J5MK1 Power tube plate dissipation

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    • J5MK1 Power tube plate dissipation

      Hi guys, I've finished building my J5MK1 kit, I had a lot of fun and it was a great learning experience, and I'm loving the sound so far.

      I decided to take some extra measurements, in particular the ones needed to calculate how much power is dissipated through the plate of the EL84. I'm reporting them below.
      The R19/R20 in parallel measures 232 ohm, and the voltage across them is 8.67 volt; this means 0.03737 ampere are flowing through the resistors.
      The plate to cathode voltage (pins 7 vs 3) is 273 volt.
      237 V * 0.03737 A= 10.2 W.
      According to the JJ datasheet the maximum plate power is 12 W. Usually you choose the 70% of that (8.4 W) as a safe upper limit to have a decent tube lifespan.
      Is my math wrong, or am I driving the output tube way too hard? Should I increase the cathode resistor value?
      Thanks :)
    • When I did my studies about a copy of Champ using a Mosfet
      TransChamp - Einladung zum Messen, Modden, Lernen und Spielen

      I began wondering, why a significantly larger Kathode resistor is not used more often as a very simple means to reduce output power or achieve Overdrive at lower volume, like a very simple power soak.

      You could buy some different values and try the sound. I would check, if the rising supply voltages are still acceptable for the caps.

      You can buy a spare tube too. I always do so.
      Gruß Christof
    • Hi Rigel, welcome to the forum!
      You're mixing up different setups - for a push-pull configuration (say 2x EL84 Class AB1) you set the quiescent plate current to get 80-95% max power dissipation (with cathode bias) or 70-85% (fixed bias). But the J5 is a single ended (one output tube) class A amplifier, so the power dissipation is roughly constant from running empty to full power, so you can set the quiescent plate current to reach nearly 100% max power dissipation. BUT you also need to make sure, that the max screen dissipation is not exceeded, this is more dangerous for the tube! So the operating point of the EL84 in the J5 is set to be on the safe side both for plate and screen current.

      @Christof: this will indeed result in earlier overdrive of the output tube (moving the operating point by changing the cathode resistor results in unsymmetrical clipping of the output tube) so this might be interesting with single ended amps but for the said reason you also reduce clean headroom which is not too big anyway in a 6V6 or EL84 single ended configuration...
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      The post was edited 1 time, last by MAD ().