Tubes Do Something Special Page 2
Fig.1 25W transistor amp into 8 ohms, sinewave just into clipping, 17V peak (positive), 5V/vertical div.
Fig.2 25W transistor amp into Audio Note E loudspeaker, 0:05 into track 1 of Touch, driven to maximum produced 18Vp, 5V/vertical div.
I then simulated the tambourine stroke by an 8ms train of 600Hz sinewaves, then tried a single-period 600Hz burst, then a different speaker as a load, then 8 ohms and bursts, then tried to push the amp harder (to see if there was anything "beyond clipping"). In every case, I ended up with the same 18Vp.
I then connected my 300B amp and repeated the tests, with the 'scope's scale set to 10V/div. Fig.3 shows that this amplifier went into mild clipping (estimated THD 3%) into 8 ohms at 14Vp for the positive, least-clipping side, and 11Vp negative. This suggests a maximum output power of 11W RMS. Doubling the amp's input voltage produced heavy clipping at 17Vp positive.
Fig.3 11W 300B SE tube amp into 8 ohms, sinewave just into clipping, 14Vp (positive), 10V/vertical div.
I replaced the 8 ohm load with the speaker and tried to see how far I could crank up the volume with this passage on the CD until no further increase in output occurred. I got fig.4: certainly distorted in comparison to fig.2, although I could hear nothing at all problematic. But look at that 36Vp in the negative half of the picture—it would take an 80W class-A transistor amp to allow such a voltage excursion! Fig.4 also suggests that if the 300B output stage were dimensioned differently and optimized for these transient conditions instead of the usual steady-state sinewave condition, the heavy positive clipping could have been avoided. This deserves investigation, but that means a whole new project...
Fig.4 11W 300B SE tube amp into Audio Note E loudspeaker, 0:05 into track 1 of Touch, driven to maximum produced 36Vp (negative), 10V/vertical div.
I then tried to simulate the tambourine stroke with 600Hz single-sine and 8ms bursts. The results were roughly similar into the speaker load, but the amp bottomed out at 19Vp into an 8 ohm resistor. I tried the Touch-at-0:05 signal and the 8 ohm load: same 19Vp. So it had to be an interaction between the tube amp and the speaker. I connected a different speaker, an old Wharfedale Denton 2XP, and got an astonishing 39Vp.
Herein lies a key to another mystery: It is well known that tube amps, and especially single-ended variants, are choosy with respect to the speaker they have to drive. High sensitivity (preferably >90dB/W/m) is a necessary condition, of course, but not a sufficient one. Pick a speaker that, impedance-wise, looks like an 8 ohm resistor (the theoretical ideal!) and your tube amp will sound restrained.