2A3 at 45 operating points

[Dr. Toobz]

Are there any sonic disadvantages to running 2A3's at the 45 operating point in the Stereomour? I recently plugged in a pair of Psvane 2A3B tubes, which sound great, but the amp is still wired for 45's. My understanding is that the power output will of course be lower than running the 2A3 at its normal operating point, but what about distortion levels, damping, etc?

I am assuming that the effective plate impedance is pretty much the same at either point, so I am going off the assumption that bass/treble balance shouldn't be any different under thae lower points. The tubes are being run at a lower dissipation, and are pretty damn linear, so I am also assuming that THD is not going to be much different, either. So, is the only difference really the lower power output?

I of course don't need the extra power from the 2A3's - at least not in my current condo - so my interest in this question is more theoretical than practical!

[Paul Joppa]

Actually, the optimum load impedance is higher for the 2A3, so when using the 2A3 you will get a bit more distortion at max power. But you'll also get a bit more power, so they kind of trade off. The other difference is, when set up for the 45 you are getting twice the inductance from the plate choke (compared to the 2A3 setup), and the stock parafeed cap is more protective of tube overload, both of which provide reduced distortion from deep bass peak signals.

Too much information, right?!  :^)  All these effects are fairly small, and within the range of good design selections.

[Dr. Toobz]

Your answers are always great! I really do learn something every time I read this board.

The bass is much, much better right now (running the 2A3's as pseudo-45's) than it was with the true 45 tubes, even if I don't need the extra reserve power of a 2A3 (or even a 45! I listen very quietly). The Klipsch speakers have really "fleshed out" and the bottom end has filled in nicely, with actual bass notes to be heard again. Thus, I may just leave the amp as-is. If I need the extra watt or two in the future, such as when I own an actual house, it will be easy to go back to the standard 2A3 arrangement. Right now, I rarely go past 11 o'clock on the volume pot.

I like these 2A3's way more than the Sovteks, and they happen to suit my speakers much better than NOS 45's. A win-win!

[Paul Joppa]

Ooohhh - I left out that the lower plate impedance means a better damping factor with the 2A3. I expect that's why the bass sounds better.

[Dr. Toobz]

I suspected that as well. The 8-ohm taps are being used right now, and as I've probably complained about 100 times over the past few years, my Klipsch speakers seem to be really picky about damping. With the 45's, bass was too light on the same tap. Now, it's just right. Lower impedance taps "dry out" the sound too much for my tastes, quickly catapulting the Klipsches back into their "thin and shrill" tendencies. I'd get rid of them, but they were pretty much made for my living room (see attached photo!)