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A problem that doesn't sound like a problem and may not even be a problem

Discussion in 'Shock Brother's DIY Amps' started by Lavochkin, Jan 19, 2021.

  1. Lavochkin

    Lavochkin TDPRI Member

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    I built a Champ Micro a year ago and have no complaints about the sound. I did notice today the the power tube (12AU7) was noticeably hotter than the preamp tube (12AY7). The preamp tube was "if it was much hotter I should let go" and the power tube was "LET GO IMMEDIATELY". Turned the lights out and they looked the same, only the filament glow.

    I was pretty much assuming that they would be about the same temp but since this is my first tube amp I am not sure. Do I have a problem that needs further investigation?

    TIA!
     
  2. tubelectron

    tubelectron Tele-Afflicted

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    IMHO @Lavochkin , this probably comes from the fact that the power tube (hence its name) endure a higher plate operating dissipation than the preamp tube, so that's why it is hotter.

    For safety, I just would check the currents and voltages on your 12AU7 power tube and see if they are still correct versus the schematic values, and if the datasheet maximum dissipation ratings are not exceeded (ECC82 = 2.75W plate dissipation per triode, that is to say 5.5W for a complete 12AU7).

    -tbln.
     
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  3. dan40

    dan40 Friend of Leo's

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    The 12au7 definitely runs much hotter than the preamp tube in the micro amps' that I have built. As Tubeelectron stated, its running a higher dissipation than the preamp tube so the extra heat is normal.
     
  4. DJLockjaw

    DJLockjaw TDPRI Member

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    I built a studio one which uses a 12au7 for the power tube, and it does get super hot. It shipped without a tube shield for the socket, which makes sense given how hot it is. If your amp is working the same as ever, I wouldn't worry. It's probably been hot this whole time.
     
  5. Lavochkin

    Lavochkin TDPRI Member

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    Thanks tubelectron, dan40, and DJLockjaw
    I'll do a checkup tomorrow and see what the dissipation is. Since I made it I've had it in a cobbled together cabinet. In prep for making a proper cab, I had it out to get some dimensions and decide to play, which is when I noticed that the power tube was hot. As DJLockjaw wrote, it's probably been hot this entire time and I didn't notice because the tubes were more inaccessible.
     
  6. tubeswell

    tubeswell Friend of Leo's

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    Oxide-coated cathodes operate at about 800-1000 °C, but the heat from the tube envelope is mainly a function of plate dissipation (if the envelope was 800 °C, you wouldn't be able to touch it without getting instant 3rd-degree burns). A 12AU7 operated as an output stage with both triodes in parallel in a single ended amp may dissipate around 5.5W. But operated as a pre-amp triode it may dissipate a lot less, depending on the bias and load resistance (plate load resistance eats up supply voltage, so the plate-to-ground* voltage is typically 1/2 to 2/3 of the High Tension voltage)

    *cathode voltage is 1-2V in a typical 12AU7 preamp stage, which is 'next to nothing' c/f plate-to-ground voltage (although technically, plate-to-cathode voltage provides a more accurate calculation of plate dissipation)
     
    Last edited: Jan 20, 2021
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