Did I Just Blow Up My OT?

Discussion in 'Shock Brother's DIY Amps' started by 72_Custom, Aug 1, 2020 at 2:27 PM.

  1. 72_Custom

    72_Custom TDPRI Member

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    I'm working on converting an Epiphone Valve Junior into a handwired 5F1 Champ with a solid state rectifier (as I'm trying to reuse the existing transformers). During the initial startup/testing procedure, everything checked out OK without tubes and then with only the 12AX7. Finally, when I popped in the 6V6 and brought the variac up to full voltage, I saw a puff of smoke. (Unfortunately, I moved to shut off the amp so fast that I wasn't able to discern where exactly it came from).

    Now, I'm getting a loud hum, which initially led me to believe the issue was 120Hz hum from the power section or a grounding issue. After hours of poking and prodding, the only thing that I can find wrong is the output transformer. I'm measuring about 500 Ohms on the primary, about 1 Ohm on the 4 Ohm secondary and 1.8 Ohms on the 8 Ohm secondary. I know the DC resistance is a bit less than the AC impedance, but it shouldn't be THIS low. Right?!?

    Before I buy another OT, is there anything else obvious that I should be investigating? I'm not quite sure what was wrong in the first place and I'd hate to blow up another transformer.


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  2. Intubator

    Intubator Tele-Holic

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    Smoke is generally not a good sign. Was the speaker plugged in by any chance, or did you have a dummy load on the OT?
     
  3. sds1

    sds1 Tele-Afflicted

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    You are measuring the resistance across a very short length of wire, we expect the resistance should be approaching zero.

    Remember, the "8 ohm" secondary is for connecting to an 8ohm speaker load. This says nothing of the resistance/impedance across the winding itself.

    Every time I try to blame a transformer for the trouble I'm having, it's not the transformer. I would say keep lookin!

    You might have gotten a bad tube out of the box. This exact scenario has happened to me before, tube smokes out of the box. Probably a million times more likely than you wrecking a transformer that quickly.
     
    Last edited: Aug 1, 2020 at 3:26 PM
  4. jhundt

    jhundt Doctor of Teleocity

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    so, you aren't using a light-bulb limiter? Maybe you should try it.
     
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  5. tubegeek

    tubegeek Friend of Leo's

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    See whether the primary and the secondary of the OT read as shorted to each other. Do this with the power off and your output tubes removed.

    The secondary will read as short or very very small ohms from end to end as @sds1 said. The primary should measure no more than about 1K ohms between any two wires, probably much less. Any primary wire to any secondary wire should measure as an open circuit - your meter should read the same whether you touch those two wires or touch nothing at all.

    Examine the output tube sockets closely - you are looking for evidence of an arc-over, where you'd see some carbon residue between parts of the socket. That would be one possibility of a smoke event that might be hard to spot.
     
  6. tubegeek

    tubegeek Friend of Leo's

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    @jhundt - if it was an arc-over on a socket, a tube failing short (internal arc-over), or an insulation fail within the OT, a LBL might not have saved him anyway - didn't happen until the variac reached full voltage - and then, sudden-onset - so that might not have been indicated by the LBL since it tends to reduce voltage everywhere.
     
  7. jhundt

    jhundt Doctor of Teleocity

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    perhaps not.

    But for $5 worth of parts, I would certainly make one and use it.

    Maybe I've been lucky, but I never had an arc on a socket or in a tube, or an insulation failure in a transformer so I can't really say what would happen in a case like that.
     
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  8. tubegeek

    tubegeek Friend of Leo's

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    Agreed on the $5 risk/reward. The LBL will protect you from many other, much more common faults. Gotta have one.

    Arced sockets are not worth trying to fix - replacement of the socket is the way to go.

    Here's a LBL that would've cost more than $5 if I hadn't gotten dumpster-lucky.

    Small Edison-base fuse box with a cool Frankenstein blade switch. From a Brooklyn apartment renovation. "GE brings good things to life."

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  9. 72_Custom

    72_Custom TDPRI Member

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    Thanks for all of the input folks. To answer a couple of the questions posed: yes, there was a speaker connected and yes, I was using a light bulb limiter.

    It sounds like my measured resistances were all in line with what is to be expected. I just tested and there is no continuity between the primary and secondary windings. I learned something new about transformers today and saved myself a cool $30 in the process! It doesn't seem like that's my issue, so I'll keep looking around for other issues.
     
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  10. 72_Custom

    72_Custom TDPRI Member

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    Quick update:

    Looks like I found the root of the problem: bad solder joint on the 6V6 tube socket. Whew!

    I’ve still got a small amount of 60Hz hum, so I’m going to play around with some different grounding schemes to try to get it as quiet as possible. But it’s certainly a relief to go from “uh oh” to operational in such a short amount of time. Thanks again everyone.
     
  11. elpico

    elpico Tele-Afflicted

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    Grounding schemes influence 120hz buzz much more than 60hz hum. If it's 60hz look to the heater wire lead dress (and any wires running close to the heater wires) and consider simple changes like a hum dinger and elevated heater voltage.
     
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  12. jhundt

    jhundt Doctor of Teleocity

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    good one!
    @elpico what is a hum dinger?

    OK - I looked it up. Hum balance pot, like Fender used on my Princeton Reverb II.
     
    Last edited: Aug 2, 2020 at 7:26 AM
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