Adjustable Bench Power Supply - Redux

Discussion in 'Tube Audio' started by kward, Dec 22, 2016.

  1. kward

    kward AK Subscriber Subscriber

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    All during the year of 2015 I worked on a design for an an all-transistor bench power supply that I could use to diagnose and build tube electronics. The idea was a fully contained unit that offered an HV supply following the Maida topology, but adjustable from 10V to 480V at 300 mA, a negative bias supply adjustable from 0V to -100V at 30 mA, a filament supply for both 6.3AC and 12.6VAC duty, and bullet proof--ability to survive short circuits on any of the supplies as well as unconditionally stable. Those are tall orders for a weekend engineer.

    In my original work, the high voltage supply caused me some problems. No matter how I tried, I consistently blew up the power transistors, whether they were mosfets or BJTs. I was at a dead end, and being burned out on the project, I put it aside for a year.

    What I wanted to try was swapping out the power transistors for big power pentodes, but would a Maida-style regulator still work? Since I built the whole thing on modular boards, all I would need to do is cook up a new board for the power tubes, figure out how to power the filaments and screens, and figure out how to interface the regulator board to the power tube board.

    So for the past several months I've been building a pass tube array board and a screen supply board. The whole Maida regulator approach is a floating design, so to make the pentodes work in place of the transistors I would also need to float the filament and screen voltages.

    In the original Maida circuit, Interfacing the LM317 regulator to the power transistors requires the use of a Zener diode to force enough of a voltage drop across the 317 so that it can regulate. Maida originally used a 6.2V Zener I believe, so that the 317 always had 6.2V of headroom with which to regulate. But since power tubes run the grids at a good number of volts less than the cathode, the needed voltage drop is already implicitly provided, so I did away with the Zener all together. My prototype never runs the grids at more than 20V below the cathodes, therefore there is never more than 20V across the 317, so I felt this was safe. (Note a higher screen voltage may exceed the 37V limit of the 317.) Under short circuit conditions, the 317 flops out of regulation, but who cares? It's a short circuit, not useful for real world situations anyway.

    I've now got the prototype mocked up on my bench and for the past several days I've been beating on it. Where power transistors fail, vacuum tubes shine! One torture test I tried was a metal rasp to implement a short circuit by connecting ground to one end of the rasp, and dragging the HV lead across the teeth of the rasp. In my previous attempts with the power transistors, this test would immediately blow out the transistors, usually with a mighty bang, and take out the LM317 in the process. But not with the pentodes, a few sparks were flying across the teeth of the rasp, but the 6550s were cool cucumbers, dutifully carrying out their responsibilities.

    Here is a picture of the test bed with the HV boards shown. Boards top left to top right are screen supply board, power tube board, and regulator board. The shot shows the power supply delivering 270 VDC across a 5K load (54 mA).

    IMG_2664.JPG
    I'm using three 6550 tubes with a semi-regulated 116V screen supply. Screen regulation is not strictly necessary I think, but it was easy to provide and helps the supply deliver more constant current under varying loads.

    Current limiting is achieved through the use of the screens of the pentodes themselves. Not only do they allow the tube to swing to near zero volts voltage drop so the regulator can efficiently deliver high output voltage, but they implicitly limit current too. It's not perfect current limiting (current limit changes slightly based on output voltage selected) as the pentode curves are not quite flat, and the 6550 curves have a kink down near 100V plate, but hey, close enough, as long as the thing doesn't go up in smoke under a short circuit test!

    I chose the screen voltage quite carefully. Several parameters needed to be met simultaneously: a) never exceed max screen dissipation under any circumstances, b) never exceed max plate dissipation under any circumstances, c) never exceed 37V drop across the 317, d) maximize efficiency by having each tube deliver as much current as possible while conforming to the first three conditions. At the screen voltage I chose, each tube contributes a max of 100 mA to the total output current, so 3x tubes gives 300 mA max current.

    At the raw HV voltage I chose (510 VDC), under short circuit conditions each tube dissipates 42 watts, which is design-center max dissipation for a 6550. I built the board for 4x tubes, but the power transformer I'm using only supplies 250 mA on its HV winding. I think I can drive it a little over that without problems, so I am limiting my unit to 300 mA via three 6550 tubes. The regulator will work with anywhere from one to four 6550 tubes installed. If you want 100 mA max current, remove all but one 6550 tube, etc. I could have built a variable screen supply to be able to limit current from the front panel, but this is more work than I am willing to do right now.

    Up next is more testing. There might be certain conditions under which the supply will oscillate, so I need to investigate that and determine how to address those cases, if possible. Under normal resistive only loads, the supply is superbly stable and quiet--noise is down under 1 mV. I think you sort of expect a very quiet output given the use of the LM317, since it rejects something like 60 dB.
     
    Last edited: Jan 26, 2017

     

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  2. chuckworkb

    chuckworkb AK Subscriber Subscriber

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    :lurk: and Learn. Thanks.
     
  3. moewillis

    moewillis just to recap, i love recapping

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    When I grow up I want to be able to accomplish scratch built projects like this. Nice work! Can't wait to see it all done!
     
  4. knockbill

    knockbill Addicted Member

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    Been wondering what happened to this project!! figured you were regrouping and redesigning,,, Glad to see you got it licked... Its gonna be a welcome addition to bench...
     
  5. Brice

    Brice AK Subscriber Subscriber

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    This is a great achievement. Congrats. What's next with your tester?
     
  6. thorpej

    thorpej AK Subscriber Subscriber

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    FANTASTIC. "Regulated bench supply" is on my to-do list for 2017, so your timing is perfect :thumbsup:. I also planned to use power pentodes for the main HV supply (and a couple of TIP50-based supplies for less strenuous applications, such as pre-amp tubes / screens). With an AnTek AS-4T400 power transformer, there would certainly be no shortage of current capability.

    Can't wait to see how this turns out!
     

     

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  7. kward

    kward AK Subscriber Subscriber

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    Thank you all for the interest.

    This morning I ran the high voltage supply through its paces with varying types of capacitive and resitive loads. I tried cap only loads from .01 uF to 100 uF, various combination of resistive and capacitive loads in both parallel and series arrangements, while varying the output voltage from min to max, and I see no problems. Yesterday I tried the high voltage short circuit torture tests (those tests can make you sweat bullets), as well as previously I tested the negative bias board for stability and short circuit conditions, so all is looking good.

    The chassis I selected for this build is a 4U rack mount type, so one "hard" thing I still need to do is cut a 3" hole in one of the internal chassis walls, to which I will mount the fan that will force air over the tubes to keep them cool. Gotta do this by hand--no fun. Other than that, it's now just "crank turning" so to speak to assemble the unit. Then after that, I want to do a full battery of specification tests (load regulation, line regulation, etc.). I'll post some pics with the schematic and full specifications over the next week or so.
     
  8. thorpej

    thorpej AK Subscriber Subscriber

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    If you have an air compressor, an air nibbler will make short work out of that. Even a manually-operated one shouldn't be too bad.
     
  9. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Very interesting.



    Thanks for the update and the pictures.
     
  10. kward

    kward AK Subscriber Subscriber

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    No air compressor. Maybe Santa will surprise me this year (I doubt it).

    ===
    I got the hole cut on the inside chassis wall and the fan mounted. Here is a shot looking at what will be the front of the unit.

    IMG_2665.jpg

    And here is a shot of the front plate. I built the front plate thinking I was making an all transistor supply, but with the conversion over to tubes, the constant current adjust knob and the constant current LED are not going to function at this stage.

    IMG_2669.jpg
     
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  11. thorpej

    thorpej AK Subscriber Subscriber

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    Looks great!
     

     

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  12. BinaryMike

    BinaryMike Pelagic EE Subscriber

    I could have loaned you a 3" Greenlee hole punch, but I see that you're not waiting around for anything. Nice work!
     
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  13. kward

    kward AK Subscriber Subscriber

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    Looks like I have a bad transformer. It was working fine for a while and then I heard a fizz and the 6.3V winding suddenly is putting out only 5.2V under load of 4.5A current draw, where originally I tested it under full load as putting out 6.3V. Filament winding is rated for 6A.

    I thought these Edcor power transformers were hi pot rated at 2K volts. I am biasing up the filament winding to as much as 500V, but that should put less stress on the voltage difference between the filament winding and the other windings. Even tried different tubes just to be sure it wasn't some sort of funky short in the tubes. Good news is there is no continuity between any of the separate windings. It just feels like one or two of the turns on the filament winding shorted together

    Anyway, project on hold until I get a new transformer.
     
  14. thorpej

    thorpej AK Subscriber Subscriber

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    Oh, wow, that's a bummer. I would definitely contact Edcor about it.
     
  15. kward

    kward AK Subscriber Subscriber

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    Another symptom I discovered is suddenly when this happened, the power supply wouldn't regulate. It would adjust voltage up and down just fine but it wouldn't regulate--passing through the ripple right to the output, and dropping significant volts (maybe 20VDC) under a 5K load at a dialed in 350VDC output. That should be no sweat at all for this supply to handle. So I think there is some coupling also going on between the HV winding and the filament winding, even though it measures clean with an ohm meter out of circuit. I have a TVS diode across the LM317 for transient voltage suppression in case of events like this, so no harm done there.

    That being said, I have been putting extreme stress on this thing doing the short circuit torture tests. The metal rasp test is a punishing short circuit test since it simulates a switching load at maybe 10 to 15 cycles per second (depending on how fast you move the lead across the teeth). But still, that shouldn't have killed the transformer since current never exceeds 300 mA under any conditions. The HV winding is rated for 250 mA, but I can't imagine that 50 mA overage would have killed it. I did purposefully leave it in short circuit mode for about 10 seconds once, simulating how long it might take in worst case conditions to notice it, but I still have a hard time understanding how my testing could have caused this. The transformer never even got hot, staying at room temperature the entire time.

    It's no big deal cost wise, it's an XPWR-133-120 model, they are not really that expensive, but the issue is the lead time to get another one. Last time I had a bad transformer, I got put at the head of the queue and got it in like two wekks, so maybe they will do that again this time.

    If I had a bigger chassis, I would use a separate filament transformer, but it just won't fit, so I must use a multi-wind transformer.
     
    Last edited: Dec 24, 2016
  16. knockbill

    knockbill Addicted Member

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    That's a tough break... I think I would try to extend the back of the chassis and use separate TXs rather than wait for and risk another PT from Edcor... any possibility the PS circuits some how caused the break down?
    Personally I would be leery of my design,, but unlike me, you know what you're doing!!!
     

     

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  17. thorpej

    thorpej AK Subscriber Subscriber

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    Do you have room for 5" diameter? AnTek AS-4T400 has //more// than enough current capacity.
     
  18. kward

    kward AK Subscriber Subscriber

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    That was my idea originally also, unfortunately in the move to the power tubes instead of the mosfets, I also need a screen supply that is on a separate winding so that it can be floated. So ideally, for the three power tube version, I need one transformer with the following secondaries:
    • 400V at 300 mA DC
    • 6.3V at 4.5A AC
    • 60V at 75 mA DC (anywhere from 60V to 80V will work).
    I might try a custom wound transformer, or maybe a deeper case. My case is 16" x 12" x 6", and if using separate transformers I think minimum size would be 16" x 16" x 6".
     
  19. kward

    kward AK Subscriber Subscriber

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    Turns out I do not have a bad transformer. Finally pinpointed the filament voltage drop at the alligator clip leads I was using. These were store bought leads, with wires crimped to the clip but not soldered. Under 4.5A current draw, the faulty connection readily made itself visible.

    I've been doing some more testing around current limiting. When the supply enters current limit mode it oscillates at a frequency of 16 MHz and at an amplitude of about 50 mV (peak). It's not related to my florescent lighting (as has sometimes bit me in the past). No amount of extra capacitance hanging off of the output addresses it, nor does more aggressive low pass filtering (i.e., a significantly larger grid #1 resistor or screen resistor).

    So I'm sorta out of ideas.The oscillation is not a show stopper because I intended current limiting only as a safety feature in case of an accidental short circuit, but it would have been nice if constant current mode was as clean as constant voltage mode.
     
  20. thorpej

    thorpej AK Subscriber Subscriber

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    Glad it's not the transformer! I guess you're using a voltage multiplier on the screen supply? If you didn't have the bias tap available, you could use one of Triad's small split-bobbin 117V->120V transformers, like the F5-120.
     

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