of parts in the junk box.
Lots of respect for the guys who design and build that sort of stuff though.



Brett, looking at the photos you linked was quite an inspiration and has influenced me very positively to get back into more home brewing projects.  Your receivers are a class act, very nice construction.  I especially like the way you mounted the IF transformers on sockets so that you can swap and troubleshoot.  I will do the same thing on mine.  Your modular approach to transmitter building, with variable supplies and extensive metering is the way I like to build.  Your attention to detal and organization produced equipment that you can be proud of.  I would like to see more photos of the 4x250 based modules, either modulator or RF stages.  Looking at your shack, it is mostly home brew, very little in the way of manufactured rigs.  Way cool!
[/quote]

[Steve - K4HX]

Very impressive!

[steve_qix]

This is mostly in response to Rick's query.

If you are going to use a tube PW modulator device, it is *significantly* easier to float the RF amplifier's negative rail than it is to float the modulator's.  For the RF, you essentially have to bypass the cathode circuit to RF ground (and make that capacitance part of the last element if your PWM filter), and drive the grid with a dc (and audio) isolated feed.  The grid circuit tuning components need not float at all - only the DC grid components (grid leak resistor, RF choke and metering).

The existing PWM generator board I use for the class E rigs is a proven, easy to assemble circuit that is perfect for any PW modulator application.  The circuit is published, and if you want a board and parts, they will again be available when I do the next run of PC boards which is planned for later this month.

For the PWM switch tube, use the highest current tube available.  I've built a lot of PWM tube rigs, and have found tetrodes are somewhat easier to work with, but triodes are fine and also work.  My first PWM tube rig used a pair of 250th tubes modulated by a single 833A.

The PWM driver is the hardest part of it, but it's not THAT hard.  The driver must produce enough output to drive the grid as hard as it can be driven without damage, and the driving waveform should be reasonably unaffected by the grid load.  A source follower is the best way to accomplish this.

You really should have analog compensation in your PWM driver.  This is discussed somewhat at length in the PWM (PDM) document - a link to which is in one of the previous posts.  Analog compensation drives the tube harder as the On-time of the pulse increases, and drives the tube less hard as the on-time decreases.  This both linearizes the tube (tubes are not perfect switches), and works to allow the transmitter to be modulated to 100% negative cleanly.  Analog compensation is done in all of the tube PWM broadcast transmitters that were made by Harris, etc. for this same reason.

Anyway, a worthy project for sure.

Oh, one more thing - I've been asked a number of times if I started with a low power transmitter for the first PWM rig.  No.  It's almost the exact same amount of work to build a big transmitter as it is a smaller one, so what the heck!  Go for the big one if it's no more difficult 

[w8khk]

This is mostly in response to Rick's query.

If you are going to use a tube PW modulator device, it is *significantly* easier to float the RF amplifier's negative rail than it is to float the modulator's.  For the RF, you essentially have to bypass the cathode circuit to RF ground (and make that capacitance part of the last element if your PWM filter), and drive the grid with a dc (and audio) isolated feed.  The grid circuit tuning components need not float at all - only the DC grid components (grid leak resistor, RF choke and metering).

The existing PWM generator board I use for the class E rigs is a proven, easy to assemble circuit that is perfect for any PW modulator application.  The circuit is published, and if you want a board and parts, they will again be available when I do the next run of PC boards which is planned for later this month.

For the PWM switch tube, use the highest current tube available.  I've built a lot of PWM tube rigs, and have found tetrodes are somewhat easier to work with, but triodes are fine and also work.  My first PWM tube rig used a pair of 250th tubes modulated by a single 833A.

The PWM driver is the hardest part of it, but it's not THAT hard.  The driver must produce enough output to drive the grid as hard as it can be driven without damage, and the driving waveform should be reasonably unaffected by the grid load.  A source follower is the best way to accomplish this.

You really should have analog compensation in your PWM driver.  This is discussed somewhat at length in the PWM (PDM) document - a link to which is in one of the previous posts.  Analog compensation drives the tube harder as the On-time of the pulse increases, and drives the tube less hard as the on-time decreases.  This both linearizes the tube (tubes are not perfect switches), and works to allow the transmitter to be modulated to 100% negative cleanly.  Analog compensation is done in all of the tube PWM broadcast transmitters that were made by Harris, etc. for this same reason.

Anyway, a worthy project for sure.

Oh, one more thing - I've been asked a number of times if I started with a low power transmitter for the first PWM rig.  No.  It's almost the exact same amount of work to build a big transmitter as it is a smaller one, so what the heck!  Go for the big one if it's no more difficult 

Steve, thanks for the detailed response to my questions.  I definitely would like to order the complete set of boards and parts for the PDM generator when your next production run is available. 

I have been studying both the  PDM document you linked, as well as the newer documents on the class E website.  If I understand correctly, the analog compensation is required only for the tube switch, but not for the mosfet switch.  It appears this circuitry is documented in figG.  It is not clear to me whether that part of the circuitry is based on a board, or just manually wired. 

In any case, I think I will build the transmitter in a modular fashion, so I can use the PDM generator with either a larger or smaller switch, filter, and RF deck.  None of my RF decks are easy to isolate the cathodes for high voltage, so I will just build a new deck, I have all the needed parts available without purchase.  I am confident that I can calculate values and construct the PDM filter portion of the transmitter, but I expect I will need a bit more guidance on interfacing the PDM generator to the switch tube.  We can address that once I get started building the generator.  (I am thinking maybe a pair of 4-400A RF tubes modulated by either a 304-TL or 833A switch tube).

I really appreciate the resource of knowledge and boards/parts you provide to the AM community for this type of equipment. 

[K1JJ]

  Tom,

   If the audio response  problem continues after using the SS PDM, look at the 6146 self modulated screen circuit. Best I can follow in your earlier post is you have a 25K screen dropping resistor, and four .001 uf capacitors in parallel bypassing the screens to the cathode cage. The reactance of .oo4uf at 5 Khz is about 8K. Perhaps the self modulated screen concept was getting phase delayed, and at reduced level as you swept above 1700 Hz?

Jim
Wd5JKO

Hi Jim -

Yes, 8K certainly seems a low impedance compared to the existing screen resistor. (I currently use 15K for the screen dropper)


As you suggested, when I test the rig with the new IGBT modulator, I will keep this in mind.  I could always reduce the four screen bypass caps to 250 pf each.

I should have some results before Friday if the parts come in. Right now I'm setting up the modulator heatsink.  Looks like I will be mounting the IGBTs on Sil-Pads, since the ones I have (pink)  are good for about 5KV or so.  I noticed that even though the IGBT pad itself is good for 4KV, the leads are only spaced 0.118" from the heatsink.  Sounds like an arc waiting to happen without the pads.   Never used IGBTs before and too expensive to screw them up.


BTW, Rick, your plans sound like a very cool PDM project - 440As X an 833A.   You will love PDM once it is running right.  I remember the first PDM rig I built in 1991. It was a pair of 4X1s X a pair PDM.  The filter coils were about 40" long and wound on 8" diameter PVC pipe - thousands of turns. The coils were hung from ropes and did pull-ups in the rack when I hit lows according to Frank/GFZ.   Tron/ HLR came over for a visit and just stared at the rig as it was on the air pouring on the soup.. He had a glazed look in his eyes and said, "That's the way to do it - I like it!"    

T

[steve_qix]

Ok, sounds good.  The 833A will be MUCH easier to drive than a 304TL. A 304Th - maybe.

My biggest PWM rig was a pair of 450TLs modulated by a single 4PR1000.  That was quite the transmitter !!!  The PWM filter coil - I still have it - is an air wound coil that's around 30 inches long and 12 inches in diameter.  Wound with #22 magnet wire.  Worked great - and that's more or less the kind of coil you want.  It took me a few hours to wind it.

The 2nd coil can be wound in layers, and that's how I made mine.  I put waxed paper between layers to prevent arcing between them.

The first inductor of the PWM filter *must* be large enough to completely contain the PWM waveform at the smallest duty cycle - typically 1% or 2% on-time and 99% or 98% off time.  If the waveform collapses or otherwise begins to distort from a square wave, the PWM filter input coil is too small.  It should be JUST big enough to contain the entire waveform when the filter is properly terminated and no bigger.  Making the coil of a higher inductance than necessary will compromise the efficiency of the modulator somewhat.

Anyway, just a few pointers.  It'll be interesting to see how the project progresses.

Tom, you may need an active pull-down with your solid state modulator to maintain the waveform.

[w8khk]

The first inductor of the PWM filter *must* be large enough to completely contain the PWM waveform at the smallest duty cycle - typically 1% or 2% on-time and 99% or 98% off time.  If the waveform collapses or otherwise begins to distort from a square wave, the PWM filter input coil is too small.  It should be JUST big enough to contain the entire waveform when the filter is properly terminated and no bigger.  Making the coil of a higher inductance than necessary will compromise the efficiency of the modulator somewhat.

OK, So it appears to me that I must decide on the approximate power level of the RF final (Plate voltage and current) before calculating the filter component values.  One of the reasons I thought about starting smaller is the higher voltages involved on the plate side of the RF deck, typically double what you would normally run as a linear or conventional plate modulated final.  Not only is it necessary to float the filament side, it is also necessary to provide adequate spacing of the DC components in the plate circuit to avoid bypass and blocking capacitor breakdown and arcing. (I don't think the pi net components need to be oversized, as they will not see the higher DC potential.)  Extremely high B+ voltages are more dangerous, even in an enclosed rack.  So I still might start smaller with three or four 4D32s in the final, running lower voltage but maybe higher current.  If I do go with a pair of 4-400As, either the 833A or a 3-500Z should work fine as a switch tube.  Lots of options, just need to nail down the best compromise.  Thinking out loud, maybe its time to tear apart and rebuild a new RF deck from the dual 4-400A G2DAF linear that I built in the air force barracks in 1967! Although I never received any negative signal quality reports, it certainly does not meet today's IMD goals.

[steve_qix]

Not only is it necessary to float the filament side, it is also necessary to provide adequate spacing of the DC components in the plate circuit to avoid bypass and blocking capacitor breakdown and arcing. (I don't think the pi net components need to be oversized, as they will not see the higher DC potential.) 

The voltages across the DC blocking cap will be the same as in a standard transformer coupled plate modulator.  The only difference is that the (dc) voltage is there all the time as opposed to only on peaks.

I ran a 10kV power supply with my rig - 4000V across the RF amplifier and 6000V (dc - filtered) across the modulator.  This allowed for 150% positive peaks.

Typically, a 2.5 to 1 ratio is used, with 1 being the DC voltage at carrier, and the 2.5 being the total power supply voltage.

A pair of 813s modulated by either an 833 or a some other tube makes a nice combination because the 813 is a relatively low voltage tube.  You'll need a 5000V power supply, and the rig will run 800 watts DC input (2000V at 400mA).  Not bad  - a good medium power rig and very practical in all respects.

[w8khk]

Typically, a 2.5 to 1 ratio is used, with 1 being the DC voltage at carrier, and the 2.5 being the total power supply voltage.

I see.  I was thinking a 2-1 ratio, but that would just barely allow 100 percent peaks.  2.5 to 1 makes sense now.  

Based upon power supply components I have on hand, I was initially looking at keeping the total power supply voltage down to 3500 or 4000 volts.  I could go higher but that would mean using new rather than available parts.  (A bridge rectifier and a pair of oil caps in series would put me around 8000 volts, but I would rather not do that initially.  For safety, for this rig I prefer a dedicated internal power supply, rather than a shared external one.)  So maybe my initial build might still be best to go with a quad of 4D32s, and maybe two or more 4-65A tubes for the switch.  I have 6 NOS 4D32s and 10 NOS 4-65As.  That would allow me to build it on two decks and use convection cooling.  Power supply components on the bottom floor of the rack.  So many different options, but modular construction will allow me to scale up later.  

I am wondering if the switch driver would be considerably different for 4-65A switch tubes as compared with an 833 or 3-500, and is that driver part of the printed circuit board or hand wired?  I am sure these details will be worked out in time.  Lots to build during the cold winter months.

By the way, one of the things that I really like about the transmitters that Brett, N2DTS builds is the plug-in coils for the pi-net.  Easy to get just the right inductance for any band with no band-switching on the plate tank.  I will definitely build that way.  The ceramic pillar insulators and the metal plate holding two banana jacks allows for short or long coils, simple, neat, effective.  We can sure learn a lot from just looking at photos of other folks innovation!

[steve_qix]

Sounds good except for the 4-65As.  I believe the 4-65A would make a poor switch because it is a relatively high impedance tube.  The 4d32 would be better, or if you can find them cheaply (and I think there are a lot of these out there) the 6DQ5 - which is a very good switch !

I've had some experience with 4-65As in various types of service, and they definitely like a lot of voltage, and are a bit anemic in the current department.

[DMOD]

Here is one such PDM transmitter using two 6883s (12V version of 6146).

http://www.lu8jb.com.ar/PWM_6883.htm

The switch tube grid is switched in turn by a HV bi-polar transistor with its base switched by the PDM signal of about 5V.

Phil - AC0OB

[steve_qix]

The audio in that particular implementation will have some distortion due to the lack of analog compensation, and in fact the drive may actually decrease somewhat with increases in the on-time (duty cycle) due to a resistor pull-up to the modulator tube grid (the opposite of what is required).

One thing about PWM, shorcuts are costly with respect to audio quality.  But this is amateur radio, not broadcasting, and there's a lot of stuff on the air that will sound a whole lot worse    (and some that will sound a whole lot better, too!).  Depends on how much work you want to put into it and what the final goal is.