Living With A Class A Tube Amp

Amplifiers employing tubes often present a larger, deeper sound stage that some term ‘halographic.’ Tubes also have a distinctive distortion signature that I notice most with guitar treble. Jimi Hendrix and Stevie Ray Vaughan solos sound like they’re playing different instruments when reproduced with transistor vs. tube amps. The transistors result in clearer, cleaner sounds; but the tubes sound like what I remember from the 70’s (in Jimi’s case). That reverberating, echoey sound can’t be reproduced by transistors, in my experience. 

There was a time when this was true but no longer. I'm not saying that all solid state amps can do all the things that tubes can, but there are some out there and notably, are not as expensive as tube amps of the same power. For me this is about good low level detail retrieval, with an involving sound that is smooth without harshness or brightness. Tubes used to rule the roost on this but that has changed.
 

Thank you for providing some information from an owners perspective.  I watched one of Rogers videos and it was instructive. 

@jrcotner I'm also an owner and purchased a Rogers KWM-88 two and a half years ago and I absolutely love it! Coincidentally, I also replaced a Prima Luna Dialogue, which I still have. I'm pushing a pair Focal Sopra 2s and the results are absolutely amazing!! I get the best of both worlds, power and dynamics and holographic imaging and deep soundstage. This will improve your SQ substantially and then your focus will be on improving your source and other components. 

I've been running the same two pair of KT88 and KT150s and I'm sure the 150s must have 3000 hrs and still going strong. Yes, I listen a lot, mostly because I can't pull myself away until the wee hours. 

I think your placement of the amp should be fine, the same as any other tube amp. It may run slightly warmer but not to the point of requiring anything special. 

You've received a lot of great information on this post from some of the best minds in the business but at the end of the day, we typically have one amp/system and we want that system to be engaging (when it all comes together).

Finally, as @wsrrsw mentioned, Rogers provides great support if ever needed. His amps really are built like a tank and will last generations. If you haven't already, check out some of his videos you'll get an idea of the quality, engineering and extensive  testing he puts into his product.

Good luck on your decision and enjoy the music.

You most likely have either ultralinear or pentode

pentode being a purer circuit without all the feedback as in a Ultralinear circuit.

I always thought that ultralinear was synonymous with pentode, and that triode was the purer circuit. Was I mistaken?

Amplifiers employing tubes often present a larger, deeper sound stage that some term ‘halographic.’ Tubes also have a distinctive distortion signature that I notice most with guitar treble. Jimi Hendrix and Stevie Ray Vaughan solos sound like they’re playing different instruments when reproduced with transistor vs. tube amps. The transistors result in clearer, cleaner sounds; but the tubes sound like what I remember from the 70’s (in Jimi’s case). That reverberating, echoey sound can’t be reproduced by transistors, in my experience. 
 

I have both tube (Rogue Audio Stereo 100 Dark) and transistor (McIntosh MC252) amps fed by tip-of-the-line preamps of their respective lines. I find them to perform equally well, but they are not identical. Currently, I find myself leaning more toward the sound of the tubes, but it is so close; at least with the different speakers, each system seems offer its own unique contribution and I frequently run them both at the same time. 
 

Note: my response was prompted by @rooze ’s contribution, I didn’t intend to cop the thread at all. 

@jrcotner 

1. Class A tube amps have desirable sonic qualities, but other current tube or solid state amps can produce equally good sound quality without the drawbacks listed above.

That came from @atmasphere and while I agree with the majority of his posts, and learn a lot from him, the point above is too subjective. I've heard him comparing the latest Class D amps to tubes and using the 'equally good sound quality' comment. I've yet to have that experience. (But I also haven't heard Atmasphere's own Class D amps.)

There's a couple ways that good tubes present themselves that I haven't heard from solid state/Class D. 

PS There’s an 88 on ebay. EBay takes 13% that a phone call will save you that and maybe more. 

Here’s one on Agone

I have one of his AMPs (KT-88) driving Sonus Faber Guarneri Evolution Loudspeakers. Like you I had a PL (HP) amp. The Rogers is a terrific upgrade. (Pls see my system page). You not only will get a great AMP but if needed Roger is there. Rarely is the volume past 9 oclock on the dial. I’m N Cal if you are here too. 

Heat rises so as long as what’s above AMP is clear yr. good. A heavy table should be Aok

My tube set is 4-5 years old.  The few other Rogers owners I have had contact with are all happy campers. Dig daddyoh?

A 200 Watt amp will use 0.2 kilo-watt-hour in 1-hour. So, 5-hrs/day x 7-days/week gives you 7 kWh. At current Minneapolis rates that's about 17.7 cents x 7 = $1.239 cost per week, or $5.31/month. No biggies.

I second having Brandon build you a pair of SET amps.

The tubes will last a long long time. 
I’m powering my 92 db speakers 4 ohm with  833 monoblocks at  33watts per channel and they have more thanks enough power. 
 

I use a push pull with a quad of KT88 tubes; it consumes about 2-300 W and has an output of about 35 wpc. I definitely generates some heat. The tubes themselves are like exposed light bulbs. The majority of the heat seems to come from the transformers. Mine sits on a shelf with at least 6 inches of free space around it and nothing above. If you put a hand within about a foot, you will feel significantly radiant heat. 

I have 4 single ended amps, 2 SET (triode), 2 SEP (pentode) ranging from 8 wpc to 23 wpc driving several different speakers that range from 88dB to 99dB sensitivity.  I’ve never had the volume pot on the 23 wpc “beast” past 11 o’clock on any pair of speakers I own.

The Sonus Faber VIII looks to be 90dB sensitive, and the Rogers @ 55 wpc. Unless you’re seeking concert hall or jet engine sp, the Rogers should drive the Sonus Fabers just fine.  You’ll likely only push 10-12 watts, or less, at normal listening levels, its the dynamic transients that you need extra headroom for.  
 

Rogers specs the EHF 100 @ .01% THD @ 55 watts, which is incredibly low for a tube amp. A good 300b SET will hit 3-5% THD at 8 watts. You’ll have little to no distortion or clipping with transients with those specs. Rogers looks to be a very solid build, my guess is it would drive the h**l out of the Sonus Fabers! 

Thank you for all of your responses.  I sifted through them and here's what I've been able to discern:

1. Class A amps produce more heat than typical amps.  Placement may need to be considered depending on wattage and style.
2. The additional heat may reduce tube life by as much as half. 
3. Operating costs will be higher due to more frequent tube replacement.
4. Class A tube amps have desirable sonic qualities, but other current tube or solid state amps can produce equally good sound quality without the drawbacks listed above.

I can't believe there is a guy called Roger at Rogers High Fidelity. 

You most likely have either ultralinear or pentode 

pentode being a purer circuit without all the feedback as in a Ultralinear circuit.

no need to bias tubes.

Don’t you still have to adjust the bias on the one output tube per side that you do have? That is, unless it is self biasing?

One of the best ways to leverage class A: 10 watts or less of single ended triode with a speaker of 97db or greater sensitivity.  Minimal heat, minimal warmup time, long tube life, no need to bias tubes.

Any Rogers owners out there want to chime in?

@immatthewj FWIW, 'pure class A' is a marketing phrase. As far as engineering goes, a circuit is either class A or its not. None of this 'enriched class A' stuff you often see (such amps are class AB). In your case my guess is class A1.

I thought after I read your postings on the different categories of Class  A that "pure Class A" probably was the manufacturer romanticizing the owner's manual.  Thanks for the info. 

@immatthewj FWIW, 'pure class A' is a marketing phrase. As far as engineering goes, a circuit is either class A or its not. None of this 'enriched class A' stuff you often see (such amps are class AB). In your case my guess is class A1.

@atmasphere  , thank you for the explanation--that is interesting and makes me a better informed hobbyist.  

"Pure Class A" was a direct quote from the owner's manual, so that is all I know about it, as my ability to comprehend electrical theory is limited.

I always thought the room got warmer when I used to crank it way up, but that typed, these days I listen at low to moderate levels, and after a couple of hours, if I put the palm of my hand on one of the transformers, it is hot enough to make me remove it (my hand) fairly quickly.  And, after a few hours of listening at low to moderate levels the room still gets warm.  So I do not know for sure that  making it scream had it running hotter or not--I just felt that it must be; and I also do not know which category of Class A Cary was referring to by "Pure Class A"; until now, I was not aware of the different categories.

If it makes a difference as to how hot it gets, these days I always run it in triode.

@immatthewj Class A, if we are talking about tubes, comes in three flavors, A1, A2 and A3 (recently patented).

If the amp really does get hotter when you push it and it really is class A, then its class A2. In class A2, grid current can exist during part of the audio waveform so the driver has to be a bit more robust in order to maintain linearity during that portion of the cycle. Our OTLs are class A2.

@atmasphere  ,

quoting from my owner's manual:  "CIRCUIT TYPE:  Push-Pull Ultralinear Amplification in Pure Class A"  then "POWER OUTPUT 50 watts-Triode  100 watts - ultralinear."

As always, I defer to your knowledge on these subjects.  I had just assumed that when I was blasting away at ear bleed levels and the room was getting warmer the output tubes were taking a beating.  I also acknowledge that a lot of my preconceived notions are often wrong.  

I will say that this current set of EL34s is holding up better than the last set, but I am sure that there could be other reasons for that.

For your consideration

https://www.jadis-electronics.com/en/i70-57.php

Sometime in early '20 I replaced them with a set of the reissued Russiian Tung Sols and they have held up beautifully (however, I don't run them as hard and no mishaps involved).  

@immatthewj If the amp really is class A, you do run them as hard. How loud you play the system has no bearing on the heat or tube life.

If the amp is class A1, if you were to put an Ammeter in series with the AC line, you'd find the current draw unchanged regardless if the amp is at idle or full power.

This is not a class A amp. It is a push pull.

@carlsbad2 Push pull amps can be class A no worries. The Dynaco SCA35 using a pair of EL84s was a class A PP integrated amp. We’ve been making class A triode PP OTLs for decades.

If you were not aware that amps other than SETs can be class A, now you are. The trick is just the same as in an SET- the power tubes do not go into cutoff during any portion of the output waveform right up to full power. So you can also have class A solid state PP amps too.

@atmasphere  , thanks for clarifying that.

@jrcotner  , my old Cary V-12 is switchable between 100 wpc ultralinear and  triode, and in triode Cary says it is 50 wpc class A.  Yes, it does run hot, but I have not had to do anything special with it that I didn't do with my two previous tube amps.  I bought a set of 12 EL34s (reissued Russian Mullards) near the end of 17 and after a bit two years of being pushed pretty hard and a couple of mishaps they were sounding tired and the bias was doing funny things.  Sometime in early '20 I replaced them with a set of the reissued Russiian Tung Sols and they have held up beautifully (however, I don't run them as hard and no mishaps involved).  

As long as it doesn't get way too close to the curtains, I think you will be okay.

Thanks for the additional information.  I always learn so much from these posts. 

This is not a class A amp. It is a push pull.

@carlsbad2 Push pull amps can be class A no worries. The Dynaco SCA35 using a pair of EL84s was a class A PP integrated amp. We’ve been making class A triode PP OTLs for decades.

If you were not aware that amps other than SETs can be class A, now you are. The trick is just the same as in an SET- the power tubes do not go into cutoff during any portion of the output waveform right up to full power. So you can also have class A solid state PP amps too.

@jrcotner Tube amps running class A have hotter running power tubes and that always means shorter tube life on account of the heat. To get the most out of the tubes they should be kept free of dust and fingerprints, have adequate ventilation and the speaker load should be benign (they will run hotter if the speaker is 4 Ohms on the 8 Ohm tap for example, because more of the power they make will be dissipated in the tubes themselves rather than in the load).

There are amps of other classes of operation now that have all the good properties of the best of class A; this is true of even some class D amps now.

The reason for class A is to maximize linearity out of the output section; the less work the feedback (if any) has to do to correct the output the better. In the old days I think one concern was crossover distortion but I’ve yet to see a PP amp that actually had a problem with this as long as it was biased correctly.

In case you’re not aware of it, the differences you hear between amps is mostly the different ways those amp make distortion. IOW the distortion signature of the amp is also its ’sonic signature’. Once you know this, then its easy to see that the distortion signature is actually what is important and not the class of operation.

Thanks for the useful information.  I'm not concerned about household power usage, since the amp is on no more than four hours a day.  But I do want to have my eyes open if the tubes need to be replaced more often, which is an expense worth considering.

I have Sonus Faber Amati which I am powering with 70wpc of triode tube power. It is more than sufficient. So, I would think the Rogers amp should be fine.

I cannot imagine there is anything wrong with your placement. The tubes are in the front. They are not going to get that hot. Running in class A will reduce the tube life.. so the output tubes will need to be replaced more often… maybe 2,000 hours… ask Mr. Rogers, and check on line about the price of replacements. I doubt it is a big deal.

Finally. Sure they use more power. So what… how long do you have them on a day? An hour? When my air conditioner is on round the clock it cost two or three bucks a day… and the amp is going to be a fraction of the power.

Uh Huh....

You may notice a bump  In your electricity bill Kwh usage and increased heat output compared to the PL- that's about it.

No additional accommodations necessary based on how you describe placement.

This is not a class A amp. It is a push pull.

From the Rogers website, see below:

’’Class A Design – The EHF-100 MK2 runs in full Class-A, which means the output tubes always pass current. This ensures that the power tubes are always operating in their optimal range, and yields the ultimate accuracy in music reproduction.’’

See website here!

Mike