![]() With Imax = 320mA and (Vb-Vmin) = 375V, Pdiss=15.8W per tube, somewhat lower than the 18W result from the simulation. It is interesting to compare the simulation results above with basic pentode plate dissipation theory (RCA Receiving Tube Manual) which predicts an average plate dissipation per tube of: Of each tube, in agreement with the result above: The average value over a cycle divided by 2 gives ~ 18W for the average dynamic plate dissipation The upper result is the instantaneous power dissipated by both tube plates. The instantaneous power plots over 2 cycles are shown below. Speaker load minus the power delivered to the screen grid circuit (less than 1 W). Instantaneous power goes through a peak and decreases as expected from theory.Īveraging the power over a full cycle yields the dynamic plate dissipation for each tube of ~ 18W, within the maximum plate dissipation specification for the 6L6 tube.Īlternatively, the plate dissipation can be calculated from the total power supplied by Vp minus the output power delivered to the The instantaneous plate power dissipation curve below, for a single tube, shows that the Half this cycle, the tube is not conducting and therefore dissipates no power. Under the maximum signal swing, the plate power dissipation of the tube is the average plate power over a complete cycle. The plate power dissipation at zero signal is the product of the quiescent plate voltage 450V and the plate current 36mA or 16.2W as shown in the zero In the simulation.The plate voltage curve shows some distortion in the slope corresponding to cross-over distortion centered around Vp=450V. The output power into the 8Ω load is P out= 60W in agreement with the power delivered to the transformer primary for the ideal transformer model used The simulation below shows V out=31Vpeak. ![]() To the transformer primary (combination of both tubes) is P p = 60W. Which, for an ideal transformer, must equal the output power delivered to the (speaker) load, where V out is the peak output voltage:įrom the load line and also the simulation curves for plate voltage and plate current, (Vp-Vmin) = 375V and Imax = 320mA. Swing for a half cycle is (Vp-Vmin) and the corresponding peak current swing is Imin, the averaged power delivered Plate-to-plate impedance Rpp of the transformer primary R L = 1/4*Rpp because each half of the center-tapped transformer has half the turns. Of the load-line, is just 1/4 of the full The effective load impedance R L that each tube experiences over it's conducting half-cycle, which is the slope The power delivered to the transformer primary can be calculated from the voltage-swing (or current-swing) along the load-line for the half-cycle thatĮach tube in the push-pull circuit contributes. The plate characteristic shows the single-tube load line for the 6L6 tubeįor the maximum power condition with maximum plate voltage swing corresponding to the grid voltage Ec1 swinging to zero. The schematic diagrams below show component values and zero-signal DC voltage levels. Except for finite transformer inductances, the simulator model assumes an ideal transformer with zero DC resistance and no internal The center-tapped output transformer is characterized by L1 = L2 = 14.4H (half side of primary) and 元 = 0.1H (output secondary) corresponding to a full The peak control-grid signal voltage is 37Vpeak. The zero-signal plate dissipation per tube is 16.2W. The plate supply voltage Vp is 450VDC, the control-grid bias voltage is -44VDC, and the quiescent plate current per tube is 36mA. These circuit values correspond approximately to the 65 Fender Twin Reverb Ampīut with two tubes (instead of 4) and with an 8 Ω speaker load (instead of the 4Ω load for 4 tubes). The power tubes are standard 6L6 pentodes. The Microcap simulator was used to produce the graphs for a signal frequency of 1kHz. The results below present circuit simulations for a push-pull pentode 2-tube power amplifier.
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