Magnatone

Discussion in 'Shock Brother's DIY Amps' started by Clintstone, Jun 20, 2021.

  1. Clintstone

    Clintstone Tele-Meister

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    A little upgrade to the power supply, now just waiting for my tubes to arrive to give it a real test. The 12DW7 is quite the oddball with half of the tube being a 12ax7 and the other half being a 12AU7 gain factor of 17.
    20210703_183851.jpg
     
  2. Clintstone

    Clintstone Tele-Meister

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    Does this capacitor look like an electrolytic, I should probably change all these out if I am going to try to resurrect this beast. 20210714_124635.jpg
     
  3. NTC

    NTC Tele-Holic

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    No that isn't. It is a film cap.
     
  4. Clintstone

    Clintstone Tele-Meister

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    So it should be ok?
     
  5. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Yeah, usually. You may want to measure it to see if it’s still around the value printed on the cap.

    All electrolytics (usually the power filter caps and cathode bypass caps only) should be swapped, coupling caps are usually fine.
     
  6. Clintstone

    Clintstone Tele-Meister

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    My amplified parts order came in, time to resurrect this beast.
    20210725_113917.jpg
     
  7. Clintstone

    Clintstone Tele-Meister

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    Took a bit of work but that ugly old can is out 20210725_121046.jpg
     
  8. Clintstone

    Clintstone Tele-Meister

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    Glad I found this bad joint
    20210731_120720.jpg
     
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  9. NTC

    NTC Tele-Holic

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    After how many years? That looks factory original...
     
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  10. Clintstone

    Clintstone Tele-Meister

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    20210801_121523.jpg
    Here is the cap I replaced, would this be era correct?
     
  11. Clintstone

    Clintstone Tele-Meister

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    Embarrassing story time

    So yesterday after replacing all the filter caps I decided to test out the amp and see if I could bring this beast back to life. I used my light bulb limiter and switched it on. Nothing caught on fire so I assumed I had all the caps installed correctly. I installed the tubes and proceeded to measure some more voltages. Power amp B+ 435 measured, should be 400 according to schematic. Reverb power amp 362 measured, should be 340 on schematic. Mixer driver/phase inverter 258 should be 220 on schematic. Preamp measures 157 should be 140 on schematic. I assume these increased levels are probably due to higher modern wall voltage (mine measured 121 ) so I am not really worried.
    So I plugged in a guitar and slowly turned up the normal channel volume, NOTHING. So I checked a few obvious things then went back over the schematic, assuming I had wired something wrong. Nope, everything is ok, so this morning I hooked up my oscilloscope to the normal channel and plugged in a 600 hz tone from my handy signal generator smart phone app. As soon as I switched on the tone button from my phone the amp almost blew my eardrums out. So I plugged my guitar back in, once again nothing. So I took my guitar and cable into my studio and plugged it into my Marshall, dead quiet.
    Moral of the s 20210801_124759.jpg tory, before you spend hours troubleshooting the amp check your cheap guitar cable first.
     
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  12. mrriggs

    mrriggs Tele-Meister

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    I once rewired a semi-hollow guitar (fishing through f-hole) cause I thought the output jack was bad, only to figure out after the fact that my cable had a bad end.
     
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  13. Clintstone

    Clintstone Tele-Meister

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    So after walking away mad at myself over the instrument cable episode I decided to go back to the magnatone. The normal channel seems to work fine but the reverb channel is dead. I hooked up my signal generator and my oscilloscope to the normal channel and got a baseline waveform working on the screen, then I swapped the signal generator output to the reverb channel. I had a shop speaker hooked up and when I touched the scope probe to pin 2 of V2 I had sound. I removed it and the sound went away, aha bad connection I thought. So I powered everything down and discharged the caps then broke out the soldering iron. As soon as I touched pin 2 the connection popped apart. 20210823_120102.jpg
     
  14. Clintstone

    Clintstone Tele-Meister

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    Further inspection of the old tube socket revealed the terminal underneath was missing as well, probably been like this for decades as my friend who owned it before me just used the clean channel with pedals. 20210823_125255.jpg
     
  15. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    Hopefully an easy fix swapping the socket.
     
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