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.


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!
Thanks, Marty. I look forward to your next post. The 100s are a top contender in my speaker upgrade (currently Vandy 1Cs.). FYI, I use a tube pre with a ss amp.
Perhaps overall linearity could be improved with KT90 tubes. I am very fond of these in when power and even response through the audible range is desired. They can usually be subbed for a KT88.
Very interesting stuff.

Marty, thanks for taking the time and effort to sort through the Ohms with tube power realm.
Interesting results and I realize that your experiment was from the OHM's point of view. I have been experimenting with various tube types, but from the Prima Luna's point of view. I have owned the Prima Luna PL2 and PL5 for 3 years and have spent the better part of the last year experimenting on the PL5 with different tube types. What prompted this experimenting was the upgrading of my speakers from Acoustic Research 302 (3 way, 10 inch woofer, acoustic suspension) to Opera Platea floorstanders (2 way, 6 inch woofer, front ported). The other PL5 associated equipment is a Musical Fidelity CD PRE 24 (ss preamp/cd player) and a SONY SCD 555ES SACD player.

The output tube types tried were KT 88 (Svetlana Winged C), 6550 (Svetlana Winged C), EL 34 (Electro Harmonix), and KT 90 (Electro Harmonix). The speakers were tried only from the 4 ohm tap. Each tube type was used for at least 2 months.

I found the KT 90's fairly sterile sounding and not very engaging. All types of music sounded pretty lifeless. I wasn't expecting this result.

I felt that the music was the most dynamic sounding with the KT88's, only slightly less so with the 6550's. There was more than sufficient detail, but the music was not particularly sweet sounding. What was strange, was that both tube types didn't seem to last very long ... a few hundred hours at most. There was no particular pattern to it, the tubes would just seem to get excessively scratchy and noisy.

The EH EL 34's have become the keepers. The sound is dynamic, lively, and sweet. All ends of the spectrum are reproduced with good detail. The tubes seem to be lasting the longest.

Anxious to see your next post. I plan to try this experimenting with the PL2 next.


marty thank you for this detailed report! I am running a hybrid integrated amp with my Ohms but the tubes are all in the preamp section and the power amp section is SS, so no elaborate tube rolling experiments for me.
"Out Of The Cradle" is another excellent Lindsay Buckingham recording to experience on OHMs.
Since the thread is back, I'll provide a part II (or at least a part 1 of Part II):

I recently re-jiggered my system to allow me to fire up my ARC VT-130SE amp. This unit is balanced operation only. It has no phase splitter so I couldn't use it with my Joule pre-amp. My ARC LS-25 is back from service, so the ARC chain is now on line. The rest of the current system:

Sources are Cary 303 cdp
QSonix server with Benchmark DAC-1 or Cambridge DAC Magic
Oracle V/Graham/Nightingale or Acoustic Solid/Rega/Lyra Dorian via Aethetix Rhea

High pass is active at 80hz via NHT X2
Low pass/Room Analysis/PEq is at 74hz (for Ohms, 80hz for Maggie SMG) via Velodyne SMS-1
2 X SPLR 8 subwoofers


The ARC produces app 125 WPC via 4 X 6550 output tubes. Rich pretty much nails my take on 6550s. Like KT88s, the upside is dynamics and the downside a little less tubey sweetness. This is true of the ARC, which sounds pretty neutral - not obviously a tube amp from the tonality. However, this amp has a certain body to the mids and imaging that shows that its heart is indeed made of glass.

Unlike the Prima Lunas loaded with KT88s, the ARC doesn't sound slightly "hard" through the mids, merely less "sweet" than P/Ls with EL-34s. In short, it's a mid way point that's (tonally) awfully close to the P/L with KT66s that I had previously settled on. I'd be pretty hard pressed to describe much in the way of difference in sound, except that:

The ARC significantly improves the dynamic performance of the system. It's only 3db of headroom vs the P/Ls, but this is a case where the result sounds like more than that. The LS-25 also allows higher gain than the Joule so it's possible that some of this result is attributable to the switchover in pre-amps. Either way, the dynamic improvement is welcome and separates this amplification chain from the previous choice - for this application.

As a side note, there was much less tonal shift with tap changes on the ARC than on the P/Ls. Don't have a clue why, but I'm not sure that I could tell which taps are active on the ARC by ear. Subwoofer matching was pretty straightforward - although some of that may be my improved experience at this task. The NHT X-over got the A/D/A conversion out of the main signal path. I think it sounds better and I know it provides improved peace of mind.

In sum, the ARC chain will be the primary choice for my bi-amped speakers. (The improvement in dynamics is also obvious when I roll in my SMGs to replace the Ohm 100s.) I'll probably switch the TAD SS amps and/or P/L monos along with the Joule Pre back in at some point to refresh my reference point, but as of now, the high powered ARC tube amp seems like the best choice for my Ohm 100s.


Do you think the ARC tube amplification would still be preferred running 100s without a sub or is the sub key to getting good results with the tube amp?
Good question. My room has bass issues so I haven't tried (non EQd subless) full range operation. I could shlep the whole system downstairs (like I did when the Ohms first arrived) and try full range operation in my living room. OTOH, I'm probably too lazy. So, a guess will have to do.
The subs relieve the main amps of a lot of their work. It's plainly beneficial with the subs doing their thing, so I suspect that it's as (or more) beneficial stand alone. The question of how much juice you need will also turn on your listening habits and your room, but I think, a good general statement for Ohms is:

"More power is better, tubed or SS."

ASL hurricane