Tube Amp Questions

Hello all!

I've decided to build my system around my Thiel CS 3.7 speakers. Currently, I'm using an ARC LS-7 tube line-stage (purchased just 3 weeks ago) feeding my old Denon PMA-2000 IVR integrated. Sources are laptop running Tidal & Roon, as well as Denon 2900 CD/SACD player.  Musical tastes are varied, but I listen mostly to classic rock, the blues, classic jazz & Latin/bossanova. 

My questions are regarding tube amps. One, in real-world experience, are they really that labor intensive to maintain & operate?  Two, the particular amp im considering is an ARC VS110. According to ARC, it draws 400 watts at idle, 620 at rated output & 900 maximum. How big of an impact does running an amp like this have on an electric bill? 

These are newbie questions, I know, but after a really long time as an admirer of the "High-End", I've finally decided to leave "Mid-Fi" behind, albeit by going the vintage/pre-owned route. 

Thanks for any input or reccomendations!


I, too, am new to tubes. I went with Atma-Sphere, and am super happy with my choice. 
The ARC is no slouch and would probably sound better with a better amp (tube or SS). IMHO.
I would PM Ralph Karsten (atmasphere) and get his opinion/knowledge.

I have a VT100 which is similar in size to the VS110 as far as output tube complement (8 6550 output tubes). They do generate a lot of heat and need some open space for cooling. If you're going to run it 8 hours a day you'll notice it on your bill. 8 hours a week who cares.

For the most part they are plug and play but they do need some occasional interaction. You'll need to check tube bias occasionally and every few years replace the tubes depending on how often you run it.

If you're of a mind to get a European sports car get a tube amp. You can tinker on occasion but it won't leave oil spots on the floor. And you can get years of enjoyment and pride.

If you're more a sedan type guy I'd look at something more solid state. A little more reliable and almost no maintenance. Nothing wrong with mixing a tube pre with a mosfet power amp.

As far as tube preamps are concerned there's not much maintenance until the tubes start to wear out and that can be years. Replacement is easy.
My suspicion is that the VS110, as well as many other tube amps, would be a poor match for the Thiel CS3.7. From Stereophile’s measurements of the speaker:
...the CS3.7’s impedance remains between 2 and 3 ohms over much of the audioband (fig.1), and that there is a demanding combination of 3.8 ohms and –40° capacitive phase angle at 60Hz. Thiel specifies the impedance being nominally 4 ohms, with a minimum of 2.8 ohms. I actually found the minimum impedance to be 2.4 ohms at 125Hz. The difference between 2.8 and 2.4 ohms is academic, either mandating use of an amplifier that has no problem delivering high currents.
Figure 1 at that link, which depicts the speaker’s impedance magnitude and phase graphically, further reinforces my belief that the CS3.7 is not a tube-friendly speaker.

Also, as stated in Stereophile’s measurements of the VS110:
The output impedance from the 4 ohm tap was a fairly high 0.72 ohm across most of the audioband, this rising to 1.3 ohms at 20kHz. The equivalent figures from the 8 ohm tap were 0.93 ohm and 1.9 ohms, respectively. As a result, the modification of the amplifier’s frequency response due to the Ohm’s Law interaction between its output impedance and the way in which a speaker’s impedance changes with frequency will be quite large.
So IMO, generally speaking, a solid state amp would stand a much greater chance of being an optimal match for that speaker. Although in choosing one keep in mind that its input impedance should conform to ARC's load recommendation for the LS7:
Recommended load 60K ohms and 100pF (20K ohms minimum and 1000pF maximum)
(The pF part won’t usually be a problem, unless the interconnect cables are unusually long. But many and probably most solid state amps will have input impedances below 60K ohms, although I wouldn’t worry about it if the input impedance is around 30K or more).

It may be worthwhile for you to search Audiogon’s "Virtual Systems" section to see what amplifiers others are using with the CS3.7 or similar Thiel speakers.

Good luck. Regards,
-- Al

almarg +1. Thiels love SS amps. The tube pre is O.K.
Speaking as a former Thiel owner (1.6 an 2.4SE).
150 wpc minimum.
I would suggest a Pass X..5 or .8  series amplifier size would depend on budget. Thiels like current and need to be feed properly to sound their best. 

Just FYI. I am using a Belles Reference 150a V2 amp with really really good results with my Thiel 2.4's. So if your budget is like mine this my be a very good amp for your 3.7's. Some people think that it's sound doesn't suggest a strong association with solid state, but sounds somewhere between tubes and solid state in a good way.
What about class D amps?
I use a Belles Reference 150a Reference V2 as well . You won’t do better under 5K and with a good tube pre more than that that .
Just took a pair of Theta Prometheus in Trade. $7500.00 a pair.
Hello all!

Thank you so much for all your feedback and suggestions...they were all truly helpful.

The big thing I'm taking away from this discussion is that, as one moves away from the more "standard" stereo equipment & into the realm of Hi-Fi, equipment matching is so, so crucial.  I now know why so many of us in the hobby agonize over the choices that make up our systems...the impact of any change is magnified on a whole new level with equipment that is so capable & so revealing.

I am going to take my time & try to audition several possibilities.  Will definitely keep you all updated.

Thanks again so much!

You are on the right track, regardless of what anyone tells you or the specs your ears are the ultimate judge. Have fun and trust in what you hear! And as you said take your time getting it right will be worth the time spent.

Interesting dweller-

which loudspeaker did you move to from the CS 2.4SE ?

Happy Listening!

New York Audio Labs NYAL Moscode 150 or 300. Superb sounding tube/MOSFET amps designed by George Kaye. Available on the second-hand market for quite reasonable prices!
The listing I posted is sold! My bad, but you get the idea. 

You didn't specify a budget but given the ARC option you mentioned, I assume you're looking for something in the $2K price range. With that in mind, there are lots of options out there if you're willing to go used, maybe even an older Krell or Mark Levinson, something that can handle a 2 ohm load without breaking a sweat.

How's your current 80wpc Denon managing such demanding speakers?


Well, the Denon doubles to 160 watts @ 4 ohms, so it works ok.  Truth be told, I got the speakers at such a good price locally that I'm starting with them as the building block and going from there.

The ARC VS110 I offered on went to someone else, hind sight, it was prob for the better as the majority of feedback I've gotten from here as well as from friends is that SS is a better option for Thiels.  

You're right:  I'm shopping in the $2K - $3K range.  There are two options that are catching my eye right now, a Krell KST-100 & a Pass Labs x150.  Both are Class A for some of their range, then switching to Class A/B.  

My concern now is my ARC LS-7 line stage.  How do I know if it will "match" these (or any other) amplifiers?

Arvin, according to its manual the Pass X150 has a specified balanced input impedance of 22K, with no spec indicated for its unbalanced input impedance. The unbalanced input of course is what you would be using with your LS-7. Most likely the unbalanced input impedance is much lower than 22K, quite possibly 11K. Per my earlier comments even 22K would at best be marginal for use with your LS-7, and 11K would be completely unacceptable. So I would not recommend that amp.

On the other hand, I looked at Stereophile’s measurements of the KST-100, and I see no compatibility issues whatsoever. The only concern would be the condition of the specific amplifier you may purchase, given its age.

Good luck. Regards,
-- Al


Hello Al!  Thanks for both of your posts...incredibly useful info!!!

If I may, one more question for you:  As I'm so new to the whole idea of component matching, using the example of mismatched impedances between preamp & amp, what does the result "sound" like?  In other words, how do I know by listening that my equipment just isn't "right" for each other?

Thanks for any additional insight you may be able to share!

The majority of tube-based preamps, including the LS-7, have coupling capacitors at their outputs. The impedance presented by a capacitor rises as frequency decreases, which can result in the output impedance of the preamp being much higher at deep bass frequencies than its specified value, the specified value usually being based on the output impedance at a mid-range frequency such as 1 kHz.

So if the input impedance of the amp is not high enough, and keep in mind that for the LS-7 ARC recommends 20K as a minimum with 60K being preferred, the interaction of that input impedance with the rise in the preamp’s output impedance at low frequencies will result in weak bass. Depending on how the impedances of the specific designs vary at other frequencies there might also be adverse sonic effects at higher frequencies, but their sonic character won’t have much if any predictability since those effects will be dependent on the specific designs.

The commonly cited rule of thumb to assure impedance compatibility for a line-level interface is a 10x minimum ratio between the input impedance of the component receiving the signal and the output impedance of the component providing the signal. However, what is often overlooked is that the 10x ratio should be applied at the frequency within the 20Hz to 20kHz audible frequency range at which output impedance is highest. And as I said manufacturer specs commonly just indicate a nominal output impedance that is presumably at 1kHz or some other mid-range frequency.

If the preamp or other component providing the signal has been reviewed by Stereophile, John Atkinson’s measurements usually indicate the worst case output impedance, i.e., the highest output impedance at any audible frequency. If only a nominal output impedance is known, though, IMO a minimum ratio of at least 50x, and ideally 75x, should be used, especially if the preamp may have a coupling capacitor at its output as most tube preamps and at least a few solid state preamps do.

All of which is not to say that there will necessarily be a problem if those ratios aren't satisfied; as I indicated it would depend on how the impedances of the specific designs vary as a function of frequency.  But it is to say that satisfying those ratios assures that there won't be a problem.

-- Al

Tube amps have a beautiful sound to my ears as compared to solid state but certain manufactures run the power tubes at such a high voltage that tube life can suffer.  Tube replacement and the change in sound they experience over their useful life are to be expected.  I recommend  sticking with a tube amp that has auto bias and sophisticated real time monitoring systems.
As always Al,s posts are incitefull and incredibly informative and I have never been steered wrong by his responses to some of my more "needy" threads.
Listen and heed his advice and you cannot go far wrong.
I have definitely learnt a lot from his and many others information passed along in these forums.
Good luck with whatever route you decide to follow.
Hey Arvin I say this with all sincerity if you don’t audition the PrimaLuna components you are doing yourself a tremendous disservice. I have owned and auditioned several tube and solid state gear the past 20 years and nothing has brought the musical joy as my PrimaLuna’s. Save yourself some coin and do yourself ur ears a favor. Unless your looking at uber over priced Reference components trust me you will not be disappointed with PL. Please give Kevin Deal a call at Upscale Audio and let him share his years of experience with you. Also visit the PrimaLuna website and look at there product line, reviews and videos. 
Hello all,

Just to provide some closure to this topic, I went with an Audio Research D240 Mk II. I was able to audition it in my system & loved how it drove my Thiels. There was a definite increase in clarity over the entire frequency range, but more noticeably, more power, “slam” & control at the bottom end. For most of the music I listen to, gaining this power & control was probably a good trade off between getting a “warmer” type of sound with a tube amp. 

In the end, I now have tubes in my LS-7 line-stage & in my PH-3 phono pre-amp to go along with the SS D240. 

Thanks again for everyone’s help & suggestions!

Thanks for the follow-up Arvin! You'd be surprised how many people don't bother...