Power Transformer Questions

Discussion in 'The Workbench' started by Chris-in-LA, May 3, 2021.

  1. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I recently built the Mojotone Bassman kit. It uses their Mojo752ex PT. Their wiring diagram shows 355v at each of the red wires that go to the rectifier socket. I think that my PT is possibly much too hot. The b+ is reading at almost 460vdc while the amp is running. But I blew 2 500v filter caps today, blew one, replaced it with a spare 450v cap and blew that one as well. I decided to take out the 5AR4 in favor of a 5U4-GB to drop the b+ but that tube started to sizzle and pop immediately even though it’s rated for 450vac. I took out the rectifier and measured the voltage at one red wire at the socket, red probe to the socket, black wire to ground, it read 426vac. That can’t be right, can it?

    Old Bassman diagrams show 425vdc at the OT, mine shows about 460vdc at that point. I modded it with cascaded V1 but voltages there also seem a little high. Plate voltage at v1a is 282, and is 208 at v1b.

    Any feedback is appreciated.
     
  2. neikeel

    neikeel Well-Known Member

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    You should not be blowing filter caps or 5U4 in those circumstances. If you have too high a load (first cap/resistor/choke network that will kill a rectifier. Whose filter caps are you using? Was it the mains one that died?
    Do you have a solid ground on the PT centre tap? What are you reading on your heaters (6.3 and 5v)? That might tell you if your primary selection is adrift.
     
    Last edited: May 7, 2021
  3. Pete Farrington

    Pete Farrington Active Member

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    PT secondary VAC will measure higher when unloaded than when loaded, falling as the degree of loading is increased.
    Unless otherwise noted, schematic voltages are noted at idle, no signal.
    The HT winding is effectively unloaded at initial power up, because cold valves can’t conduct HT current.

    With the amp at idle, please measure VAC at each PT primary and secondary winding, ie across the winding to which its load is connected. Check that the battery in your meter is ok.
     
  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    Something is very wrong there. What voltage is across the terminals of each filter cap? And if you have a scope, what is the waveform? Half wave or full wave?

    You've got to be careful when wiring up the capacitor stack. It's very possible to accidentally make a capacitive voltage doubler circuit.

    Look at this for a point of reference. Just swapping two wires would make this a series capacitive filter which is what should be in a Marshall type power supply.

    If the voltage feed from the rectifier is across ONE capacitor, the other will create the doubled voltage.

    If the voltage feed is across BOTH capacitors, that voltage doubling won't happen.

    So double check that wiring.
    [​IMG]
     
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  5. neikeel

    neikeel Well-Known Member

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    He is using a valve rectifier with centre tapped HT secondary so the situation above is almost impossible.
    As per Pete’s point my Fender PT has 335-0-335 and I’m using a 5U4. With only preamp tubes unloaded it has 460vdc on first node but with a pair of JJ 6v6 biased at 70% it drops down to 420v. That is 240v in on 240v tap and dead on 6.3v heaters.
    You need to do the same measurements (without the rectifier first).
     
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  6. Chris-in-LA

    Chris-in-LA Well-Known Member

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    The amp has been working great for the last 10 to 15 hours, since building it on 4/27.

    In the morning I will re-check voltages with no rectifier. I will check both red wires and the heater circuit. The amp was running on the lightbulb limiter after I replaced the first mains cap, it’s part of a pair on the first node. it also worked well on my variac at 110v. The replacement cap blew when I got the voltage up to 120.
     
  7. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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  8. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I think the PT is toast. At 97vac from the variac, heaters read 3.3 and mains read 343vac. At 120vac, heaters read 4.01vac and mains read 460. I did use the white and black wires for the hookup and the amp had been working for hours prior to the failure.

    I did have an arcing GZ34 rectifier when I first built the amp on 4/27 but it was fine after I put in a 5ar4. The problem must have just started as the heaters seemed unusually bright yesterday afternoon just prior to the filter cap blowing. Everything was fine yesterday morning.

    I plan to contact Mojotone to see if I can get them to replace the PT.

    PS - The amplifier works perfectly when it’s on the variac at 97 volts, I just ran it for 20 minutes without any problems.

    @neikeel @Guitar-Rocker @thetragichero @Matthews Guitars
     
    Last edited: May 4, 2021
  9. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Sorry about the build.
     
  10. Chris-in-LA

    Chris-in-LA Well-Known Member

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    It’s fine, I believe I know what’s wrong, just a matter of getting a new PT. But, no word from Mojotone yet.
     
  11. Pete Farrington

    Pete Farrington Active Member

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    I don’t see how the transformer’s winding ratios can go from ok to bad without it drawing fault current, ie due to developing a shorted turn in its primary winding?
    What’s does a light bulb limiter do?
    How much mains current is it drawing?
     
  12. neikeel

    neikeel Well-Known Member

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    To me I doubt the PT is bad, as such, more that the primary windings are mislabelled. So I agree with Pete.
    You need to find the correct primary for 120v in that gives you 6.3 heaters and 335-0-335 HT and you are good to go.
     
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  13. Chris-in-LA

    Chris-in-LA Well-Known Member

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    I am using the right leads for 120, the white and black. The other leads are capped with heat shrink individually and bundled together but it’s worth a look at that, there may be a short there.

    The amp now only works when it’s on a light bulb limiter or the variac. The mains initially read 355 and the heaters read 3.2 and the amp was working perfectly for almost a week plugged into the house 120ac. However when I took it off standby the other day it started to make a clicking noise and the heaters looked exceptionally bright and one of the first two filter caps blew. It blew the replacement when I ran the variac up to 120.

    Checking the amp at 120v without a rectifier showed that the mains were now 460 and the heaters were up to 4vac. The only thing that brought the voltages back to normal range was to turn down the voltage with a variac.

    There is one more detail that may figure in here. The original gz34 arced and the fuse blew when I first built the amp and took it off the lightbulb limiter and played a loud power chord. I changed out the rectifier, fuse, and power tubes from a working amp, turned down the volume, and it ran fine. A check with my meter showed voltages were in the expected range. However, there was at least two occasions in the first day or two that the amp made a loud buzz or clicking when coming off standby. But turning it off, and then back on was enough to solve the problem. I wrote it off as being a problem with possible corrosion on the input jack or loose ground wire.
     
    Last edited: May 5, 2021
  14. myersbw

    myersbw Well-Known Member

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    "Heaters at 4vac"...any chance you got the 5vac rectifier winding and the power/pre 6.3vac windings swapped?
     
  15. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Not a chance. They measured at 3.2 initially, everything was fine until the last day. I have a feeling that when the gz34 flashed and the fuse blew it on the initial startup that weakened the PT. It basically worked flawlessly for almost a week after that. The day the cap blew the voltages had gone up and the heaters got very bright. The amp ran less than a minute in that condition. I have a feeling that the 120v tap and the 100v were compromised and possibly shorted together. The amp works perfectly at 98 volts on the variac, heaters back to 3.2, mains back to 355. I told Mojotone about the problem and they are sending a new PT no questions asked. They have great customer service.

    upload_2021-5-5_19-9-29.jpeg
     
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  16. Pete Farrington

    Pete Farrington Active Member

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    But shorted turns, especially on the primary, should cause it to draw fault current, ie significantly above its max rated current.
    Because the inductance would collapse.
    Hence the mains fuse should blow or light bulb limiter stay bright.
    :hmm:
     
  17. Chris-in-LA

    Chris-in-LA Well-Known Member

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    You’re probably right. But something changed inside the PT. I looked closely at the capped wires, unbundled them, wiggled them, no change in how the PT dealt with the incoming AC. It will only work right now with 100v input. Very strange. Mojotone did not ask for the old PT. I’m tempted to take off the end caps and inspect for damage.
     
  18. Pete Farrington

    Pete Farrington Active Member

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    So the 120V primary tap is open circuit or something?
     
  19. 2L man

    2L man Member

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    When you get new transformer measure all primary wire resistances against neutral. They should follow logig increasing towards 240VAC. There happens mistakes when transformers are manufactured.

    Then connect neutral and 120V to variac and put 30VAC and then measue all primary wire voltages against neutral again. Then you can verify that voltages come logical just like resistances did. To 240VAC wire there comes about 60VAC because of transformer inductance. Then measure all secondary voltages and they too need to follow logig having about 1/4 voltages. I recommend using 30v input because it makes working with wires safer.

    You can do same to the transformer you now have to check its condition!!!

    Then add current limiter resistors to both rectifier anode circuits and also diodes if you haven't yet. Without them you shorten rectifier life and possibly other components too as has happened.
     
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  20. mickeydg5

    mickeydg5 Well-Known Member

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    The higher measured voltage than the indicated voltage on the old Bassman schematic is due to the time frame in relation to technology of the day. I have mentioned this before, tried to explain it and is how I got my signature below. I was told I did not know what the fuck I was talking about. Man I despise ignorance.

    In a nutshell:
    I do not believe they teach old technology circuit operations in basic or general electronics any longer.
    In the old days test equipment was a bit more primitive especially for the general off the shelf user.
    What we are dealing with is the volts-per-ohm rating of the meter being used to measure voltage. Old or vintage test equipment had lower rating due to their circuitry and that in turn loaded down the circuit being tested.

    So the 432V DC shown on the original schematic would be low when measured with one of todays multimeters. 460VDC is more correct.

    My 1959 5F6-A, all original components included, with 5AR4/GZ34 averages about 475-480VDC at the power tube plates and I bias at about 29-30mA idle with a wall voltage of about 120VAC. It has always been like that. However, I suspect my transformer is running on the "+" side of the +/- tolerance.
     
    Last edited: May 6, 2021
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