Recommendation for higher power tube amp


AudiogoNers - I am interested in trying more tube power with my Thiel CS2.3s, which are hard to drive (~3 ohm load over 100Hz), budget is ~$3000-3500, used is fine as long as reliability is not a big risk, what would you recommend?

Currently running a BAT VK-55 and love the sound of tubes with these speakers, first thought was to get a second VK-55 and run them in mono, but realizing there are several other potential contenders in this price range, including Sonic Frontiers Power 2, Music Reference RM-200, a few VTL models, Antique Sound Labs Hurricane, Jolida Fusion, Manley Snappers, and more ARCs then you can shake a stick at, I thought I'd post the question.

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My recommendation always tends towards finding the best value. Spending $3500 to hopefully make a pair of $1800 difficult to drive speakers maybe sound a little better. 
You love the sound of the tube amp you already have. Imagine how much more with better speakers. 
Why are you trying to put a square peg in a round hole?  Theil's love and perform best with high current SS amps. 
If you want to stay tube, for ~$3500 you could get a Rogue Stereo 100, which outputs 100W into 8 ohms when in ultralinear mode, 60W in triode.  I have one paired with a RP-7 preamp and I love the combination, but I'm not running a difficult load.
What sets the Music Reference RM-200 apart from (as far as I know) all other tube amps is that it's power output does not decrease with speaker impedance---it puts out 100 watts at both 8 ohms and 4 ohms. It also has a low (for a tube amp) output impedance, a good thing with loudspeakers (such as your Thiel's) having a varying impedance. The original version of the RM-200 goes for around $2000 used, the Mk.2 for around $3000. Reviews of both (by Michael Fremer) available on the Stereophile website.
Post removed 
would you consider a CAT preamp with SS amp?
If your up to putting together a kit  (you can buy them already assembled if you pay extra) look at the VTA-125. 125 watt monoblock tube amplifiers from Bob Latino. I have these and they sound wonderful. Here is a link: http://www.tubes4hifi.com/bob.htm#M125 
Bob Carver 350 monoblocks.  It may be a bit out of your price range, but you may be able to find them used.  Just incredible amps! 
@erik_squires Keep in mind his budget...
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@thosb
A used RM-200 would be a good amp. BUT - tube power is expensive, which is why in the old days there were horns. Without going that extreme, I am wondering if it might be more practical to change out the speakers instead of changing the amps.

The VK-55 is about 55 watts. The RM-200 is thus about a 6dB increase in power. The Thiel has a sensitivity spec of 87dB, but being a 4 ohm load this makes its actual efficiency about 84dB - borderline criminal as with such low efficiency, its hard to find an amp that can make enough power and *sound like music* at the same time. The RM-200 would be on a *very* short list!


How this works is sensitivity is a **voltage** measurement- 2.83 volts on the speaker measured at one meter. If 8 ohms, that's 1 watt; at 4 ohms that's 2 watts- meaning that the speaker's efficiency is 3dB less than its sensitivity. IOW it takes 2 watts to make 2.83 volts on a 4 ohm speaker.


Now efficiency is a **far** more useful spec when dealing with tube amps- this is because tube amps don't double power as impedance is halved; instead cutting power in half as impedance is doubled (if running a lot of feedback). In a nutshell efficiency lets you know if your tube amp makes enough power to work with the speaker. Sensitivity can fool you.


Its far more practical to run a speaker of higher efficiency. Let's explore this:
A speaker that might easily keep up with the Thiel (from a sonic perspective- you don't have to give up sound quality with higher efficiency) might be one of the ZU Audio offerings which you could easily find used within the price range specified. Now ZUs are pretty easy to drive and about 98-99 dB with 1 watt. So let's do a **tiny little** math:

With a 50 watt amp the Thiel can make x volume at the listening chair. Now each doubling of power is only a 3dB increase. The ZU is about 15 dB more efficient. I don't want to lose anyone here by going too deep into the math of decibels, so let's go with a 6dB difference- to make the **same volume**, a speaker 6dB more efficient needs only 1/4th the power. So if we had a speaker that was 90dB, a 12.5 watt amp would play the same volume as a **50 watt** amp on the Thiels! But the ZU is about 9 dB more; if we break that up in 3dB chunks and start with 6dB more, to make the same volume level as a 50 watt amp on the Thiel we would need only about 3 watts. Cut that in half again since we have 3dB left and you see how impractical it is to throw power at a speaker that inefficient!

The actual math- 55 watts- a 15 dB increase in speaker efficiency- results in a 1.739 watt amp able to make the same sound pressure. Think about it.


With 55 watts you would have nearly unlimited power- which is what you want- at normal volumes the amp will be loafing (which is also what you want) and so your tubes will last a lot longer.


+1 on VTA M-125.

I've owned several KT-88 amps since Citation II way back. These are gorgeous sounding. Drive 4Ω Spica TC-50 with ease.

If you are at all handy, buy the kit and assemble yourself. I'm an engineer, could have easily done, but couldn't be bothered. Now, I wish I had. The build was not equal to the images and cost 3x what it should.

PM for details
I just walked over to my 12wpc tube amp and yelled at it to let it know I'm aware of all the loafing into my 99db speakers. I used to have it matched with 90db or so speakers, and you could make that amp sweat bullets...now it just loafs along...I hope I didn't hurt its feelings.
Wow, great responses, here's what I am thinking.

Different speakers - I have considered this as a next step, and was thinking about Zu, Devore 3XLs, and Joseph Audio RM-25si, all of which I think would get my amp closer to the loafing so enjoyed by wolf_garcia's Carys.  Any others I should think about?  Maybe start another thread...

atmasphere, thanks for the math/design explanation, conceptually I knew this but the details are VERY illuminating.

erik_squires and testpilot, I would love to end up with combo of tube pre and ss or d power, but that's too far of a $$$ leap right now. I have also considered a Schitt Freya through a Krell 305il, an integrated that I don't use as I very much prefer the BAT with the Thiels.  Low cost, low risk, plus I could finally try a BAT/Schitt system.

I am very much intrigued by the ideas of building a kit amp, but imagine I would need to tune up (learn?) some soldering skills and have some time set aside to do this, but if I did, I imagine I would go down the 300B route, as I was inspired by Herb's recent review of the EleKit, which in turn gets me back to high efficiency speakers.  Eventually I do want to hear what the fuss is about with 300Bs.  But I will look into the VTA.  Bonus here is that if I built an amp from a kit, I'd have the confidence to rebuild the xovers on the Thiels!

And fyi, Underwood's recent offering of a Music Reference RM-200 mk2 (and Fremer's review, and my hunch that the current capabilities of the VK-55 are what makes it sound good with the Thiel's to begin with) is what got this ball rolling.

Thanks all - cheers and enjoy the music!


I ran my Thiel 2.4s with a VAC PA 100/100 that I had factory upgraded to the same front end as the phi 200 VAC. Has a 2 ohm and 4 ohm speaker tap. I would look to spend a little more if you want the iron (transformers) to handle your speakers. A used VAC phi200 could be on your list with a stretch. 
Years ago I owned the Pass X250 amp w/the power meter on the front and borrowed a pair of 2.3's from my local shop they had for sale used and it was amazing how they buried the needle on the power output needle on the faceplate ie took a huge amount of grunt to power them.  After that I owned (unfortunately:) a new pair of 2.4's that weren't quite as tough the load on the Pass speedo that the 2.3's were.  I'd go for the RM200 if you're stuck on the 2.3's.  If you're gonna do tube amps get some speakers that will do them justice.  cheers & have fun
Different speakers - I have considered this as a next step, and was thinking about Zu, Devore 3XLs, and Joseph Audio RM-25si, all of which I think would get my amp closer to the loafing so enjoyed by wolf_garcia's Carys. Any others I should think about? Maybe start another thread...
Watch the impedance. If you want the best performance out of a tube amp, the impedance best not be under 8 ohms especially in the bass.

Sorry, but why You need 50-100 W, that You never use, and pay too much money?....Better variant is - speakers with 94 and more sensitivity and quality SE with 4-8 W /that is better than PP with sharp and metal sounding, caused by switching signal between first and second shoulders of PP construction/. 
So...

I may have what you need.  I have an ARC VT100 mkiii, an ARC VT200 and a pair of ARC VTM200’s.  All are recently retubed and in 9/10 condition. I am the original owner of the VTM200’s.  The other 2 were demos.

What I like about these amps is the they have 4, 8 and 16 ohm taps. I would use the 4 ohm taps for your speakers. You can check them at www.arcdb.com.

chip houst




OK- we can do the math again. 200 watts is about 4x the power of the amp in the OP. That's about 6dB.


So the speakers are ***84dB 1 watt/1 meter***! If a speaker that was only 90 dB (which is easy to find) the amp he has now would make the same sound pressure. The thing is, the bigger you make a tube amp, the harder it becomes to make an output transformer that has bandwidth. Because big tube amps are expected to make bass, often its the highs that suffer but often the bass suffers a bit too. The only exception to this rule is if an OTL is used; then its possible to scale the amp to any power without loss of bandwidth.


But a 200 watt OTL on this speaker is impractical- and that would be only to maintain the bandwidth he has already. So it makes more sense to change out the speaker for something more efficient than it does to change out the amp- the latter is nearly a Sisyphean task. Even with 200 watts this speaker isn't going to be able to make satisfying sound pressures in most rooms if you want to play anywhere near realistic volume levels.

But you don't have to lose any resolution (in fact many think that you gain resolution) with higher efficiency speakers. In my example of my first post on this thread, I showed that an amp of 1.7 watt could keep up with his BAT amplifier if that amp was on a set of ZU Audio loudspeakers. This means that a 7 watt amplifier could keep up with a 200 watt ARC in the same circumstances.


The bottom line here is its very obvious that the speaker needs to be replaced if tubes are going to be used. 


All this talk about volts and sound pressure! If you are looking for loudness, get a DJ set and make your dishes rattle. If you are looking for good sound with good dynamic range WITH YOUR SPEAKERS, don't stop with calculations.

Math is only 1/2 or less of the equation. Tube amps react (!) to the internals of speakers in ways that are difficult to quantify - inductance, reactivity, impedance curves etc.

Efficient speakers MAY help, but an efficient speaker may introduce a difficult load for that particular amp. The crossovers or some other feature may confuse the amp.

I know this doesn't help, but the only way to know if any amp will be happy driving your speakers is to listen to it, with your speakers.

More often than not, I have been disappointed with the sound of an amp, after I've done all the math and "calculated" what will sound good with my speakers.

Living in a rural area with few nearby dealers, I finally resorted to taking my speakers to the Big City and arranging to audition a few amps with known sources and familiar material.

Better yet, find a dealer who will let you take 2 - 3 amps home for several days to listen in your environment.

Or you might try a used Citation II from one of the updaters - they handle difficult loads with strange impedance curves pretty well. KT88s and legendary transformers. Just have spare tubes on hand. (But that's the price you pay with any tube amp.)


OK, my takeaways are - 
1) Get different speakers (currently favoring the Devore 3XLs based solely on web based research) or
2) Go tube pre and very high power amp or
3) Try a Music Reference RM-200, no holding my breath for sonic epiphany (and also being wary of falling boulders, while Persephone (my wife?!?) looks on from the kitchen).

mikapen - I would love to find that dealer w/ amps I could demo at home, but living two hours from civilization makes this tough.  I did actually find a dealer in the big city who just happened to have the 2.3s sitting around, let me try several amps with them, which is how I started down the tube amp road in the first place.    

Great discussion and insights, this has helped me make informed choices, plus entertaining as always, thanks all.
Have you considered running a powered sub and limiting the signal to the Thiels?   I have the Thiel PCS speakers, which are basically the top end of the 2.3/2.4's, with a powered-Sub, all driven by an ARC VS-55 (puts out about 45-watts a side) that gives me near concert level volume.  Fyi, I use a pre-pro to handle the speaker setup.

btw, I would highly not recommend buying anything that you can't return based on internet reviews.  I have been utterly destroyed by that in the past.  It's pretty amazing how erudite some def people can be!
@atmasphere  and OP,
Wouldn't using a pair of Zero Autoformers help with the low impedance issue?
Bob
Efficient speakers MAY help, but an efficient speaker may introduce a difficult load for that particular amp. The crossovers or some other feature may confuse the amp.
IME this is an *extreme* rarity. Usually efficient speakers are a much easier load; in fact I've never seen an efficient speaker that was harder to drive. FWIW most speakers of higher efficiency don't have weird phase angles; that is more commonly seen in lower efficiency speakers. So the crossover comment seems right out.
Wouldn't using a pair of Zero Autoformers help with the low impedance issue?
No. If anything, the terrible inefficiency of these speakers is more of an issue than the impedance, although the impedance isn't helping.


I have thought about using a sub fed from pre-pro, that could be another potential path, any recommendations?  Is it worth it to go the whole enchillada room correction route with something like the DSPeaker anti-mode 2.0 or similar,or is any preamp with dual outputs ok (already have a sub).
You have to check the input impedance of the sub in parallel with your amps and then see if the preamp is OK with that.
Yeah and the more I think about it, if adding power is not going to significantly change SQ, why would offloading the bass to a sub do much either?
If the amp that drives the Theil no longer has to make power for the bass, there will be more power for everything else.
IME this is a really tricky way to go since active electronic crossover (which is what you need to see to it that your amp for the Theils does not have to deal with bass) are another block in the signal chain and so some immediacy is lost.