Saturday 1 June 2013

Calculating the plate dissipation

How hot is your amp running?

The EL84 was designed to be running at about 12w - that's to say that at this output it will last pretty much forever.  Obviously part of the appeal of valve amps is the overdriven sound at various points - clipping preamp stages, power tubes running hot and (to a lesser extent) speaker distortion.  So running the EL84s at beyond their rated dissipation adds to the tone, at the expense of their operational lifespan.  This can be controlled by changing the value of the cathode resistor, but it's probably a good idea to know how to calculate it and make a judgement based on that, rather than just eyeballing the tubes to see if they're glowing red hot.  These are my calculations which I think are correct, based partly on the Amp Maker testing notes.

                                                   Cathode voltage (D)
Cathode current  =                 --------------------------
                                               Value of cathode resistor (120 ohm)

                                          Voltage over Cathode resistor (D-E)
Screen current   =            -----------------------------------------
                                              Value of can cap resistor (1500 ohm)

                                            Cathode current - Screen current         
Plate Current =                   ------------------------------------

Cathode-to-plate voltage  =  Plate voltage - Cathode Voltage

Plate dissipation   =   Plate Current  x  Cathode-to-plate voltage


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