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I Want a cheap and easy "Confidence Builder" amp kit.

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  • #16
    Originally posted by Guitarist1983 View Post
    I measured Voltage between EL84's pins 7 & 3 (plate & cathode) at 245.7 VDC. The OT primary at 226 ohms, and Voltage across OT primary at 5.64 VDC. So, using a calculator Plate current is 5.64/226 = 25 mA and Plate dissipation is 245.7 VDC * 25 mA = 6.14 watts. If I'm correct, the EL84 is idling at ~50% maximum, right? If so, can I fiddle with the Cathode resistor to increase the plate dissipation?
    A simpler way to find the current is to just take the voltage across the cathode resistor and divide by the resistance. This also includes screen current but it changes the reading very little.
    The schematic calls for about 28mA current. (5.5V divided by 200 ohms). So you are right in line with the recommended bias. You can play around with it if you want, but if there is no big benefit (tonally) to your ears, the lower current will increase the tube life. You may find a compromise somewhere between the factory spec. and the maximum.

    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


    • #17
      What Helmholtz and g1 said^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

      Yes you can lower the cathode resistor value for more current. 100% rated tube dissipation at idle for class A in your case is fine. As mentioned, class A means that the one tube is biased so that signal swings from the center of saturation and cutoff. So you can only expect half peak voltage at either end. Since your voltage is a little on the low side I think you shouldn't expect more than about five watts. A bit less actually because tubes are never operated in perfect, on paper circumstances and there are losses in the circuit elsewhere.

      "Take two placebos, works twice as well." Enzo

      "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

      "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A