Magnavox 6AQ5 mono amp shorting/burning after recap!

Discussion in 'Tube Audio' started by HiFi-Stereo, May 17, 2018.

  1. HiFi-Stereo

    HiFi-Stereo Active Member

    Messages:
    209
    Okay, when I plug in the amp, a few seconds later one output tube socket starts sparking with smoke and a burning smell... a couple times it played for a few minutes till it started sparking.
    Checked and rechecked this amp after replacing all the caps and resistors...
    One time the 22K resistor between the 20uf and 10uf fried.
    All the right values, connections are tight, tried two sets of all the tubes,
    nothing touching something it shouldn't...

    Could it be a short in the output transformer?

    Also, when I test the electrolytics afterwards, the 40uf says "Overload" on my Klein multimeter, and the 20 doesn't test... if i unsolder the leads, they all test fine.
    It's such a simple circuit, hoping someone can offer a good suggestion... IMG_5477.JPG IMG_5482.JPG
     

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  2. smccrary

    smccrary Low-Budget Collector

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  3. kward

    kward AK Subscriber Subscriber

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    The schematic is really hard to read. Is the 22K resistor that is frying R12? If so, I think first thing I'd do is pull the audio amp tubes (both output stage 6BQ5 tubes and frontend 12AX7 tube), but leave in the rectifier tube), and fire up the amp (after replacing the fried 22K resistor). Thus you're checking the power supply and associated wiring by itself without interaction from the audio amp section. If it still fries that same 22K resistor, then it's something in the power supply or associated wiring, such as C1 is shorting or wired backwards, or perhaps R3 is significantly smaller resistance than what the schematic says it should be.

    If it doesn't fry that 22K resistor, then while your there, check the voltages in the power supply. The voltage at the center tap of the output transformer should be in the neighborhood of 275V to 290V DC in this unloaded condition. Then check the voltages on the screen pins of the 6BQ5 sockets. Then check the voltage on the positive side of C1. They all should read approx the same voltage, give or take, in this condition with the audio tubes removed.
     
  4. HiFi-Stereo

    HiFi-Stereo Active Member

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    209
    Thanks so much, excellent information, will check first thing tomorrow, almost midnight here! I am always impressed with the great technical knowledge people have here, to me it's a really fun hobby, but I've a long way to go in understanding how and why it all works...
    Will report back tomorrow!
     
  5. primosounds

    primosounds SE KT120 w/ 6J5G drivers. LCLC ps + 2 x 5ar4 Subscriber

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    Although the amp is rather simple rebuilding can easily introduce wiring errors by even the most experienced builders. It is a good first step in troubleshooting to try and explain what is wrong with your amp. The give and take of exchanging information is often the catalyst for discovering an error. Go through your rebuild and the circuit from power supply to output tubes then output tubes to input tube. Make sure there are no parts shorting on hot leads. It seems like you may have made a wiring mistake.
     
  6. knockbill

    knockbill Addicted Member

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    +1,,, The wiring and solder blobs around the PS caps look suspicious,,, make sure the cap polarity is correct and there aren't any shorts...
     

     

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  7. HiFi-Stereo

    HiFi-Stereo Active Member

    Messages:
    209
    Okay, just figured it out!!
    After lots of looking things over a hundred times, and taking all the good advise given,
    my last option i figured was to take the amp apart...
    I first did what Kward suggested, and left only the rectifier tube in place, but still had sparking.
    Checked all the connections , polarity and the power supply, as Primosounds and Knockbill suggested, and even reflowed all the connections.

    Turns out it was a factory error... one of the heater wires going to the 12AX7 was never soldered, it looked like it was, but only very tightly crimped.
    You could never see it unless you took the socket out! Which I did.
    There were two heater wires going to one pin, the #8 or 9 pin of the 12AX7. It was the wire underneath that was not soldered.
    I can see why the worker doing the soldering would think she got it.
    Hard to believe, that's why I was going crazy... Now it sounds perfect!!!
    Thanks for all the help as usual, couldn't get through it without you guys!
     

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  8. BillWojo

    BillWojo AK Subscriber Subscriber

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    So your doing Magnavox warranty service work? Good to know. LOL

    BillWojo
     
  9. kward

    kward AK Subscriber Subscriber

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    I had a very similar issue with my Maggie or maybe it was the Curtis Mathes amp. One of the filament wires was crimped but not soldered. It wasn't causing issues though, but I'm still glad I found it.
     

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