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Thread: Powered up the build for the first time... I'm getting troublesome readings

  1. #1
    Senior Member SoulFetish's Avatar
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    Powered up the build for the first time... I'm getting troublesome readings

    So, I just got back and had a buddy come over while we powered up my build for the first time.
    First things first, I used a 25W incandescent limiter to take some readings of the secondary AC and DC voltages with the tubes pulled to take some no load readings on my secondaries. (roughly 1mA resistive max load).
    Using a true RMS meter, I first tested the mains voltage, which read about 118VAC. After a great start, it turns out that was the only measurement that gave me the voltage I expected. My secondary voltages are troublesome. The no load AC voltages measured pretty much exactly what I designed them to be under full drive conditions.
    The secondary which supplies the main amplifier is spec'd for 245-0-245 under a load of 150mA. I was getting 241VAC to center tap from each leg and I turned to my friend and said "bullshit the regulation is that good". I told him I expected to see around 252V-260V.
    Also, the other secondary supply was spec'd for 100-0-100V at 100mA for my +/- voltages. Under no load I was getting about +97VDC and -98VDC (which totally f-ed my bias calculations).
    I then switched to a 60W bulb and put in my put my preamp tubes and driver in, and set the grid voltage for the output tubes before installing them. But the DC was well under 300V, and my secondary fuses on both legs blew open. I suspect they may have fell victim to repeated in rush when switching power on and off. They are 200mA fast blow 2AG.
    What going on do you think? Just a dud power transformer? I'm thinking of calling the manufacturer, but I want to do this again and make sure I get the same figures to be sure. But the fuses blew because of something and I want to find out why before I make any decisions about where to go from here. You know, I really never anticipated having to replace a mains transformer for any other reason than destroying it somehow.



    Funny thing is, I used to bitch about designers who soldered leaded fuses in a circuit because it... well, because its a pain in the ass. That's why. But, I decided to call an audible towards the end of the build and solder these particular fuses in because I was worried about the littelfuse fuse holders being too brittle. So, I soldered those fuses to the board. Figures those were the ones to go!
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  2. #2
    Supporting Member eschertron's Avatar
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    I could only hazard a wild-@ss guess. But I do wonder what the filament voltages are, and what the secondary voltages are now after the fuses blew (upstream of the fuses, of course!). Consider this just a request for additional data points.
    Quote Originally Posted by Steve Conner View Post
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  3. #3
    Old Timer J M Fahey's Avatar
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    You get lower than expected voltages BECAUSE you are plugging into a lamp bulb limiter.

    And if you blow fuses is because you pass more current through them than what they are rated for.
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    Juan Manuel Fahey

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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by eschertron View Post
    I could only hazard a wild-@ss guess. But I do wonder what the filament voltages are, and what the secondary voltages are now after the fuses blew (upstream of the fuses, of course!). Consider this just a request for additional data points.
    Heater voltage measured 3.2-0-3.2VAC with the tubes pulled. I want to double check but, I think with the tubes installed, the voltage dropped under 3V to centertap. The AC voltage on the secondary, POST blown fuse, was 241-0-241VAC.
    I'm goint to mount some fuse holders, instead of using pigtails.
    Then, I'm going to pull all the secondary leads from the circuit, and retest the true unloaded voltages, and try and isolate whether the power transformer needs to be replaced. Which, as far as having to replace singular thing in the amp, is by far the worst case scenario. But, thems the breaks. I'll hold on to this transformer, and design a circuit that uses 250V on the plates, bias it in class A2, and drive those grids into orbit.
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    And if you blow fuses is because you pass more current through them than what they are rated for.

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    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  6. #6
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    You get lower than expected voltages BECAUSE you are plugging into a lamp bulb limiter.
    Huh. It's a series resistance, so I see the logic here.
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  7. #7
    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by SoulFetish View Post
    Besides that silly remark, doesn´t that make you think about "what am I doing wrong"? .... which is the point of my suggestion.
    Just sayin´

    I'm probably being a smart ass..
    Well, will that help you solve your problem?
    Juan Manuel Fahey

  8. #8
    g1
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    For reference, and for your own peace of mind, when you measure your wall voltage, also measure your AC coming out of the bulb limiter.
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    Certified Dotard

  9. #9
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Besides that silly remark, doesn´t that make you think about "what am I doing wrong"? .... which is the point of my suggestion.
    Just sayin´

    Well, will that help you solve your problem?
    Juan, you have always been helpful. You are one of my go to guys here because the experience and knowledge you freely share. That and you have a good sense of humor. I was only trying to bust your balls a little. I was hoping it came across in light hearted fun. But if it came across like I was being a dick, I'm sorry. I appreciate you, man.
    Last edited by SoulFetish; 05-10-2017 at 10:14 PM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  10. #10
    Supporting Member Chuck H's Avatar
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    Juan can be touchy sometimes, but he's worth it. Group hug!

    Ech. I just threw up a little in my mouth.
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  11. #11
    Old Timer J M Fahey's Avatar
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    Cool
    The idea was to move you out of your stalled position, and think other possibilities.
    Specifically the line saying:
    my secondary fuses on both legs blew open. I suspect they may have fell victim to repeated in rush when switching power on and off. They are 200mA fast blow 2AG
    That instantly makes me think of 4 possibilities, all of which can be checked and discarded, until just one stands.
    Which one?
    Dunno, crystal balls not much use here, but [testing rulez!!!!][tm] :
    to be confirmed or discarded:
    1) 200 mA is not enough, try next higher.
    2) fast blow is inadequate: try slow blow.
    3) both of the above
    4) 200mA is a mistake. Where did that value come from?
    * Schematic?
    * Screened on board?
    * Old (blown) fuse was 200mA? That doesn´t mean it was the proper value (doubly so if found blown )
    * You just misread it? I have *always* that problem, my bread and butter 100W SS amplifier uses a 1.5A fuse (220V mains) ; for the very good reason that 1A nuisance blows now and then and 2A is "way too much" . At least 40% customers replace them with 15A ones Some claim (and I believe them) : "Juan, I brought the old fuse to the shop and the attendant gave me the 15A one".
    * The circuit, for some reason (excessive bias?) might need more current, yet it´s not really "wrong".

    Please test the different possibilities and tell us what you found.
    Good luck
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    Juan Manuel Fahey

  12. #12
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Cool
    The idea was to move you out of your stalled position, and think other possibilities.
    Specifically the line saying:
    That instantly makes me think of 4 possibilities, all of which can be checked and discarded, until just one stands.
    Which one?
    Dunno, crystal balls not much use here, but [testing rulez!!!!][tm] :
    to be confirmed or discarded:
    1) 200 mA is not enough, try next higher.
    2) fast blow is inadequate: try slow blow.
    3) both of the above
    4) 200mA is a mistake. Where did that value come from?
    * Schematic?
    * Screened on board?
    * Old (blown) fuse was 200mA? That doesn´t mean it was the proper value (doubly so if found blown )
    * You just misread it? I have *always* that problem, my bread and butter 100W SS amplifier uses a 1.5A fuse (220V mains) ; for the very good reason that 1A nuisance blows now and then and 2A is "way too much" . At least 40% customers replace them with 15A ones Some claim (and I believe them) : "Juan, I brought the old fuse to the shop and the attendant gave me the 15A one".
    * The circuit, for some reason (excessive bias?) might need more current, yet it´s not really "wrong".

    Please test the different possibilities and tell us what you found.
    Good luck
    Juan, you letting me know that the limiter was the reason for the low voltages shed some light on several things which could have caused the fuses to blow. I didn't know to expect that, although it makes perfect sense now. I'm glad the fuses where there when I plugged it into the mains with the bias voltage where it was. (Lets just say it didn't have a "-" in front of it)
    I was leaning towards the bias being the reason the fuses blew. But, all the things you mentioned above are still certainly at play as well, among other things.
    The 200mA fast acting value was a conservative value I chose for the power supply. It's my own amp.

    I've been at work on this and here is where I'm at so far:
    Pulled the power transformer secondary leads from the board and ground from the chassis to test for shorts. So far all the resistances seem normal. No continuity where it shouldn't be.The bleeder resistor seems fine. Filter capacitor values come up reading exactly what they should be.
    When I was going to power it up again, I remember thinking that I should have had the tubes tested. Then I remembered... Shit! I HAVE a tube tester! I got a Triplett in great condition for $50 a few months ago and totally forgot about it. It tests heaters, cathode emission, and shorts, which is fine for this purpose.
    So, the tubes were all tested and ruled out as causes.
    After cleaning up some work and checking some other things, I decided to do the power on procedure again. With the amp plugged in to the limiter, and no tubes, no load, I was getting the voltage readings I was expecting to see with my secondaries right at spec. The B+ was a little low, but certainly acceptable. So far, so good.
    installed the preamp tubes, still, everything looks good. The limiter lights up during inrush, then fades away. So far, so good.
    So I pulled the output tubes and plugged it into the mains. Flipped the switch, and FLASH, the fuses blew again. Hmmm.

    Lets go back and double check my choice of fuse.
    I ran some simulations on my power supply and it looks like 200mA is too low. I ran simulations during inrush at 10ms, 500ms, and 1000ms; as well as simulated conditions at idle and full drive.
    I got an I^2t value of 0.0193 at inrush, with an RMS current of 497mA @ 100ms, 356ma @500ms, and 192mA under continuous full drive. Quiescent current is around 108mA.
    I will end up using a larger value going forward anyway, but would this alone cause the fuse to blow? What do you guys think?
    Last edited by SoulFetish; 05-16-2017 at 04:05 AM.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  13. #13
    Senior Member SoulFetish's Avatar
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    here are the results of some simulations.





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  14. #14
    g1
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    I think you could still have a bad power tube. A tube tester can't guarantee a tube is good, only bad.
    Many power tube faults only show up at higher voltages that the tester usually does not provide.
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    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by g1 View Post
    I think you could still have a bad power tube. A tube tester can't guarantee a tube is good, only bad.
    Many power tube faults only show up at higher voltages that the tester usually does not provide.
    I may. But there were no power tubes plugged in when the fuse blew. So thats not the reason for the fuse blowing.
    If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.

  16. #16
    g1
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    Ah, sorry, I misread and thought you said you installed them; where you said you pulled them then the fuse blew.
    Do you have a meter with 'peak hold' that you can put in series with the fuse?
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  17. #17
    Senior Member SoulFetish's Avatar
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    Quote Originally Posted by g1 View Post
    Ah, sorry, I misread and thought you said you installed them; where you said you pulled them then the fuse blew.
    Do you have a meter with 'peak hold' that you can put in series with the fuse?
    I do. I'll take some peak measurements tonight and report back.
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  18. #18
    Senior Member SoulFetish's Avatar
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    In my first pass at trying to take a peak current reading, I set everything up. I pulled the lead going from the fuse to the rectifier and set my Meter to the 10A position for a series connection.; put the meter on MAX to display the peak AC current. Went to flip the power switch ON. Stopped going to flip the power switch ON. Plugged my amp into a load and said to myself "shit, that was close." Flipped the power switch ON and *flash*, I could see the fuses blow again. Looked over at my meter to get a reading and there was no readings displayed. No indication of any current reading at all. So, I though, "I probably shouldn't have placed the series(meter) connection after the circuit break connection.
    Down to my last two F-200mA/2AG fuses, I switched it to the other side of the fuse and powered it up again. Still no reading.
    The fuses are blowing too fast for my meter's to be able to display any meaningful data.
    I really do like the idea of the 2AG (5X15mm) size fuses. More so, because the fuse holders are small and free up valuable real estate within an enclosure or chassis. But , I have to concede their limitations in selection and availability compared to the more common 5X20mm. I decide to make an adjustment, with that in mind, and switch from the 2AG to 5X20mm. I was going to have to find a way to make it work with whatever space was available in there now. Worst case scenario is I would use leaded fuses on this particular build.
    But I got lucky!
    By some frigging miracle, these 5X20mm fuse holders I''ve been holding on to for a few years fit PERECTLY into a set of terminals I put in at the end of the build for current limiting resistors!





    With the fuse holders soldered in securely (and looking kind of cool as well), I could install some appropriately rated fuses and try the startup procedure again. Using the data I posted above, I opted for 315mA fast glass type. I still think this is a conservative rating (albeit, terribly generic without any manufactures data). But, honestly, I chose all components rated to survive a dead short to anything but itself. So, I feel confident that this will protect the transformers, which their job.
    After installing the fuses, I powered it on first with the bulb limiter to insure no mistakes had occurred during the modification. Everything looked good, and it was ready to be plugged into the wall. Was this going this going to fix the problem? Or where these going to burn up as soon as the switch was thrown, indicating something else was going on?
    Find out next month...
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  19. #19
    Senior Member SoulFetish's Avatar
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    No, I'm just kidding. Everything worked out. All Voltages and biasing indicating the power supply is working exactly as it was designed.
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