The AM Forum

I have 4  3300 vac hv transformers.Can i put 2 of them in series to double the voltage and 2 in series and parrel to the first 2 to double the current.

They are the same make and part number

Putting two in parallel to double the current should work fine if they are indentical.

Putting two in series may or may not work.

It depends on the insulation ratings of the secondaries.

Some old plate transformers, the UTC S series for example, are intended to operate with the center tap of the HV winding grounded.

Putting two of those is series, and then using either a full wave or bridge circuit would result in high voltage on the CT. So you would likely have trouble.

Do you have working voltage specs for the seconary windings?

Dave

all i know is that its a 3300 volt ac center tap.I would use a full wave bride.if the insulation cannot handle the voltage i wont try it

Do you mean a 1650-0-1650 transformers?   If you use a full wave bridge (sorry 'bout that) with out touching the center tap you will get 3300 out but at reduced current.

Assuming 1650-0-1650
If you parallel the primarys and series the secondarys you can get 6600 volts out, with multiple taps (the existing CT's and the one you make by putting the two together).  If you series parallel for more current things will get tricky.  You'll have to keep it all in phase of course.  

Isolate the frames by mounting on some think plastic away from everything else.  

It'll be big and messy, but should work.  I'd derate the power out a bit just to be safe.  ie: if you think you can get .500 A out, only draw .300 A for a margin.  

Good luck and keep one hand in your Pocket OM!!!

 

3300 Volts is pretty beefy anyway, what do you need the 6600 for???

yeah thats what i have 3300 volts total 1650 per leg.Each transformer is rated for 1 amp ccs 3300 vac.With the 4 transformers i would hope to get 2 amps @ 6600 volts ac.I think i may just go with a full wave doubler and 2 c filter capacitors just to play it safe.Its for a project i have over here i have them so might as well use them.Will be legal limit of course.

Be aware that some center tapped transformers are not designed to be use with a full wave bridge-the insulation is designed only for use with the center tap grounded and will fail with the center tap floating.  This is the same reason why you can get into trouble mounting the filter choke in series between the center tap and ground since this places the center tap at the ripple voltage above ground and may lead to insulation failure.

Unless you have the specs from the maker that state it is acceptable to use the transformer in a full wave bridge then proceed at your own risk.

There is no free lunch with using a full wave bridge on a center tapped secondary; even if you are able to safely rectify the entire secondary voltage to double the B+ DC output it will have to be at half the current rating.  So in either case the wattage will be the same and the difference will be at what voltage and current you are able to draw the wattage.

With center tapped transformers I doubt that there is any way you will be able to run the secondaries in series in a full wave bridge arrangement without having secondary breakdown problems.

Why not rectify to DC, THEN apply series / parallel?

Seems this might be a workaround for the phasing and CT's, etc.

Each xformer would operate as it's designed to, then rectify, and instead of grounding the neg side, could you put it in series (or parallel to increase current rating)?

Put Cfilter after the entire mess, so it's at full DC potential.

Someone please point to the flaw in this logic, I'm SURE I'm missing something stupid....

I'm the LAST one to talk about legal limit, but even I can see the flawed logic in 13 grand input for 1500 output :)


--Shane

If that transformer has a full 3300V secondary and with a CT then it is rated at 1650V at 1A aka 3300VCT @ 1A

So which do you have CT or no CT??

Carl
KM1H

I think he should move to Yahoo Shane, then you and Jim can educate him ;D

Carl

im sorry i can only tell you what the manufacturer told me which is what i told you before.The transformer weighs a little over 50 pounds and have compared it to a 4 kva transfomer with the same secondary voltage.The 4kva transformer weighed 10 pounds more same style core.Yeah the total kva rating is kinda high but i like over kill.I have seen much larger power supplies out there.

it does have a center tap

I also have some henry 2000d rf generator transformers that are single phase.They are 4kva they have a center tap they use the center tap for the tube driver plate voltage and for the final tube plate voltage they use the 2 outer taps on the secondary side of the plate transformer.

They are able to get away with using the outer taps on that transformer.

I have learned this the hard way!! Ever since the first transformer blew out in my 4X1 rig,
(It lasted 9 years with FWB and negative lead filtering), I have had transformer after transformer fail trying to run transformers that were NOT rated for FWB in that configuration.
Also putting the choke in the negative lead took one out as well. (I was not sure if the choke's insulation would take it or not, so I put it in the negative lead, and the ripple voltage took out the transformer.)

Quite a few people had told me NOT to do it, but I was bull headed and learned the hard way. you can usually get away with running FWB rectification on low or lower voltage transformers that are not rated for it, BUT..........................

DONT TRY IT WITH PLATE IRON UNLESS YOU KNOW IT IS DESIGNED FOR IT! !  With the higher voltages incurred with big plate iron the chance of having an insulation brakedown is greatly increased. If you dont know whether or not it will take it, dont even trust a HI-Pot test. I have tested them and they showed up as OK, and broke down shortly afterwards.

Transformers are usually somewhat "design specific". If it has no center tap, it is most likely designed for FWB service. If it has a center tap it is most likely designed for the center tap to be grounded, and the internal insulation is configured the same way. If it has a CT run it with the CT grounded, or you'll most likely be hating life shortly afterwards.

Good plate iron is a terrible thing to waste! ! !

                                                                The Slab Bacon

His capacitor bank isn't large enough for the barrier to entry :)

You know us, John Lyles is the only guy in existance that has more joules stored :)


On a more serious note, would rectifying and THEN series / parallel work in this config?

--Shane

Don't series even after the rectifier.

so why is it we never hear this 13KV rig on th eeast coast?

I talk am.

I think therefore I AM.

Frank Slab,

this vy timely for me; i have old thordarson iron with CT, 200 lbs 1A 3250 - 0 - 3250 & 872A s.s. replacements

rob

That will give you about .85 to .9 RMS for DC out with a choke input with better dynamic regulation than the common commercial amp FWB or doubler iron and 25-30 uF cap.

Carl
KM1H

The "free lunch" you get with a full wave bridge is the fact that the required PIV rating of the rectifiers is the same with a given total secondary voltage whether the transformer is used in a bridge or full wave midtapped circuit.

For example, the maximum rating for the 866A or 872A MV tube is 10 kv PIV.  With fullwave midtap the maximum DC output voltage is 3100 volts.  Using 4 of the same tubes with the same transformer (assuming it is insulated to withstand it) in a bridge circuit, the maximum output voltage is 6200 volts.  You get twice the current rating with the conventional full wave because each side of the transformer is operating at 50% duty cycle.  With the bridge circuit, the entire transformer is working at 100% duty cycle. The bridge makes more efficient use of the transformer than does the midtap circuit.

With tube type rectifiers, it is about a 50-50 trade-off, since the bridge requires 4 rectifier tubes and their associated filament transformers instead of just a pair.  But solid state rectifiers are cheap enough that the additional set of diode stacks for the bridge is not so significant in terms of the overall cost, and that tips the overall balance towards the bridge.

tnx Carl; I don't want to hijack this thread; I'll read about choke input & if I have any questions I'll get back to you.  this way I don't waste ur time with questions I can find answers to out of books.  

don, in this case I bet the Thordarson iron was not made to withstand FWB across the whole secondary.  it is old tranny made b4 s.s. diodes; a CHT looking one judging by the feet and end bells but painted gray so it looks like it might be from the 50s.  the part no. is T47000.  if I had a decent camera I'd take a picture of it.  Anyway after all the work (I have friends thank heavens) getting it into the basement I'm not gg to take any chances hi hi.

73

rob

update:  checked and closest thing I have to filter choke for this is 10 H 800 ma Energy Systems choke.

SCHWEEEEEEEEEEEET! ! ! ! ! ! ! ! !

That sounds like a really nice chunk of iron! ! ! ! ! !

I HAD a pair of those 25 years ago. I loaned them to a friend and told him just to use them at 3KV until he could find something better. He reconfigured them to FWB and turned them to scrap.

Late BC-610 oil filled and T-368 iron can withstand full wave bridge operation with a choke input even though they have a center tap. The center tap is insulated.

Thanks Frank, yeah it looks like it gg to be a good one.  There is also a filter cap that's huge.  12 inches tall, 18 inches deep and 3 or 4 wide.  looks like a small gray briefcase except for the two porcelain insulated contacts on top.  The porcelain is at least an inch tall.

Yeah Dave I would have done the same thing maybe had I not read the warnings here.  That stinks what happened to ur iron.  The stuff is heavy enough when it works but heaving that weight when it is dead is the pits.   I sure have learned a lot from you guys.

Rob

Rob,
      If that is an old, open framed transformer, and hasn't been used for a loooonnnnggg time, I would seriously consider baking it out and dunking it in a bucket of varnish before using it for any length of time. It is cheap insurance that it will continue to work without an insulation brakedown failure. At 6500vac old insulation will be a lot more likely to show its age.

If it is a modern open frame transformer I wouldn't worry too much about it, as the insulaing material will be something much better than "fishpaper". If you run a cap input filter (C-L-C) you will probably still get 3500vdc (or better) under a load. You just need to cobble up a step-start to make it easier on things when firing it off.

Dave,
        Whak kind of voltage was he looking for? ? ? ?  ???  :o  ::)

Rob, look in an old HB, either brand, and read up on "critical inductance" in the filter. With too low an L in C-L-C and too big a cap the transformer voltage and current can soar to destruction even with no load. You also need a decent bleeder whenever the HV is on.

You can only suck so much POWER out of the transformer and C-L-C or L-C it doesnt change. Since you cant use a fat input cap with vintage iron the peak voltage is the only difference and the L-C offers superior regulation and less distortion on SSB and AM modulators.

Carl
KM1H

6KV

A quad of T-368 xfmrs with secondaries parallel and primaries across either side of the mains later filled the bill.

Okay Frank, it looks like one of the CHT style ones--it has the ends closed; not open; I was gg to run it through a 20 to 30 A variac at least at first;

Tnx  Carl; I should have mentioned this earlier--I'm starting out building a single 4-1000A linear amp; later perhaps adding a second 4-1000 & using a Ranger as an exciter; or modulating the single 4-1000A but for now, I'm just gg to try getting a single 4x1 on the air in g.g. triode service and learn about the supply, TR switching, matching the cathode 110 ohm, the tank circuit and all the rest.  But, having said that, I have the old handbooks and am adding to the collection; just purchased a 1943 Terman handbook--i was doing the power calculation last night; am glad to have been through some of this with pair 3-500 re Ep, IMD, plate dissipation, Ip, supply power and current for reasonable parameters for clean AM linear operating (not to mention the need for augmented cooling) so I have been reading the 4x1 data sheet and getting a feel for how to handle it.  However the data sheet I have seen do not cover g.g. triode use not surprisingly, so I'm glad to mine the archives here and you guys who have been down that road  ;D  looks like a single 4x1 can be run at 3.5 KV for around 300 w. and easily dissipate the input loss but that was just a first number crunch.

Rob

Here's some photos....

That looks like the one I fried about 26 years ago trying to run FWB on a 3CX1000A7 2M amp. It lasted about 2 weeks of very intermittent use 1500W SSB. Im still using a pair of 12uF @ 5kV oil filters as in your photo on that amp.

Not sure if its the replacement xfmr or the caps but that PS has better regulation than anything else here. 3900 to 3700V no load (except for 150K of bleeder) to full load of 900 ma.

Carl
KM1H

I was wondering the specs on the capacitor.  A friend guessed 5 KV and ventured 50 uF.  nothing is on the cap to give any information and the schematic for the partially built 4x1 amp has nothing on it for the h.v. supply.  it is a schematic for the standard single 4-1000 g.g. linear amp.   So, I'm glad to know what kind of cap I have.  So, 12 uF.  I have to look at the bank of bleeder resistors.  don't know what is there off hand, but from what I know, 12 uF isn't enough; I need about three times that, or around 30 uF I believe.

Rob

A capacitor meter or a DVM with a capactance scale should give you the answer.

But, no way to tell the voltage that I know of.

The voltage might be higher than 5KV given the big insulators.

yeah, stupid me I should have thought of that.  duh.  i have a fluke 115; it will tell me.

Don't forget to leave a set of handcuffs across that bad boy when you work on it.

They DO recover chemically after discharges and can zap a person to next Tuesday.

Nope

 
I didnt mean to imply that yours is 12uF, I was just going by apparent (eyeball estimated) case size. It could be 4uf @ 10KV as suggested. I have a pair of 25uf @ 2500V in the same case with big beehive insulators like yours. They were utility company phase balancing caps. With 24 uF the ripple is down in the noise on my amp.  BTW, dont lay them down when using.

Carl

Well you darn close Carl; you know ur stuff--got all set to put meter on the cap when what looked like faint writing on the side caught my eye.  It was down on the side away from the transformer but with a flashlight and mirror I read 10 MFD 8 KVDC.  Mfr. is Tobe.
 ???

Lesson here is never ever make assumptions about someone else's homebrew hi hi.  I knew that going in and when in doubt I'm using my parts.  This won't be a problem.  Caps are easy compared to getting iron.  QSL re upright use.

That's Tobe Deutschmann. Look him up on the web, pretty interesting..

Tnx John, yes vy interesting.   Would have never found it myself.  I googled Tobe and did not come up with much.

Rob

He is the guy who gave Bill Halligan his start at a real job before he (Bill) left Boston for Chicago and immortality.

Tobe was into anything that he could make money from, a rather colorful character from what Ive read and heard from his family and friends.

Carl