What s the best Class A integrated tube amp over 80W

I read that biasing VT100 tubes is a big headache. Why is it such a headache?

I’ve heard the VT100 a few times but never biased it so I do not know the process. But one of the tube amps I have is a Canary 301 mk 2. To bias the Canary, I need to flip the amp and remove the bottom cover. Then with a volt meter, put the probes on specific locations in order to bias each power tube. The first few times I did this convinced me to sell the amp. But I missed the Canary so  I purchased one again. Now that I’m more familiar with biasing the amp, its not an issue for me. I set the bias only once per year and that’s good enough. Canary has since made biasing procedures easier on all their amps.  I like easy biasing but sometimes you can’t have everything. Also, most of my amps are very easy to bias so if I have 1 or 2 that involve a little skill, I can live with that and it makes this hobby even more interesting. 

@chungjh ... do a search in the Forum for old posts about the ARC VT 100.  I recall that some years back, a couple of techie members explained why biasing the VT100 is so hard, ... and a tad dangerous for non-experienced, non-techies. 

I don't recall if you mentioned your budget, but someone mentioned the ARC Ref 150SE as a possibility.  I own that amp and it is a stunner.

I have question for you that should be answered by a techie.  If your ESLs have an impedance of 30 ohms in the low end of the spectrum and 3 ohms in the treble, is a tube amp a good choice for you?  Your ESLs operate almost like a giant capacitor.

I ask because many tube amps have high'ish output impedances.  Even the Ref 150SE has a "low'ish" (for a tube amp) .55 output impedance off the 4 ohm taps in the bass region; the amp's output impedance creeps up with frequency. 

I mention this because a tube amp's output impedance may color the sound based on Ohm's Law principles.  See, e.g., John Atkinson's bench test report on Ref 150 at: 
Audio Research Reference 150 power amplifier Measurements | Stereophile.com   

Based on the impedance figures you mentioned, I calculate that if you used the REF 150SE as your amp, the treble could be shelved down by approximately 2 db as compared to the bass.  Check with a techie.  You could even call ARC customer service. 

Or, if Ralph Karsten (Atmasphere) catches this thread, perhaps he could weigh in.  Ralph manufactures high quality OTL tube amps.  I recall that Ralph posted a while back that he uses his amp to drive ESLs.  Maybe Ralph could explain the whole impedance matching business. 

Good luck with your search.

Bruce
       

I have talked to Ralph (AS) about this. SS output doubles with every doubling of impedance. Therefore my Pass Labs X250.8 SS amp produced about 60W at 30HZ with 30 ohm impedance. On top of that bass frequency takes more power to make the sound than treble. Therefore, Soundlabs sounds too bright at treble with most SS and sounds better with tubes which don't double output with halving of impedance.

I don't know why but I don't know anyone who uses an ARC amp with SL but many who use Atma Sphere amps.

@chngjh,

I would never contradict Ralph.  However, I think you may have somewhat reversed what Ralph may have said about amps that operate under the so-called voltage paradigm (i.e., constant voltage at varying speaker impedance levels -- most SS amps which have near zero output impedances) versus amps that operate under the so-called power paradigm (i.e., constant power at varying speaker impedance levels - generally, tubes amps that have high'ish output impedances).  Ralph wrote a white paper on this topic.

Just an fyi, the reason the ARC Ref 150 SE operates as though it has "low'ish" (but not zero) output impedance off the 4 ohm taps (.55 ohms in the lower frequency region) is because the amp uses 14 db of negative feedback (NF).  Some folks and manufacturers prefer amps that use zero NF.

In the case of a SS amp, voltage generally remains constant at all frequency levels, but power output (wattage) will increase as speaker impedance decreases and amp power will decrease as speaker impedance increases.  So, for dynamic speakers that present low impedance levels in the bass region, a SS amp will produce more power than in the frequency regions where the speaker impedance is higher (often at driver crossover points). 

The reason the SPL of a good speaker remains relatively flat at most frequency levels if driven by a SS amp, despite the varying speaker impedance levels, is because the speaker was designed and voiced to be driven by a SS amp. 

You posted that: 

SS output doubles with every doubling of impedance. Therefore my Pass Labs X250.8 SS amp produced about 60W at 30HZ with 30 ohm impedance. On top of that bass frequency takes more power to make the sound than treble.

That seems somewhat odd and contradictory if you were using a SS amp with near zero output impedance.  This is because your ESLs present 30 ohms of impedance in the low frequency region and just 3 ohms of impedance in the high frequency region.  If you used a SS amp, I surmise that bass would be attenuated and treble bright, ... which you kinda indicate in the quoted text below.   

That said, you also posted that:

Soundlabs sounds too bright at treble with most SS and sounds better with tubes which don't double output with halving of impedance.

On the other hand, it may be that your ESLs were voiced to be driven by a tube amp that has a "high'ish" output impedance.  For example, I think many Atmasphere amps have an output impedances in the 4 ohm space. 

Rather than me being disagreeable for the sake of being disagreeable, I think what is important in the end is whether a particular tube amp will mate well with your ESLs.  Aside from electronic stability issues of mating a particular tube amp (or any amp) with your ESLs, I would expect, if it was my setup, that the amp/ESL combo would produce a relatively flat SPL over the audible sound spectrum.  

So returning to your original question, your OP asked for Class A tube amp suggestions to mate with your ESLs.  I think there have been several suggestions already offered.  I do not think ARC amps are Class A, but some type of hybrid Class A/Class A-B variant.  Further, if your ESLs mate better with high'ish output impedance amps, ARC amps may not be a good fit because many of the amps in the ARC line have low'ish output impedances.

In any case, I'll add to the list Atmasphere tube amps, especially if Ralph thinks they would mate well with your ESLs.  I would also have a chat with Sound Labs.  The manufacturer may have some good suggestions too.

Good luck with your amp search and happy listening.