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Author Topic: Control transformers for amp HV supply  (Read 5250 times)
WD4HXG
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Posts: 182




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« on: November 10, 2007, 03:45:37 PM »

I have been searching for HV transformers for a homebrew amp using a pair of 4-400a tubes. The prices of course are kind of high to say the least.

Looked at Microwave Oven Transformers and concluded they were not a good choice.

Control Transformers are readily available at the right price and are intended to step down typically 480 volts to 120 volts. VA Ratings are in the 1000 range thus a couple of these would deliver 960 volts at 2 KVA if the 120 volt windings were series connected and driven with 240 VAC.

Anyone know if this has been done? If so were the results satisfactory?
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KE3WD
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« Reply #1 on: November 10, 2007, 04:13:52 PM »

>>VA Ratings are in the 1000 range thus a couple of these would deliver 960 volts at 2 KVA if the 120 volt windings were series connected and driven with 240 VAC<<

In series connection like this, the VA rating would remain the same as for one transformer.  960 at around 1KVA then.  Series would double the voltage but not the power handling.  Parallel connection would double the VA, give or take a few.  Rethink.  

Control transformers are specifically designed to meet the high inrush industrial applications where electromagnetic components such as relays, solenoids etc. are used.  

I'm not versed in the impact that would have in an amplifier, though.  That info will have to come from someone who is or has experience with it, hopefully one will come along here.  

Be sure that whatever type you specify that it is indeed also an isolation transformer type and not an autoformer type, there may still be some of those lurking around somewhere, dunno.  

Rethink the VA rating thing and see if you can find a pair to re-specify.  


KE3WD
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WD4HXG
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« Reply #2 on: November 10, 2007, 05:29:09 PM »

Since the transformer is designed to handle inrush current problems and regulation issues it appears that this particular type offers a distinct advantage thus one of the primary reasons for focusing on this particular type.

Guess I am off in the weeds with the VA rating. If the secondaries are in series the voltage doubles, the current is identical in both secondaries. So it appeared the VA rating of two transformers would be double the single transformer.

Thanks for the feedback.

Chuck
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KA5N
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« Reply #3 on: November 10, 2007, 07:56:54 PM »

You might try pricing some of the Ameritron transformers.  They will sell you replacement parts and I understand cheaper than many other sources.  All their manuals are free downloads from their site so you should be able to find one to suit the bill.
Allen
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WD4HXG
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« Reply #4 on: November 11, 2007, 03:40:39 AM »

Thanks Allen

I will look immediately.

Chuck
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WB2WIK
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« Reply #5 on: November 11, 2007, 03:09:01 PM »

960v through a full wave bridge only yields about 1.3kVdc filtered; this is very low for 4-400s.  If you're thinking about using a voltage doubler, think again: Voltage doublers place a lot of dielectric stress on the transformers and unless the transformer is designed for that, it will likely arc internally and fail.

4-400s will work over a broad range of plate voltage, but they "sing" at about 4000V.  3000V isn't bad.  2500V and you can run some power, but not nearly the full rated power.  Lower than that, and it's a bad tube choice.

I'd use a real HV transformer for the intended application.

If you want, I can look through my stock of HV transformers and probably find one for you that is appropriate.  They ain't cheap and shipping adds to the cost, since they're heavy!  I think I have a 2000V c.t. transformer that would deliver about 2.8kVdc in a full-wave, with a rating about 1 kVA.

BTW for amateur service the kVA rating isn't so important unless you're running AM, FM or RTTY.  For SSB/CW service, very often you can use a 300mA-500mA transformer at 1A peaks and it will outlive the operator -- provided you avoid continuous-duty modes.

WB2WIK/6
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WD4HXG
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« Reply #6 on: November 12, 2007, 07:13:32 PM »

Evening Steve

-----960v through a full wave bridge only yields about 1.3kVdc filtered; this is very low for 4-400s. If you're thinking about using a voltage doubler, think again: Voltage doublers place a lot of dielectric stress on the transformers and unless the transformer is designed for that, it will likely arc internally and fail.-----

I did not realize that the output would be quite that low. That makes the two transformer run a definite no-go even with the doubler. Doublers don't give me a lot of worry but triplers are another story.

----4-400s will work over a broad range of plate voltage, but they "sing" at about 4000V. 3000V isn't bad. 2500V and you can run some power, but not nearly the full rated power. Lower than that, and it's a bad tube choice.------

Thanks for waking me up. I looked at the data sheet again and realized these are not 572's. :-)

-----I'd use a real HV transformer for the intended application.------

I have a pallet of ten of the control transformers. That was one reason for focusing on them. They are designed to handle inrush currents associated with the inductive loads of solenoids used in relays and other devices.

-----If you want, I can look through my stock of HV transformers and probably find one for you that is appropriate. They ain't cheap and shipping adds to the cost, since they're heavy! I think I have a 2000V c.t. transformer that would deliver about 2.8kVdc in a full-wave, with a rating about 1 kVA.------

I appreciate the offer and will keep that as an option if I may.

Thanks again

Chuck
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K0BG
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« Reply #7 on: November 15, 2007, 06:03:34 AM »

There is another issue no one mentioned. IMD wise, tetrodes don't make very good amplifiers, especially so at low plate voltages. As Steve alluded to, you have to get the plate voltage up around 4 kV with 4-400s.  

MFJ does sell a transformer large enough, but it isn't cheap!

Alan, KØBG
www.k0bg.com
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WD4HXG
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« Reply #8 on: November 15, 2007, 07:33:43 AM »

Ah the magic words "Not Cheap". Yeah I noticed the new transformers were in the $500.00 class right now. Well there goes more of my moldy old money.

After studying the data sheet it seemed prudent to settle on about 3.6 KV as a nice round plate voltage. That provides a plate power output rating of 765 watts for one tube. Hopefully the 3 dB difference between the 2KV  and 3.6 KV plate voltage spec will be worth the effort. At least the improved intermod performance will be a plus.

Eimac indicated you can run 4 KV in typical operation but 4 KV was also the absolute maximum spec. Maybe I am to chicken for the QRO gig but running anything right against the rails seems like a good way to invite Murphy to the party in a big way.

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WB2WIK
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« Reply #9 on: November 15, 2007, 09:01:28 AM »

4kV isn't really against the rails for a 4-400.

E.F. Johnson ran a pair of them at only 2.2 kV in the Thunderbolt and some other old stuff from the fifties but they didn't run a lot of power as a result.

I've used 4-400's at 4.2 kV no-load and 3.9 kV full load many times and they last decades.  I've seen operating life shortened by too high filament voltage, however!  So that should be measured, tube in socket and power applied, with an accurate AC voltmeter and adjusted to 5.0V as closely as possible.

WB2WIK/6
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WD4HXG
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« Reply #10 on: November 17, 2007, 02:06:08 PM »

I previously picked up a variac to control the primary filament transformer voltage. Also have a digital panel meter with an accuracy of plus or minus 0.5% so will have a visual indication of filament voltage. Bench tested the filament transformer and for 115 volts AC the output measures 5.1 volts. Unfortunately the AC line voltage in the house wanders up and down from about 110 to 130 volts depending on the time of day. The high end results in 5.75 VAC at the filament. Have purchased a Sola/HeviDuty voltage regulating unit (MCR)with 3% regulation. That has actually held the filament voltage at 5.05 VAC plus or minus a few tens of millivolts so will likely not use the variac now.

It is good to know that the 4-400a's are tolerant of operation beyond the rated plate voltage as that provides a warm fuzzy about the safety margin. I will still try to keep the operation within rated specs. Just a quirk of mine. My typical experience has been when operating beyond the specs Murphy shows up for a party. :-)
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KD8CN
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« Reply #11 on: November 22, 2007, 08:40:41 AM »

Just to throw an opinion in.....

  I believe you may have slipped a digit in you calculation.  Since the current capability of each transformer stays the same, and the voltage doubles, the VA doubles(1A x 2V= 2VA). Parallel yields twice the current but only the voltage of one (2A x 1V=2VA) The VA is again double of one.

   Now, since the series winding resistance also doubles, we lose some regulation, and the total VA drops by a few percent.  

I have used seriesed control transformers in a voltage doubler circuit for several years.  They work fine, no problem with flash over.  I high potted them well over 5000v.  Make sure they are dry and have no moisture in them. That is what causes most arc-overs in used transformers in high voltage service.

Also, a control transformer VA is rated CCS, so ICAS rating will be higher.

My pair of transformers yields a little over 1100v, which yields 3000v DC unloaded.  It falls to 2850 loaded. Still, this is a little low for 4-400's. They do work much better at 3500+.  But, the few dB's you gain is not worth the shortened tube life.  Also, with lower plate voltage, there is less stress on your tank  circuit components.

Just my opinion.  YMMV


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WD4HXG
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« Reply #12 on: November 27, 2007, 10:50:26 AM »

The control transformers if I understand correctly are not likely to pose a problem as far as regulation, They are designed to handle substantial inrush currents and assuming there is adequate current capability the sag should be minimal.

---Now, since the series winding resistance also doubles, we lose some regulation, and the total VA drops by a few percent.---

I have access to a lab oven which can be used to heat the transformers for a few days to drive out moisture.

I have a handful of identical control transformers and had originally considered using two in series. However on the bench two in series only delivered 930 volts loaded and even with the doubler that would be a little low. I could series three or four but the power supply suddenly turns into a hernia maker unless I modularize it ie: set up each transformer to plug in to a base socket.

I have a 2 KVA 480 to 120 control transformer in the corner. Have mulled over the prospect of rewinding it to provide 3500 volts but copper prices and my patience are in short demand. It will wind up a contest between my reluctance to cough up the cash for a transformer made for the requirement or the cost of copper. This is a bad hobby for anyone that resists parting with moldy greenbacks.

---I have used seriesed control transformers in a voltage doubler circuit for several years. They work fine, no problem with flash over. I high potted them well over 5000v. Make sure they are dry and have no moisture in them. That is what causes most arc-overs in used transformers in high voltage service.---
 
Thanks for the feedback.
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N5RMS
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« Reply #13 on: November 12, 2013, 07:19:57 PM »

As I recall,  Motorola in the 60's used two smaller transformers, each one to a separate diode bridge, then summed the dc, for HV in base stations.  My experience with control transformers, is that they are rated for continous service at rate (K)va. I've been thinking of using 2 1KVA control transformer myself! 
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QRP4U2
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« Reply #14 on: November 12, 2013, 09:46:12 PM »

HXG,

I have a 3500 volt secondary transformer@500 mA (CCS) I removed from a Henry amp.

With a good FWB and filtering, you should get about 4500 volts DC.

Send me an email at spf@Reagan.com  if interested.

Phil - AC0OB
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