Use choke or ?

[k7mdo]

I am about to start the chassis hole drilling for a small power supply that's intended use will be for a 100+ watt transmitter (as yet on drawing board). What I have to start with is a transformer and choke combination that I "harvested" from some radar equipment from the fifties.  The transformer has CT 1600 VAC @ 250 ma as well as CT 760 VAC (possible screen supply).  It came with a matching choke for each voltage as well as the oil filled 4 mfd 1000 VDC caps for input side and load side of the choke. the capacitors check fine and the chokes are fine but I am wondering if I should simply abandon the ckokes and capacitor combinations for a simpler capacitor filter system?  What with the availability of high capacity cpacitors nowadays should I retain the old set up or not? I am going to abandon the 5R4's for solid state and the weight and chassis work would sure be simpler without the chokes.

Thoughts?
Thanks, Tom

[K1JJ]

Hi Tom,

Check out these 1800 uf at 450 V caps on eBay.

Three in series will give you 600 uf at 1350V -   or whatever you desire.

http://www.ebay.com/itm/450V-1800uF-Electrolytic-Capacitors-High-Voltage-NEW-IN-BOX-60-Off-Dist-Price-/281047573998?pt=LH_DefaultDomain_0&hash=item416fbda5ee


The reason for chokes in the old days is cuz  capacitors were difficult to make at high values like we have today, thus the chokes.

I've used electrolytic capacitor banks without chokes for years and the regulation and lack of ripple is always top notch.  Less weight and space, too.

Please post some rig pics as you build it up!

T   

[KA2DZT]

I would always use the choke.  Big difference with a cap only filter and one with at least one choke.  If your xfmr was designed to be used with a choke input filter and you then use it with a cap input filter the current rating drops by as much as 30%.  If the xfmr was designed for a CLC filter and you use a choke input filter LC,  then you could increase the current rating by about 30%.

Choke input filters yield better voltage regulation.  Cap input filters yield poorer regulation.

In my supplies I always use two chokes LCLC filters.

What voltages are you trying to get with the xfmr you have??

Fred

[Opcom]

I'm for the use of chokes having experienced their effects pertaining to the reasons above.

For low duty cycles like SSB with no voice compression, or for quite constant currents like a class AB tube power amp, C input is OK, but if the transformer came with chokes and caps, it is probably an old kind that has better regulation if used as intended.

Forsaking the use of a choke means you might see the voltage fluctuate greatly when tuning up, and when modulating the plate. That is an annoyance.

[VE3LYX]

One of the best non choke ps s I ever built is found in the old SSB Arrl book for the single tube 811a Linear. I didn't build that linear but did build a twin untuned input 811a using the PS design for power. It is there as a voltage doubler but can and I have also used it as a non doubler.( I am sure anyone can figure that out. Just take off output straight instead of from the extra cap string.) It is extremely quiet, seems unstoppable and I am more then happy with it. It is a deceptively simple design. I have had my 811A plates into the yellow zone with the one on the linear and the transformer (old TV) didn't even get warm. It has become my favourite PS design. I used a string of 450v 40ufs. In fact today I am rebuilding the PS in my suppressor grid rig BEFORE I even fire it to this design because quite simply it works so very well.
don

[K1JJ]

There's many ways to skin a cat and it can become a Chevy vs: Ford debate.  Using whatever works with the vintage transformer iron you have is best, of course.  

Chokes are fine, but don't forget that a choke contributes its own I/R drop to the circuit.  Measure it with an ohmmeter to see.    But it can be done using only big caps and finding a bigger, more modern  transformer.  The weight and space saved on chokes can be put into the bigger transformer.


A class C rig with full carrier is a wonderful load on the supply and helps provide good stability / regulation for modulation. (The steady carrier sucks down that peak 1.414 voltage factor)  

I have 140 uf in my 4-1000A  (Fabio II) supply (no choke) and see VERY little voltage sag under full modulation.

The only chokes in the shack are used as screen modulation chokes or Heising chokes - all for modulation.  I have seven homebrew rigs with at least 15 power supplies, none using chokes.  No hum, transformer overheating or voltage regulation problems here...     It would show up in the THD / IMD test data if it were so.

T

[KL7OF]

Tom....I know you have used photo flash caps in the past..where do you find them???
  Do you use multiple electrolytics ganged up to make 140uf??

[k7mdo]

I see there are opinions on this issue.  The transformer and chokes were orginally in an CLC configuration with the C's all being 4 mfd and the choke being 10 henry.

I am leaning from some of the posts toward staying with the choke/transformer combination as it was built that way in the 1950's.

The supply is intended to be used for any of several projects as they are completed.  I may even make it a "roll around" supply so I can stash it under the radio bench and move it as needed... just a thought.

So, I still wonder if leaving the 4 mfd's in the CLC configuration could benefit from higher "C"'s being available in these mordern times?

Thanks, Tom

[K1JJ]

Tom....I know you have used photo flash caps in the past..where do you find them???
  Do you use multiple electrolytics ganged up to make 140uf??

Yo Steve,

How are thangs in Tum Tum?


Yes, I use a single 51 uf  5KV photo flash in the 2 KV supply. It uses a step start with a 20 ohm 100w resistor.  These caps are getting rare, but can be found at the flea markets as well as on eBay sometimes. I remember seeing them available from the Russians a few years back. Maybe they are still there.  I love photoflash caps and never had a crap-out with one.


For the 4-1000A rig, I use a pair of 10 KV  70 ufd large oil filled caps with a step start. They weigh about 60 pounds each.  I got one from Dino / KNX and the other from Tron, I think - about 25 years ago.  I prefer not to use electrolytics in supplies bigger than 2KV if possible.  My 1700V tube PDM uses five beer can electrolytics and is FB so far.

What have you got planned?  

T

[AB2EZ]

If you increase the value of the capacitor on the transformer-facing side of the CLC filter, then you will increase the duration of the current surge that occurs each time the plate transformer is energized. This will increase the stress on the transformer.

If you increase the value of the capacitor on the load-facing side of the CLC filter, then you will increase the stress that the choke experiences each time the plate transformer is energized.

If you change over to just a C filter (with a sufficiently large C to keep the residual ripple below a few percent of the DC value) you will significantly increase the stress on the transformer each time it is energized.


Stu

[K1JJ]

I'm curious - a question comes to mind....

Are these older so-called "vintage" transfomers  really more sensitive to stress because of a lacking design compared to today or are they simply over-rated because the engineers expected everyone to run a choke, since larger caps were not available back then?   If we reduced the stamped current rating by ~30%, it would run FB with a big capacitor input, yes?

IE, if my newer transformer is rated at 500 ma using modern capacitor input assumptions, can I then rate it at 700ma (or whatever) when using a choke input?   Is this a matter of apples to apples?   Are we using the same standards here or is there really a weakness with the older transformer design?

If so, how would one identify a vintage transformer if it did not have markings stating choke input ratings?

T

[w8khk]

Today's transformers are designed differently than those made for vacuum tube rectifiers and smaller capacity filter capacitors.  

Vacuum tube supplies usually employed full-wave center-tapped transformers.  Although they could produce more voltage with capacitor input or just capacitor filtering, choke input or CLC filters were used to attain sufficient ripple filtering and voltage regulation with the smaller-value capacitors available in those times..  As a result, it was necessary to provide a higher voltage secondary, at lower current, than what we use today.  To fit the necessary turns on the core, smaller wire was used.  Core materials then were not as efficient as they are today, for example, the hypersil cores used in the Dahl transformers.

Even with the smaller secondary wire size, the IR losses were acceptable because the transformer provided a current source during a larger portion of the charging waveform.  Thus less current for a longer period results in less transformer heating.

Today, with the availability of physically small, but large uf capacitors, we commonly use capacitor input, or simply capacitive filtering, and this results in a higher current through the transformer secondary, for a shorter portion of the AC cycle, to "top off" the capacitor voltage.  Using a very large capacitor reduces ripple, but increases not only the turn-on surge, but also the charge current during each cycle. Vacuum tubes and chokes more or less eliminate the need for a step-start, unless an abnormally large capacitor needs to be charged.

Today's transformers are therefore wound with a larger size wire in the secondary, and often used with a bridge, or even a voltage doubler, significantly reducing the secondary turns count, thus providing space on the core for the larger gauge wire.

In summary, the transformers designed for choke input can produce their rated power output without significant heating when using their designed LC or CLC filters, but could be used with capacitor-only filters if steps are taken to reduce the turn-on surge, and restrict load to a current level that does not cause excessive heating due to IR losses in the secondary.

So, if I were to build that supply, I would make the filter decision based upon power requirements for the project at hand.  Then I would perform several tests....

1) Perform hipot tests, and resistance measurements to verify the initial transformer condition
2) Run the transformer in series with a resistor or light bulb, with no secondary load, to verify it is not defective.
3) Set up a test circuit, with diodes, choke, capacitor, and a load resistor simulating the planned use.  
3a) Bring up the test configuration with a variac or powerstat, and determine whether the transformer is compatible with the planned application.

If it is to be a general purpose supply, on wheels, it might be advisable to include a variac to provide adjustable output voltage.  With solid-state rectifiers, there are no rectifier filaments or heaters to consider when varying the input voltage.  The variac would also function as a "lazy man's" step start.  If no variac is used, consider connecting the rectifier filament winding in series with the primary to buck the line voltage, thus making the transformer compatible with today's higher line voltage.
It might also be wise to put in a jumper or switch, so that capacitor input or choke input could be selected for various applications, depending upon voltage and current requirements within the capability of the supply.

[K1JJ]

"In summary, the transformers designed for choke input can produce their rated power output without significant heating when using their designed LC or CLC filters, but could be used with capacitor-only filters if steps are taken to reduce the turn-on surge, and restrict load to a current level that does not cause excessive heating due to IR losses in the secondary."


Hi Rick -

An excellent post and answered my questions.   One for the archives.  Thanks!


So, in summary, an older transformer designed for choke and tube rectifier use can be used with a large capacitor input as long as the turn-on surge is handled (step start) and the current load is reduced to maintain rated core / wire temperature.

FB, OM.

T

[w8khk]

Thanks, Tom, glad my post was helpful.  I know of no way to definitively determine whether an old transformer was designed for choke or capacitor input, other than looking at spec sheets and application notes from the manufacturer.  But measuring the DC resistance of the secondary, and knowing the voltage and current the transformer will produce, will give a good indication of how much heating you might expect from capacitor-only filtering.  

I was thinking about what you said regarding all your power supplies used just capacitors for filters.  It reminds me of my first strapping linear that I built while in the Air Force in 1968,  It employed a pair of Eimac 4-400A tubes, B&W 800 plate choke, and an Illumitronics PI-195-2 Plate inductor.   The band switch that tapped this came from a surplus BC-375 Tuning Unit.  Never had one crap-out with this amp, and it still has the original tubes.  It still produces legal limit power out.

Looking back, I probably was very lucky that the power transformer never failed.  It was a UTC S-48, intended for full-wave center tapped rectifier configuration, with tube rectifiers and either choke or cap input.  I used the transformer in a full-wave bridge configuration, using the entire secondary.  Input was rated at 115 volts.   I did not use an automated step-start, but I had a 120 volt, 10 amp GR Variac in the primary circuit.  I also had a tune/operate switch that put a 120 volt, 250 watt photoflood bulb in series with the transformer primary in the tune position.  I never turned it on at full voltage without it in tune position.  

The rectifiers were large devices like used in the HV radar power supplies in the F4 Phantom, they looked like large cartridge fuses.  Those seemed to last forever if not subjected to large start-up surges.  The capacitor bank consisted of four each GE Pyranol 14uf at 2KV, configured in series parallel with equalizing resistors, for an overall rating of 14uf at 4KV.  Not considering safety margins as a teen, I ran it up to 4KV on a regular basis.  The caps still live too!

 That UTC transformer works as well today as when it was new in the 50s.  Used with due caution, these transformers will work FB without the choke, but I still prefer choke input when the parts are available.

[KB2WIG]

" The rectifiers were large devices like used in the HV radar power supplies in the F4 Phantom."

And where did someone get them.... ..?


klc

[AB2EZ]

W8KHK wrote:

"Using a very large capacitor reduces ripple, but increases not only the turn-on surge, but also the charge current during each cycle. Vacuum tubes and chokes more or less eliminate the need for a step-start, unless an abnormally large capacitor needs to be charged."

This is a key issue. With a capacitor fed, full wave rectifier power supply (large capacitor, no choke) the transformer's peak output current on each half cycle (needed to recharge the capacitor) can be much larger than the average current drawn by the power supply's load.  

Since the heating in the transformer's winding is the average value (averaged over each half cycle) of:  i x i x R... where R is the effective total resistance of the transformer's output windings...  the transformer winding heating associated with a capacitor input supply will be larger than the heating associated with a choke input supply... for a given output power from the supply.

It would not be unusual for the average value of i x i x R (i.e. the average heating of the transformer's windings) to be 3x greater than the value of i x i x R for a capacitor input supply v. a choke input supply delivering the same average output current.

Stu

[K1JJ]

It would not be unusual for the average value of i x i x R (i.e. the average heating of the transformer's winding) to be 3x greater than the value of i x i x R for a capacitor input supply v. a choke input supply delivering the same average output current.
Stu

X3?  That's quite a lot.

I don't think there is a single modern ham linear amp on the market today using a power supply choke.  Cost, space and weight reasons, I'll bet.    But there's no doubt that a big choke is a nice thing to have if the final voltage is high enough.  Come to think of it, I do have a 1A  30H choke  sitting in my HV power supply (weighs about 80 pounds) but I never use it except for testing, that is, to lower the voltage.   It's in the transformer C.T. and gets switched in and out with a vacuum relay.

All of my transformers are way overrated, so maybe that's why I've not come across this cap-input extra heating as a problem to be addressed. But there is something to be said about better efficiency.

Good discussion!

T

[w8khk]

" The rectifiers were large devices like used in the HV radar power supplies in the F4 Phantom."

And where did someone get them.... ..?


klc

Good question!  I did maintenance on the comm/nav, radar, weapons release computer, and IFF systems on the F4-Phantom.  The radar used six of these rectifiers, in a three phase bridge, at 400 cycles per second (60s terminology).  They ran at a much higher voltage, in a pressurized container, to power the magnetron via a discharge thyratron.  Not only did we need matched sets, but we also had to weed out those that could not handle the PIV.  I used a hipot tester to select those that would "fly", those that could not handle the PIV were "grounded", relegated to the scrap.  I asked that the scrap components that did not meet spec be transferred to MARS for members to use.  Not only did MARS members get these rectifiers,that worked fine under 10KV PIV, but also lots of 4CX250B and 4CX600J tubes that were pulls from the comm maintenance, but had useful life remaining for ham or MARS use.  I hated to see good stuff thrown away, when it had some life left, even though it was no longer suited to "flight status".   It did not hurt that the squadron commander and base commander were both avid hams!

[w8khk]

It would not be unusual for the average value of i x i x R (i.e. the average heating of the transformer's winding) to be 3x greater than the value of i x i x R for a capacitor input supply v. a choke input supply delivering the same average output current.
Stu

 Come to think of it, I do have a 1A  30H choke  sitting in my HV power supply (weighs about 80 pounds) but I never use it except for testing, that is, to lower the voltage.   It's in the transformer C.T. and gets switched in and out with a vacuum relay.

I considered putting a choke between center tap and ground.  But this can  be risky if the center tap portion of the transformer does not have good enough insulation to withstand the voltage encountered when the choke is in circuit.  If in doubt, place the choke between the rectifiers and filter caps.  The choke may have insulation breakdown, but better to lose a choke than a nice plate transformer.  If floating, be careful that it is not exposed in an unsafe manner.   How bout using that choke as a modulation reactor?

[K1JJ]

Yo Rick,

I use TWO HV plate transformers with their primaries and secondaries in series.  The CT is comprised of the secondary end of each transformer  So there is no risk using the sec as a C.T.


Yep, that choke might make a good Heising reactor, but I already have one working.  It is broadcast-type iron. But how would I know if that 30H choke was used as Heising reactor or a power supply choke? I seem to remember there is a difference, though I've used any rated choke I've had available in the past for Heising reactors.  

T

[KA2DZT]

I am about to start the chassis hole drilling for a small power supply that's intended use will be for a 100+ watt transmitter (as yet on drawing board). What I have to start with is a transformer and choke combination that I "harvested" from some radar equipment from the fifties.  The transformer has CT 1600 VAC @ 250 ma as well as CT 760 VAC (possible screen supply).  It came with a matching choke for each voltage as well as the oil filled 4 mfd 1000 VDC caps for input side and load side of the choke. the capacitors check fine and the chokes are fine but I am wondering if I should simply abandon the ckokes and capacitor combinations for a simpler capacitor filter system?  What with the availability of high capacity cpacitors nowadays should I retain the old set up or not? I am going to abandon the 5R4's for solid state and the weight and chassis work would sure be simpler without the chokes.

Thoughts?
Thanks, Tom

Are there any insulation ratings on the 1600 volt xfmr??  Like RMS TEST volts or working volts ratings.  What other info is shown on the xfmr??

Fred

[k7mdo]

OK, here are photos of transformer and choke (and the main shack area)...  I had kind of figured I would use the low voltage taps for an adjustable screen supply but not yet picking a tube other than the 6146 (or a pair) for the transmitter, I am not yet sure it would be any use.  The low voltage seems a little low for that but maybe?

Tom

[KA2DZT]

The xfmr is never going to make a 100 xmtr.  It's a little light even for a pair of 6146s.  I've seen this xfmr in the past.

It has to be used with a FW rectifier.  The CT must be grounded.  I would use the choke and it must be in the positive lead, not on the CT.

Fred

[w7fox]

Tom,
I have one of these transformers, they must be quite common.  It is an ideal transformer for a 100 watt class transmitter.  With choke input, it will put out 720 volts at 322 ma all day.  The lower voltage taps, with choke input, will put out a bit over 300 volts for screen or lower stages.  I prefer to use the "tuned" input choke in my designs so I can use a higher resistance bleeder resistor.  Google resonant input choke power supply design and good info should come up.  Good luck

[KA2DZT]

Tom,
I have one of these transformers, they must be quite common.  It is an ideal transformer for a 100 watt class transmitter.  With choke input, it will put out 720 volts at 322 ma all day.  The lower voltage taps, with choke input, will put out a bit over 300 volts for screen or lower stages.  I prefer to use the "tuned" input choke in my designs so I can use a higher resistance bleeder resistor.  Google resonant input choke power supply design and good info should come up.  Good luck

OK great,  if W7fox has first hand testing of the xfmr at the higher current then I agree, it would be good for about 100 watt xmtr.  Ive seen this xfmr somewhere but I don't have one here.  I do have well over 1000 xfmrs and chokes counting the small ones.

Fred