But with a speaker hooked up, the maximum peak voltage on Touch was 17.5Vp positive (fig.8) (footnote 3). See fig.7: Vp is now 12V against the 17V measured earlier directly at the output. What if I added an external 3.5 ohm resistor to the latter? The output under steady-state conditions would fall, of course. Both tube amps operated without overall loop feedback, meaning that their internal output (source) impedance was about 3.5 ohms, compared to an effective 0 ohms for the transistor amp. The interaction between speaker and tube amp haunted me. (Not bad for a tiny triode, eh?) I repeated the simulation tests, but nothing new emerged.įig.5 4W EL84/triode-mode push-pull tube amp into 8 ohms, sinewave just into clipping, 8.5Vp, 5V/vertical div.įig.6 4W EL84/triode-mode push-pull tube amp into Audio Note E loudspeaker, 0:05 into track 1 of Touch, driven to maximum produced 15Vp, 5V/vertical div. With the Denton 2XP, the peak output was 15.5V-very close to that of the 25W transistor amp. Playing Touch at 0:05, the speaker load managed to produce fig.6, with a positive Vp of 15V-not as shocking as with the 300B, but this still implies almost a doubling of the Vp under steady-state conditions, and represents a quadrupled transient output power. Pushing further leads to hard clipping at 10.5Vp output. (As this is a lower-output amp, the vertical scale is again back to 5V/div.) Defining clipping as 3% THD, the 'scope indicates a 4.2W RMS power rating. Fig.5 gives the 8 ohm situation: Vp is about 8.5V. By now I had grown pretty curious about how my 4W push-pull triode would fare.
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