The AM Forum

Was wondering which would be better for a plate supply for a high power linyer for AM. Choke inpoot or no choke.....just caps. If choke inpoot, should the choke be a swinging choke or fixed.
TNX in advance,
Bill

Hi Bill,

Most of the commercial linears today do not use  chokes in their HV supplies. Better quality older ones like Henry usually did.  The choke helps with better regulation, but it also helps to reduce hum.

I notice that most commercial amplifiers have hum on the carrier when they are tuned up on the air. You can get only so much filtering with a big bank of capacitors.  On my big amp I have about 100 ufd, but when I short the choke out I can see a little hum on the scope.

Bottom line is get a swinging choke if you can, but a standard choke will work FB if you have some decent capacitance to work with.  Hopefully your capacitor values will be the limiting factor for regulation, not a transformer that is too small to keep them charged under a heavy load.

What tube are you using  for the linear?  Is that for the 4CX-1500 ?

73,
Tom, K1JJ

i like choke input, and also use a bleeder that sucks down 100-150 mils. the common HV supply that runs one of my stations uses a 20 henry choke and 50MFD of cap with heavy bleeder. the output is rock steady from 100 to 800 mils current drain. voltage doubler supplies with cap only for filtering never seem to work well for me. i have also found you can get away with poor regulation on a linear running AM where on  SSB your signal goes to hell in a handbasket with bad regulation.

The choke supply will yield better static and dynamic regulation. Get more info on power supplies at the links below. But, if you have enough capacitance in a cap input supply, it will be stiff enough for the plate supply on most linear amp designs.

Power Supply Design and Dynamic Regulation, PDF 1 (http://www.amwindow.org/tech/pdf/geps1.pdf), PDF 2 (http://www.amwindow.org/tech/pdf/geps2.pdf), from the GE Ham Tips supplied TNX to  John, K5PRO


The Henry amps use a resonated choke input filter. A cap is placed across the choke to resonate it at 120 Hz. The two form a series resonant circuit thus presenting a high impedance to the ripple voltage from the rectifiers.

Many consider this design superior because it reduces the amount of inductance and capacitance needed in an LC filter to reduce the ripple to an acceptable level. For production this means lower cost for the components and less space needed to house them (the cap and choke will be physically smaller), so a smaller less expensive enclosure can be used. These are valuable to a homebrewer too.

This arrangement also requires less bleeder current to prevent voltage from soaring under no load/small load conditions. So you can use a higher resistance bleeder that will dissipate less power, thus a smaller, less expensive resistor. And your supply will be cooler because you are sucking less current out of it just for bleeder. The extra current can now be used to power the amp (more efficient use of the supply).

On the other hand, some people think resonated choke filters are more bother than they are worth since the choke/cap combo must be selected/tuned to really work well. And their dynamic regulation may not be sufficient for quickly varying loads (e.g. linear amp). But they seem to work FB in the Henry amps, so who knows.

So, if you already have a pretty big choke >= 10 uH, messing with a resonated design might not be worth your while. Getting large amounts of capacitance at HV is pretty easy and cheap these days, so following a good choke with a bunch of capacitance will make for a good stiff supply.

Building a amp for SSB, ideally, a swinging choke as the input choke does best. Since obviously, as your current demand increases so wil the regulation.
Another thought, is to go with a Collins idea of using a resonant circuit...
Thus a cap & choke in parallel (I think they were chosen to have a resonant freq of 120 HZ ) then a really big cap afterwards....
gl es 73 steve

It will depend on your high voltage transformer and the output voltage you need. A choke input filter will give you less voltage but better regulation. Second choice would be a cap input filter with a choke.
This way you get less ripple with higher voltage out.
My old 4-1000A linear used a pair of BC610 transformers. Choke input in the stock form only did 2500 volts out. I used a cap bank and got 3300.
2500 volts on a 4X1 is pretty useless so I needed more voltage.

Have always used resonant ( tuned  ) chokes in linears . Really the way to go as Art Collins found out. Use one in my 3CX3000 F7 running 5000 volts on the anode (plate) and only drops around 300 volts @ 1.5 amps :badgrin:  :p .On the dummy load of course. Also have a tuned choke in the HB 5868 class B modulator with excellent regulation.

4700 volts 1.5 amps for 1500 watts out........that is quite a message!

That's... 1500 BIRD WATTS... caw moan !!!!!

now that is flappin your wings bird watts!

My Avatar is a pair of 4-1000s modulated with a pair of 5868 in class B. The 3CX3000 F7 is a LINEAR for SSB and it will loaf at the leagal PEP ( it isn't restricted to what us AM folks are 1500 PEP ). Some day I will change my avatar to the HB GG / 3CX3000. It is pretty  :D

TNX for the replies guys.
Tom, the screen supply for my 4CX1500B linyer bit the dust and I am rebuilding it. While I was pondering a new design...the old one sucked....I got thinking about general HV supply design. Although the new screen supply will use these boards http://www.ifwtech.co.uk/g3sek/boards/tetrode/tetrode-1.htm and the choke question really doesnt apply to them......I was thinking about adding another 4cx1500b to the amp. If I do, Ill have to beef up the HV supply. As of now it will provide 3KV at 1amp and uses a Peter Dahl swinging choke input supply. Another tube would require a supply capable of at least 3KV at 2amps or better and ofcourse new iron. But after thinking about the whole idea......I decided to just rebuild the screen supply and leave well enough alone. Maybe in the future Ill build something different and need a bigger HV supply. Depending on who you talk to about choke input supplys, you get different opinions. I like swinging chokes because I always thought you get better HV regulation. Cap input supplys will give more voltage for a given transformer but less regulation I was told.
Anyway thanks again for all the replys. And Steve, I printed out the pages u listed....very good stuff....thanks.
Bill

Roy VWX said:


Oh yeah?  Last I checked 1500WPEP was the limit for ANY mode.

A Fine Business amp, Roy!

73 John

Hi Bill, Pay close attention to the transformer Voltages called out for that screen supply. There is no room for error. Give yourself plenty of time to build it also. It takes time to figure out the destructions. You can still have a plate to screen zorch even with the board's protection. Stuff can still get cooked. Ask me how I know this. %$#%&*%#! Also let any new 4CX1500B's play with filament Voltage only for at least 24 hours and hopefully the filament will getter any gas.
Step 47: build a triode amplifier. Step 48: retest blood pressure.
The best part about swinging choke input supplies is that the B+ won't go much higher than your full load Voltage. The higher Voltage will invite parasitics and internal tube flashovers. Some say the transient regulation of a choke input supply is poor. Same guys don't quote filter capacitance values. Same guys build hamateur amps and have to cut cost. With cheap modern small capacitors it is easy to come up with values many times larger than what was seen in a buzzard's prime. Some type of step start should be added when using a heap of filter cap. Also becomes much more dangerous. I'm sure you know this stuff but is makes for good discussion.
It would be good to see the math Roy uses to resonate filter chokes. A great way to get more of existing iron. There was an old QST article that showed how to figure actual inductance of a power supply choke with DC current flowing through it.

Dave,
Im not sure why the screen supply pooped the bed. I replaced the toob with a "new old stock" toob that had never been used. I burned it in for about 18 hours with filaments and blower only although 24 hours probably would have been better. The amp was working fine for about a week or so. One day I fired it up....let it warm up for about 5 minutes before applying HV....applied HV and all was fine. In use, the HV is applied all the time.....the toob is cut off during receive. When I applied drive...the HV overload kicked out. Reset the HV and tried again. This time the HV held but little or no outpoot then I smelled the smoke! Shut her down quickly. Pulled out the screen/bias supply chassis and found all the screen zeners shorted causing the screen droping resistor to get REAL hot!! Basically the whole screen supply fried. Checked the screen ckt in the amp for shorts but found none. SOOOOOO......I think the toob flashed but not sure. There was no protection at all in the screen supply....not even a fuse in the primary and no flashover protection.
Anyway...I hope the new supply Im building will solve any more problems. I just hope the toob is still good.
Bill

That tube flashed like Chester the Molester. That was a good working amp too. Well, you will get it back on line soon enough. Meantime maybe take some L and C out of your 4-1000.

Bill,
Make sure you use a load resistor on the screen at the tube. I think the screen can swing negative current in the 4CX1000 / 1500. This way if you blow a fuse the screen is pulled to ground with the load resistror.
I don't remember where I read this but years ago I was going to build a 4CX1500 amp before I landed the box of 4CX3000As
It might have been Eimac care and feeding of power grid pube book.
The screens can't handle much power so don't build a big supply.
Don't store a lot of energy in the screen filter.  It may have been VHF oscillation that flashed it over.  fc

What about using FETs?

That is a good point, Tolly... use a FET as a pass regulator fer the screen.  Or you could use a cheapo horizontal outpoot transistor.  I did this years ago to replace the pair of stupid 6082 triodes in a R-390.

You need to reconize and deal with the potential for negative screen current though.

73 John

Here's the goods if you want an efficient 2KV or less screen regulator
for big tetrodes. I once had use of this circuit before going to all
triodes. From Richard Measure's site -

73,
Tom, K1JJ
(http://www.somis.org/D-a-08.GIF)

This circuit needs a supply with at least 15 volts of ripple to peak charge the boot strap. Also needs a load resistor to handle reverse screen currents.

And lose that friggin LM723.  Use an LM350 three-legged variable regulator instead - just make sure you use plenty heap big RF bypass on it or it'll sing like Ethel Merman.

Still need to address the reason the original screen circuit popped. Tubes will be tubes.

Perhaps a stupid question, but how come lose the LM723 in favor of an LM350?   Durability? Stability?  I think maybe I was sleeping in class when something about the LM723 came up on the board before.

Paul - parts count, durability, power handling, simplicity.  The 723 is the one to use if you really need precision, though.  A screen voltage regulator is NOT one of those cases.

73 John

Okay.  I get it now.   Funny because I'd built a screen voltage regulator for a homebrew transmitter and the circuit, although for lower voltage, was considerably less complex than the Measures circuit.   It appears the LM723 added quite a bit to the parts count as you say, John.  Thanks for the clarification.

Paul

Wow that is a lot of parts!

Im gonna try and post the schezmatics of the tetrode boards I am using for the screen and bias supplys.....but they are BIG.........Number 1
(http://www.kc2ifr.com/photogallery/tetrode1small.jpg)

Number 2
(http://www.kc2ifr.com/photogallery/tetrode2small.jpg)

And finally number3....
(http://www.kc2ifr.com/photogallery/tetrode3small.jpg)

Well......there they are...all three.
What do u guys think.........
BTW......I already paid for em....

Lotsa protection there - if there's one criticism I could make it's lack of a crowbar for the screen.  Easy to make - see the crowbar portion on the circuit Tom Vu posted.

73 John

When it comes to high voltage linear regulators KISS is the way I would go. The power is quite low so I would do Zener shunt myself to limit energy.

Like this one?

(http://wavebourn.com/images/bc348bplus.jpg)

One thing which you want to check after a failure involving the screen supply is the screen bypass capacitor(s). I imagine yours is built into the socket. You can reject it with an Ohmmeter test but also apply some current limited DC Voltage to it maybe 20% over what you plan to use in service. Check to make sure that the dielectric has not been punctured by excessive Voltage.

Tolly is getting closer. Just put an emitter resistor on Q2 and eliminate all the negative stuff. A series pass regulator would require a load resistor to deal with negative screen current while a shunt regulator would handle that also. The screen rating is so low with 4CX1000 / 1500 you don't want a big power supply source that could deposit the screen on the plate making it a triode. Why have a 100 watt power supply for a what 5 watt grid.

Lots of good stuff from u guys.......tnx. Well I already purchased the "tetrode boards" so I might as well use them. The seller usually sold all the parts as well as the boards but for some reason the parts wont be available from him for about 3 months so I have to get them myself. Most stuff is no problem to get  from my local supplier except for a few things that I will get from mail order. As was pointed out...this toob draws negitive screen current so that has to be taken into consideration in the design. If anyone is interested.....the complete manual for the boards is available from the website.
Thanks again for the help and advice....Ill keep ya all posted.
Bill

Frank, if to eliminate negative stuff you'll loose soft start. But if it is not a problem, instead of resistor in emitter of Q2 you must add a zener, to get regulated voltage.