Ohm & Tubes -Part 1

The results of my recent experiment with this combo (Ohm 100s/Prima Luna 7 monos) were sufficiently involved that a multi-part post is on the way (if interest warrants). I started out to answer a simple question posed by a fellow 'goner and ended up opening a most interesting can o' worms.

First twist: which output tubes.

Started with KT88s because that was the specific question put to me and because maximizing output seemed like a good idea with the power hungry 100s. Unfortunately, the good news (improved dynamics and even more dramatic imaging) was offset by bad news (loss of "body" and hardening in the presence range) and mixed news (a significant bump in mid-bass output). The last bit is mixed because it wasn't unpleasant at all - the speaker moved from "Quad-like" toward "Vandy-like". Many listeners might well prefer the latter, but this also poses problems for sub integration (see below).

Switching to EL34s relieved the hardened presence region, but exacerbasted the mid bass hump and softened the "attack" and overall dynamics. Not particularly my cup of tea. So, onto KT66s.

This was the best balance - full textures, improved dynamics and oh! that dramatic stage. Whatever dynamics the speaker had theretofore lacked were largely present. (This could also be partially break-in as the speakers are just approaching 2 mos. in service.) Output capability may have been reduced vs ss amps, but the better dynamics allowed me to back off the spls a fair bit. However, the mid-bass was still too full vs. the ss amps.

Next twist: which taps.

Switching from 4 ohm taps (John S suggested this as a starting point), reduced the mid bass hump almost completely with KT66s (and a good bit with the EL 34s and KT 88s). Dyanmics were reduced just a bit, but the wonderful imaging, stage, and texture remained front & center. In fact, the KT66s at 8 ohms really began to sound like the ss amps - with just a little better imaging. Nearly back where we began!

Last twist: Stand alone vs. subs

When mid bass levels bumped up, sub integration became a real issue. Best matching at the x-over point yielded overall bass that was elevated. Further, the higher output from the subs didn't please the room, which began to act up like a sleepy three year old.

Conclusion (Part 1)

As a stand alone, full range speaker, the Ohm 100s were definitely a quirky match for tubes. I suspect many folks will like the elevated mid-bass "warmth" and prefer the KT88 or EL34 matching over ss amplification. Other will stick with the more neutral ss sound. The choice of 4 ohm taps vs 8 ohm taps will allow the user to "dial in" warmth to taste. Seamless mating to a sub will, however, be a tough proposition.

With KT66s, a sub is a more realistic option. At 8 ohm taps, the mating dance was brief and the coupling quite comfortable (marriage vs dating?). However, this was the least "tubey" sound among the tube amp options. Some may wish to forego the bother and stick with ss.

Next: A high power tube amp.

Marty

PS - I am in the midst of a full blown infatuation with Lindsey Buckingham. For a real treat, Ohm owners should spin his "Under The Skin" cd. This recording is a (decidedly non-audiophile approved) exploration of studio effects on multiple acoutic guitar, layered vocals, and spare percussion. The Ohms create a full bodied 3d acoustic space that qualifies as shameless showboating!