Features

By now I had grown pretty curious about how my 4W push-pull triode would fare. Fig.5 gives the 8 ohm situation: Vp is about 8.5V. (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. Pushing further leads to hard clipping at 10.5Vp output. 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. With the Denton 2XP, the peak output was 15.5V—very close to that of the 25W transistor amp. (Not bad for a tiny triode, eh?) I repeated the simulation tests, but nothing new emerged.

Fig.5 4W EL84/triode-mode push-pull tube amp into 8 ohms, sinewave just into clipping, 8.5Vp, 5V/vertical div.

Fig.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.

The interaction between speaker and tube amp haunted me. 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. What if I added an external 3.5 ohm resistor to the latter? The output under steady-state conditions would fall, of course. See fig.7: Vp is now 12V against the 17V measured earlier directly at the output. But with a speaker hooked up, the maximum peak voltage on Touch was 17.5Vp positive (fig.8) (footnote 3). The transistor amplifier's output at clipping had increased almost by a factor of 1.5.

Fig.7 25W transistor amp with 3.5 ohms in series with output, system into 8 ohms, sinewave just into clipping, 12Vp, 5V/vertical div.

Fig.8 25W transistor amp with 3.5 ohms in series with output, system into Audio Note E loudspeaker, 0:05 into track 1 of Touch, driven to maximum produced 17.5Vp, 5V/vertical div.

However, the little push-pull triode still fared better at 1.8x, and the 300B SE was supreme at 2.6x. So there's more at work than just the output impedance. Magnetic energy stored in the output transformer? Unlikely; I would have seen it at work with an 8 ohm resistive load also, and I didn't (footnote 4).

So tubes do something special transistors can't. But we already knew that, didn't we?

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Footnote 3: Compare this to fig.2 and note that, in both cases, the transient output level into a speaker load is almost the same regardless of the presence of the external 3.5 ohm resistor—nothing is gained, nothing lost. Would this externally modified transistor amp now suddenly sound tubelike? Anyone care to try?

Footnote 4: While measuring earlier with the Audio Precision, I did see a 1.4x increase in Vp with pulse-like signals, but as I was disappointed I made no printouts. Stupid. And I haven't since been able to reproduce this result using the H-P gear. I have no clue why. Frustrating.