The AM Forum

I just received my plate transformer from Bill, KD0HG.  Thanks again Bill!

What a monster!  It is easily 75 lbs.  3560/4580 VAC and 1440 VA.

I will modulate a pair of 813s with 813s.  I am reconsidering the power output that I desire.  I am also going to go without a heising reactor (BC-610 mod xfmr), at least initially.

I have never built a transmitter before and I have a lot of learning to do.  I want to scale back this project to minimize the troubles that I will run into but leave the possibility to "scale up" later.  For instance, do I really want to go for a 500W carrier with my close in QTH and simple inverted V antenna?  Perhaps I should go for 200W?  Perhaps modulate a single 813?  I could always add a second right?  I could always add a Heising reactor later right?  

I know y'all have tons of experience with this stuff but what would be the "sane" path for a slob like me?

At least I have more plate transformer than I will ever need.  This thing scares me just sitting here on my bench.  ;-)

Rich

Headroom! Do 500w. 2(813) x 2(813). Be done with it. The shake down will be the same whether big or small.

going 1 to 2 will cut the plate Z in half so pi network will need to change.
Better off with 2 tubes and run reduced voltage. Start with plate transformer on 120 volt line to cut voltage in half. Work your way up from 120 volt primary fed into 240 volt winding and low tap on secondary.

I agree with GFZ.....Go with 2 tubes and adjust plate voltage and loading for the desired out put....

I agree with GFZ also.  Adding a second 813 afterward may be more work than building the PA with the two 813's to start with.  Too many factors change from 1 tube to 2 tubes.  Better to lay out the chassis for the 2 tubes  than to try to fit a second tube into what is already built.

That Thermador xfmr is not too big, in fact, it might just be enough to run the PA and modulator.  I have some Thermadors, they're good iron.

I guess the 3560/4580 vac is center tapped.  1780-0-1780, 2290-0-2290.  1440VA, the xfmr is probably good for about 500ma to 600ma

The xfmr will make about 1500VDC or about 2000VDC with a choke input filter.  I would only use a choke input filter.  You'll need a good 500ma choke.  Using only a capacitor filter or cap input and choke filter is not a good idea for an AM xmtr.

You can run the xmtr on the low voltage taps to start.  Two 813's should easily make about 300 watts output with 1500 volts.  with about 180ma plate and screen current for each tube.

You won't be able to use the xfmr on 115vac, the voltage output with only half the primary voltage will not be high enough.

Good luck with the project,

Fred, KA2DZT

Say... wait a minute!!

Aren't you that famous East Coast AM'er, "Fifty-One Watt Fred"??

How come you know so much about HIGH POWER AM transmitters?
Do you have one, and are being sneaky??


                    _-_-bear

You said:

"I would only use a choke input filter.  Using only a capacitor filter or cap input and choke filter is not a good idea for an AM xmtr."


Fred,

I'm curious of your feelings on this?  For capacitor input: Assuming the transformer is rugged, enough filter capacitance and it is a common final/modulator power supply, the class C final should provide a heavy, constant load, providing good voltage regulation for the modulators. And if enough capacitance is used (at least 40 uf) there should be no noticable hum at this high level stage.  

I often switch between choke input and capacitor input to get various voltage levels for the 4X1 rig and have no problems. Maybe there is another reason I missed?

T


BTW, Rich - I would certainly use two tubes right from the start. The only reason to go with one tube is if the mod transformer is better suited to one final (ratio) or the mod iron you have will never be able to handle the power of two finals without blowing up. You can always adjust the plate current of two tubes to better match the mod xfmr, however . But because we often run into larger iron as time goes by, go with two tubes anyway. You never know.

Thanks to all for your input.  I will stick with the two tube final.  Fred, thanks for pointing out the limitations of that transformer on 115V, it looks like I will be running 220 to the shack.

I got concerned when I saw the size of the transformer I think.  I will install a variac so I can choose the plate voltage.  I was planning on using a choke input filter on the PS.

Onward, upward.

Rich

Tom,

A lot of what you say is right.  The class C final does provide a constant load keeping the peak no load voltage down.  Most common plate xfmrs are current rated to be used with a choke input filter.  If you use a cap input filter you have to reduce your current drain by 20-30% to stay within the VA rating.

Other issues,  starting up a cap input supply is more difficult, requiring a step start (something that should be used in any case).  If the builder designs the final by biasing the 813's off and leaving the plate supply on,  the supply voltage will soar to near the peak voltage, not good.  You do end up with better regulation overall with a choke input filter (if the choke you use has a low DC resistance).  Chokes with higher DC resistance cause regulation problems with changing loads (as with class B modulators).

More things I considered,  the xfmr the OP is using.  The current capacity of that Thermador will probably be just enough for a pair by a pair.  The OP stated that he was thinking about keeping the RF output down to about 250 watts to start.  Also he stated his lack of experience.  He is less likely to make dangerous mistakes with a choke input filter.

Tom, you have much more experience with screwups, explosions, crapouts, flameouts, arcs and sparks, etc. ;D

Tom,  the xfmrs that you're using (pole pigs) probably have a VA rating way above anything you're drawing from them.

I think, a supply with no chokes will always have some ripple left.  Question is, can you hear it in the audio?  Probably not if, like you say, you use enough capacitance.  High voltage caps with high capacitance are hard to find.  Easier to use a choke and less capacitance.  Especially, if you do not need the higher plate voltage a cap input filter will provide.

Tom, you have had great success with your 4X1 rig, so I will not be questioning anything that you're doing.  I'm still at 51 watts. ;D

Fred, KA2DZT

Rich,

One thing I should have clarified,  you will not get enough voltage running 115vac across the whole 230vac primary.  You can run that xfmr on 115vac with full output by connecting 115vac across the two primary windings connected in parallel.  Look at the drawing on the xfmr,  should be, connect 1 to 3 and 2 to 4,  I can't see the numbers that well to be sure.

Does the xfmr state a current rating on it or just the VA rating??  Using the xfmr on the lower voltage taps will give you more current capacity since the voltage is less.  Less current on the higher voltage taps.  That's the whole idea behind a VOLT AMP rating on the xfmr.  There are a number of other factors involved in understanding a VA rating.  But you can do what everyone else does.  If the xfmr doesn't get too hot, you're probably OK.  If it gets too hot, you're probably over loading it a bit.  The amount of time the xfmr is under load (transmit and receive) helps the overall picture.  Some modern xfmrs are designed to run hot (they have a temperature rise rating on them).  Your early Thermador was not designed that way.  Just give it the finger-on-the-core test after its been running under load.

One last thing,  if you see smoke, you made a mistake ;D

Hope some of this helps,

Fred, KA2DZT

build it big and run it down.  It will run cooler and likely be more reliable.  I have a couple of old chokes here if you need them. I believe they are 500 ma.  They where used in a supply to put 3Kv on two 4-400s in class C.

C

OK Fred -

Good point about lessening the requirement for a soft-start circuit when using a choke. All in all, it appears a choke is a good addition to the supply for the three reasons we discussed.

My point is that it's not the end of the whirl if we don't have a choke. A cap input supply will still function OK, but just needs a little more precautions. I personally use chokes for both my big and small HV supplies here for most operations and recommend their use. Though I use vacuum relays across the chokes to give me that extra voltage boost when needed. I don't use Variacs for HV supplies, so need all the tricks I can get.

Later-

T

Lets talk about why Variacs are a bad idea on an HV supply with decent current. That sounds infomative.

C

Variacs are on my mind as well.  I like the idea of being able to bring up B+ from zero.  But now I am looking at running this thing on 220V..  I have found these options:

run one 110V variac on one side of the 220v (as described in another thread by K1JJ), that gives me an adjustment range of 110V to 220V

Run an external 220V variac on the whole thing during development only

Run no variac  (smoke test)

Opinions?

Thanks again for all the help.

Rich

If you variac the HV I think ur ok, but if you variac the whole PS, then the fils don't come up and that is a problem... the variac needs ample capability.

Fred, you don't think anyone is buying your story, do you??

                         _-_-bear

Ugh, yes, dumb idea. :-)

Rich

Watch out for supplying the 813 screens when the plate volts are zero.
Why not use a variac just for the plates and also link it to the screen grid supply?
-Alternately use a high voltage diode to supply the plates slightly via the line-connected G2 supply when the plate variac is at zero. The G2 supply may need to be beefier but it ought to be heavily bled anyway.

An oversized variac is best, like a 20A one, so the regulation is good.

The cap input C-L-C filter is fine only if the plate transformer has a quite low impedance or is oversized, else the on audio peaks it will suck down the HV and compress your modulated signal. Pole pigs rule! The input capacitor will cause much higher peak currents in the transformer and rectifier and this is where the regulation will become poor.

Since you are building it, you can JS it and see which way it works best before finalizing all of it.

Rich,

You can use a 115v or 120v variac to adjust from 230 down to 115, but, you will need to run a 230 line with a neutral (three wire with ground).  The 115v variac gets connected to one hot leg and the neutral,  the wiper going to the xfmr.  The other hot leg goes to the other side of the 230v primary.  A 15amp variac will work OK.  20amp variac is better, 10amp will probably be a little light.

Two drawbacks in using variacs on the plate supply.  One, they affect the overall regulation of the supply.  Two, they draw an amount of idling current which helps to run up the electric bill.

Unless you are going to use the HV supply for more than one xmtr (that may require different levels of plate voltage),  you could probably not use one.

One reason to use a variac is, in the summer when line voltages are down due to heavy drain on the grid, you can crank up the line voltage.

The variac will serve no purpose in keying up the xmtr (PTT),  for that you will need to design a step-start relay circuit (not difficult).  As in most AM xmtrs, you will be turning on the HV supply on transmit and turning it off on receive.

Fred,  KA2DZT

Bear,

What makes you think I'm that famous East Coast AM'er ;D

How does the reduced input to the plate trans effect the transformer itself? 

C

The most common way to supply screen voltage in an AM xmtr is through a dropping resistor from the plate supply.  This way the screen voltage drops to zero when the plate supply is turned off (on standby).  The screens will be modulated from the modulated plate supply.  Using a separate screen supply is another good way to supply screen voltage but it gets somewhat more complicated.  Correctly keying up the screen supply with the plate supply and needing a screen choke to self modulate the screens.  You will also need some sort of safety feature to prevent the screen supply from turning on if the plate supply fails to turn on.

Opcom does bring up an important point,  never run any screen grid tube with screen voltage and no plate voltage.  Unless you have a large supply of tubes, you can wipe out the screens within seconds.

Fred,  KA2DZT

    Hey Freddy, you gots some FM on your carrier...

     :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D

No ill effects that I can think of.  Transformers are passive devices,  the  ac voltage that appears on the secondary depends on the ac voltage applied to the primary.  Can not say the same for electric motors,  low voltage can damage a motor.

Fred

Bear,
        He has been telling us on the air about his "Imaginary 813 Rig"  that he is building for years now but we are yet to hear it. I think he's building it in his imagination.  ;D  ;D

51w = "Bang, lookie here, squashed just like a bug!"  ;D  ;D  ;D  ;D

What happened??? Last year Fred upgraded to 52 watts!!

One thing that has been overlooked here....if using just a big cap for filtering you better waste about 10% of the transformers capability in a bleeder resistor....maybe more. Keep an eye on the HV voltmeter and make sure it doesnt soar in Standby.

Fred pretty much covered the reasons in his post.

From actual experience, I've found a "small"  HV supply is fine when using a "big" Variac. My limit to Variacs is how much weight I can easily carry around.  Running a conventional pair of 3-500Z's linear on a large Variac gives good regulation.


However, when we start drawing BIG power, the size of the Variac gets out of hand and even requires stacking them. In ssb linear operation, there are large current variations, thus poor regulation. It is less so with plate modulation, fortunately.

I once had a pair of stacked Superior Electric Variacs that weighed in at about 250 pounds. Even those showed a fair amount of sag when pushed.  I love variacs for initial testing. They can also work well when turned all the way up so that there are no windings in the circuit, just a direct set of contacts. But when set in the middle, I've measured unacceptable sag on some. Even a few volts sag at the transformer primary multiplies to big sag at the transformer secondary. For example, when using a 240V transformer with a 4KV secondary, this means a 16.6:1  step up. Every 20 volts sag at the Variac = 332 V sag at the transformer secondary. It may not seem like much, but it is preventable by carefully calculating your requirements and then choosing power supply parts and configurations to meet them.

Again, if you want to use a Variac, once done testing and playing around, set it so the contacts put the windings out of the circuit giving a direct connection from the AC mains to the HV transformer primary. This virtually puts it out of the circuit and avoids the Variac IR and core regulation problems.  This means designing the power supply so that you can use full voltage on the rig.

T

IMO  it is never a good idea to use just a cap for filtering.  You're right about the bleeder,  you would always need a bleeder in any HV supply.  Without a choke you need caps with much higher voltage ratings plus much higher capacitance.  These caps are hard to find and expensive.  It's probably cheaper to use an input choke and a cap with lower ratings.

You end up with better filtering, better voltage regulation, and it puts less of a load on the plate xfmr.

The OP is planning on using a choke, which is the right way to go.  He doesn't need the higher voltages you get with a cap input filter for the xmtr he's building.

Fred

An interesting approach to prevent supply voltage soar is to key the primary of the suppy along with the transmitter. I do this with my class E rig and the voltage stays at a maximum consistent 135VDC, whether the transmitter is keyed or not. This is a capacitor input supply. The bonus is there is no dangerous HV placed on the rig when unkeyed.



BTW, Fred, I would venture to say that the vast majority of ham commercial linear amplifiers have used capacitor input (no choke) for decades now. I do notice a quality company like Henry, in its day, used a filter choke that was even tuned to 60hz with the cap.  No doubt that a choke is an improvement, however.

.

Terry,  I did think of very large BC tubes with very high screen dissipation but, I left that out of my comments to avoid confusion.

Fred

Fred, that helps a lot!  This transformer has a jumper across the the two center posts on the input side so it was probably running on 220v.  Of course that makes sense, if you run half the windings (by putting them in parallel) you should get the same output as running them in series with twice the voltage.

This helps me a lot as running it on 220V would add a lot of work.  AND I can still scale it up later.  No way am I putting a finger on that thing!  That is what IR thermometers are for.  I am a world class chicken sh-- when it comes to HV.

Thanks

Rich

You saw that, aye  ;D ;D

Rich,

You must connect the two primary windings in phase.  Look at the drawing on the xfmr.  You'll see that the two terminals in the middle of the primary windings cross over each other,  you connect the terminal that crossed to the far end terminal of the other winding and the same for the other side.  You should end up with terminals 1 and 3 connected together and terminals 2 and 4 connected together.

If you connect the primaries out of phase the xfmr will not have any voltage on the secondary,  meaning the xfmr will not work.

Fred

More helpful advice, thanks again Fred.

Rich

Say WHAT? A cap filter is the only smart way to go if you are using a low impedance modern transformer, it doesnt matter what the mode is. You get the higher voltage plus good regulation with enough C. Its an especially efficient way to go with a modulator PS to get the most out per pound of xfmr.

With old iron with high resistance HV windings a choke is the safest route. Also if you need the lower voltage from iron already on hand. Use the lowest L to meet the critical L formula plus 20% followed by a lot of C and the prescribed bleeder current per the same set of formulas. Choke input always wastes a lot of power in the bleeder. In a C only PS you can use a string of 100K 3W resistors across the caps, it just takes longer to bleed down.

Start looking around and you will see that big caps are one of the cheapest items these days. Industry is scrapping new or almost new equipment and the surplus dealers are selling on Fleabay and elsewhere. I picked up 50 1200uF 500V 105C CDE caps with 2 year old codes for $2 a pop locally; they were never used, just attached to PC boards that were chopped off. Yep, you need a good step start for them and preferably 6A10 diodes. Many of those caps will be going into the new amp Im building. If thats too much C for you there are plenty of 470, 560, and 680 uF caps out there. Some real serious buliders using tubes with handles are using 2400uF and even bigger caps. Just calculate the energy in a 7500V PS using them, lots of serious safety is required. Regulation from no load to 2-4A is about 50V or less; that makes for some serious PEP compared to the usual 10-12% sag on peaks.

Eliminate regulated supplies for AB and B stages, use a fat cap in the bias and screen supplies, the seriously low impedance of these big caps meets the criteria perfectly.

I recently rebuilt a PS for a customers 1993 amp, pulled the 8 180uF caps and replaced with 470uf CDE 105C that had an identical footprint, low ESR, high ripple current capability; capacitor technology has come a long way real fast recently. That amp already had an adequate step start when built and PEP went from 2300 to 2700W, and the iron was only around 43#. Even Ameritron is supplying caps of twice the value and less than half the size in current production and customer replacements in older amps.

Just dont expect to stretch the average PEP out of a small transformer such as the SB-220 by very much unless its well cooled.

Carl

Carl,

Very good comments in your post.  I was referring to using a single oil cap, which can be expensive.  I don't like using strings of caps in my power supplies.  I recently bought thousands of small low voltage filter caps for nearly nothing.

I guess I'm from the old school and like using chokes in my power supplies.  I have probably over 1000 xfmrs and chokes (counting the small ones).  So, rather than leaving them sitting on shelves I build them into my power supplies.

I realize that in most SSB amps, chokes are not usually used.  Maybe using a modern plate xfmr, a cap only filter will not overload the xfmr.  My comments are primarily directed to the use of old buzzard xfmrs.

Fred

Fred, new oil caps are ver expensive but old ones are cheap at hamfests and elsewhere. I used to build all customers 4-1000A amps using them and still have a pair of parallel GE Pyranol 12uF 4000V in the 2M amp PS that I built over 25 years ago. They run right at the edge at 3900V just fine.

When using electrolytics be very careful what you buy. The majority of 450V caps are pure garbage for large PS use; they have high ESR and cant handle the ripple currents. Even good name brands have several choices, go to the manufacturers spec sheets and note the ESR, ripple rating, and expected life. For snap-in styles which fit computer grade PC board holes perfectly (read the caps spacing specs) I prefer the CDE 381LX series which are the most expensive when buying new from Mouser, etc. CDE also makes many industrial grade caps which arent in the distributor catalogs, these are what shows up on Fleabay and elsewhere a lot of the time.....good stuff.

I also use a military ZM-11 to analyze all electrolytic caps before using.

Carl

Not to hijack, but I'm watching this thread with interest, as I'm in the design stage of my 4X1 by 833s rig. I was wondering if I could get away with a cap filter also. I have a choke rated at 10H 600mA, but I'm wondering if that would be a little too light given the tubes being used? At any rate, I've been shopping around for caps, and Carl is on the money with his comments. Oil caps are out there, and they are even somewhat affordable, but a stack of electrolytics are looking pretty appealing.

It has been mentioned here that if you are using modern iron, a cap filter is feasible and even desirable. What constitutes modern iron? My plate transformer is a Dahl replacement made for a BC-1H. Is that considered modern, and would just a cap filter work good with it?

What do fairly modern broadcast transmitter designers use for filters?

I thinking of transmitters (AM. FM. or TV) with tube finals (even if the rest of the circuit is solid state)

Doesn't seem to me that a commercial designer would use a long strings of electrolytics.

There would be too many points of failure.

But are chokes still used?

Measure the secondary resistance, if it is around 100 Ohms or less a fat cap will be fine. Thats being conservative as Ive had no problems as high as 200 Ohms but I do use a fairly low value bleeder in addition to the string of 100K equalizers. Depending on the KVA rating Ive seen them as low as 10 Ohms; a 2160VAC @ 1.7A CCS Dahl xfmr I just measured is 13.2 Ohms and an older 3450VAC @ 800ma CCS is 96 Ohms and was used with a pair of 3-500Z's in an industrial amp with choke input. Id have no problem using that one on a 4-1000A with just a fat cap and a bit less demanding bleeder. A real floor weight here runs 7000VAC @ 3A CCS and comes in at 85 Ohms. Ive tested that with a 51uF 125# 10KV oil filled (and a Variac) and about 300W of bleeder current and the FWB diode blocks it came with. It ran a 14KW Pd Amperex water cooled industrial triode which I also have. It might make a nice 160-20M CW pileup buster ;D but Ive been thinking more towards a YC-156 and a 30A Variac which shouldnt bother the regulation too much with that much C and a lighter tube load. I can gang a pair if necessary, already on hand from Nearfest last year at short money.

A 600ma CCS choke is pretty healthy but not if its ICAS, how much do the 833's take at the voltage you will be using?  Voltage rating is also very important unless its mounted on something insulating. It would be pushing the luck with a full bore 4x1.

Based upon the age of tube BCB rigs Id say they were still in the choke input era even at the end. I do know that those also went up in smoke, especially the tuned ones used to keep cost down. Dahl made a lot of money in the choke and transformer business and his products werent always the best from several station engineer comments Ive heard.

Carl

10Hy@600ma choke could be enough.  The more important thing is the high voltage rating of the choke.  Do you know what it is??

I guess a Dahl xfmr could be considered modern.  But,  the more important thing to know is the current rating of the xfmr (or the VA rating).  Most old xfmrs were current rated with using a choke input filter (L-C).  Your PS with a L-C filter will give you whatever DC voltage the xfmr is designed for at the current rating of the xfmr.  When you use a cap only filter or C-L-C filter the DC voltage output will be much higher.  Because of this higher DC voltage output you must reduce the amount of current load on the supply to stay within the VA rating of the xfmr.  This reduction of current load usually should be about 20%-30% lower than the rated xfmr current.

Power supplies for SSB amps are not overloaded that easily since the current load is so intermittant (when on SSB).  Power supplies for AM xmtrs have a more steady state current load of the class C final plus the intermittant load from the modulators.

If the xfmr you have will give you the amount of high voltage you need with a choke input filter (L-C) then build it with the choke.  If you need higher voltage then leave out the choke, but you should reduce the current load.

You do get better voltage regulation and better ripple filtering with a choke input filter.

All these factors are things to consider when building a power supply.

Having said all of this,  if you have some monster size xfmr rated at 2amps DC, then disregard all previous instructions.  Build whatever you want,  nothing you build will ever come close to a 2 amp load, unless you're shooting for a 5KW output. ;D

Fred, KA2DZT

Fer what it's worth, most Peter Dahl plate transformers that i have seen are rated for full wave or bridge.

Terry,  you're right,  I think the ones I remember seeing on ebay didn't have a CT terminal.

Xfmrs actually run better with FWB rectifiers than with FW (IMO).

Fred

Yup, it's meant for FWB. No CT. I measured the secondary, and it's 24 Ohms, so I guess that's pretty low.
It's rated for 1A CCS according to Harbach.

As for the choke, I guess there is no telling. It's just labeled as a 600mA 10H modulation reactor (okay to use those as ps chokes, right?) So if I didn't want to push my luck, I'd need to find a beefier one, or go with big caps. :)

That is a factor that  would give you an idea of what wire gauge is used in it. Somewhere there's a wire table that will show the ohms per 1000 feet so you could figure out whats in it and the actual rating of the wire from your measurement.

IMHO a choke is a choke and how its used depends entirely on its voltage and current ratings with the exception of a swinging choke that would not be a consideration for a mod reactor. 

That is a healthy transformer and you can use whatever you want or need for a filter. Most ham and industrial style transformers built for the past 50-60 years are built for FWB or doubler service, no CT.

A LC PS DC will run about 85-90% of the AC and regulation is dependent on the L and the C to some extent. In a cap only PS the unloaded DC will be 1.414 times the AC and regulation primarily depends upon the C and of course the transformer kVa rating vs the actual load. The C only PS will be able to deliver slightly more power to the load since there is no choke winding resistance and the bleeder load is less. A 600ma choke drawing 600ma continuously will be the least efficient and run at or near its maximum rated temperature. It better be an open frame job with cooling.

There is no difference between the current drawn in a SSB amp versus any AB or B modulator, both are controlled by the voice and whatever level of processing is used. Average power is typically 25-50% of peak.

For the Class C final most of us can get away with ICAS ratings and maybe a small fan on the iron if pushing the specs. The blowhards need CCS but Id rather they melt down smaller iron and be off the air for a long period >:(  ;D

Carl

True.


heheheh... that's quite a message.


T

well said.  Its just not a good idea on a high power rig.  Unless everything is just right. You really start to get into trouble the more you rely on the variac to reduce the voltage.

C

About sagging voltage using a variac. Me thinks that if you used one with double the KVA rating of the plate transformer, the sag would be very low. The only problem is finding one with about a 5KVA rating or more and then the space to put it.

I guess you could just mount it on the wall or get one of those fancy motor driven types.  I've seen some big three phase jobbies that perhaps could be put in parallel.

I think a 3 phase 15 KVA variac would weigh about 500 pounds. That should do it ;D ;D ;D
 

Complete with one of those Buzzardly 12" spoked wheels to turn it and a big old cast iron and brass AC voltmeter next to it!

I did a little test today, hooked a 10A 240V variac to a SB-220 I just finished converting to 6M for a customer.

Without the variac and at 2065W DC input key down the sag from 2995V is 413V or 8.7% regulation. With the variac at 240V it is unchanged as expected. At 220V the sag from 2745V and 1840W DC input is 441V or 8.4% regulation due to less xfmr and variac loading. I didnt go any lower as the filament xfmr is off half of the primary winding. Thats not too bad for a variac of just about equal (2400W vs 2150W) rating to the SB-220 when the 150W filament is factored in. I was rather suprised.

The SB-220 transformer is rather wimpy while the variac is CCS.

Thats not to bad really.

C

Yep, makes sense when run within its ratings.  Variacs are FB on smaller rigs where the weight is reasonable. I use one (about 40 pounds) on my own pair of 3-500Z's linear amp and find it a blessing when using the amp at 1500V, loaded heavily.    

Scaled up, they should work just as well, why not?

However, if the amp gets really big and current requirements soar, you soon need a fork lift... ;D   If one can design the big amp / supply without it, it saves a lot of space and weight.

T

I read in one of my handbooks that there is a line you should nto cross when using a variac.  An example would be when you use the variac to cut the voltage down in half or less for the designed transformer. 

C

Carl,

I'm having some problems trying to follow your math.  I'm not seeing the same % regulation of the overall plate supply voltage.

Unless I'm wrong,  I always thought % regulation was;   no load V - full load V (sag) divided by the no load V times 100.

From your numbers;  I see in case one,  413V/2995V x 100 = 13.79%

                             and in case two,    441V/2745V x 100 = 16.07%

My results show a poorer regulation with the variac set at 220vac,  which is what would be expected.  Your results are showing a better regulation.  In addition, I'm not seeing how you arrived at those overall lower % numbers.

Maybe you define % regulation in some different manner.

I'm going to check a Handbook to see how the ARRL defines % regulation.

Fred

The 12.5KVA powerstats around here are about 120 lbs each. dual section 28A/240V each.  Real power supplies have handwheels. 8-)