5f1ish build making horrible sounds

Discussion in 'Shock Brother's DIY Amps' started by AnalogTwang, Aug 18, 2019.

  1. AnalogTwang

    AnalogTwang TDPRI Member

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    Hey all,

    I was bored one day and decided to make a 5f1 type amp from scrap I had laying around. Basically the same as the weber 5f1 schem here. Except using a solid state voltage doubler (delon circuit) because I didn't have exactly the right transformer. This gives a plate voltage of ~310 VDC for the power tube.

    So when I put it all together I plugged it in and, Yay there was sound! Unfortunately the sound being made was really awful! Like it would be completely quiet until you hit the strings really hard and then it would come out really distorted and nasty sounding and then fade away with a farty sound.

    So I whipped out my scope and signal gen and I traced a perfect sine wave through the preamp section, all the way to the input of the power tube. When measuring at the speaker jack (with speaker connected) unfortunately I got this:

    IMG_20190818_134838.jpg

    Well it doesn't look good to me so I was wondering what might cause this? I think I've eliminated the preamp section as a culprit. Does this sound like the bias for the power tube is completely messed up?
     
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Step 1 is always to make a voltage chart.
     
  3. dunehunter

    dunehunter Tele-Holic Double Platinum Supporter

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    Not sure, but Acom's Razor suggests the voltage doubler circuit would be a good place to look.
     
  4. AnalogTwang

    AnalogTwang TDPRI Member

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    OK so I took the voltages from the base of the 6v6. We have:

    Cathode : 49.3 V
    Plate : 319 V
    Screen : 299 V
    Grid :69 mV

    Well, something is indeed going wrong, I believe the cathode is only supposed to be around 20V! Any Idea what might cause this? Is my 6v6 likely messed? I could order another one.
     
  5. VintageSG

    VintageSG Friend of Leo's Ad Free Member

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    What value cathode resistor are you using? Is it bypassed, and if so, by what value and is it the correct way round?
     
  6. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Pics can really help the helpers help here...


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  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Is the 6v6 redplating under those conditions? Sounds like you found where to look though from the voltage readings.
     
  8. AnalogTwang

    AnalogTwang TDPRI Member

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    Well, it ain't pretty but here she is (like I said made from scrap):

    IMG_20190818_171628.jpg

    As you can see the cathode resistor is a 470R 10W, bypassed with a 22u 50v capacitor. Wouldn't be surprised if the cap is toast now, seems like its been operating near max voltage, perhaps exceeding during power up.
     
  9. AnalogTwang

    AnalogTwang TDPRI Member

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    Managed to fix it, It looks like a was the victim of a bad batch of 470R resistors. Measured it and it was open circuit! All my spares were too!

    Got some new ones and replaced it as well as the bypass cap (with a 160v this time), now it sounds awesome!! thanks everyone for their help.
     
  10. SnidelyWhiplash

    SnidelyWhiplash Friend of Leo's

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    Glad you sorted it out. Test all your components & never assume anything!
    The 5F11 is a great circuit. i wish i had the patience to assemble such.

    Play it loud enough to annoy the neighbors! :twisted:
     
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  11. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Another Acom fan here.
     
  12. King Fan

    King Fan Friend of Leo's Ad Free Member

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    LOL, yep. Acom was nearly as famous as Occam. And his name was even more parsimonious.


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  13. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Ha! I prefer Occam only because he uses less auto tune.
     
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  14. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Yeah, logical as he was, he was no Lil Wayne...
     
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  15. dunehunter

    dunehunter Tele-Holic Double Platinum Supporter

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    How will I embarrass myself next...stay tuned!o_O
     
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  16. Luthier Vandros

    Luthier Vandros Tele-Holic Silver Supporter

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    Story of my life!
     
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