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Sonic qualities of SET output tubes?

Can you readily identify a 300B, 2A3, 805, 211, etc., amp’s sound with your eyes closed most of the time?


If so, I’sure would like to hear from you.


Amplifier design and the technology utilized within its confines decides the ‘voice’ or influence it will yield as much or more so than merely the output tubes the designer has chosen to use.


I get that part emphatically. One must hear the amp regardless the type of output tube technology on hand.


And yes, some Pentodes and Tetrodes are used as Triodes but are not indeed triodes by their specific architecture. That’s OK, just focus on their use as Triodes herein, please.


There are however certain tube types, irrespective of vintage which have basic undeniable sonic colors or characteristics, apart from their electrical aspects which keep attracting people to amps which use this or that tube in its output stage.


Some love 211s. some adore 300Bs. Some love EL34s configured to run as Triodes. I have an affinity for the latter. So far anyway. This topic could change my mind.


Has your own experience informed you what this or that output tube’s natural flavor regularly announces itself to be so you can have a reasonable expectation of its general presentation?


What sonic attribute continually attracts you to a particular SET tube design, 300B or some others?


Or, conversely, what is it about the sound that would bring you to covet a 211 amp over a 2A3, for example.


Why as another example, would you pick a 2A3 amp over one using 805s or 300B, 211, etc. or vice versa?


Removing ‘vintages’ and electronic or electrical qualities from the argument, what sonic attributes for the more popular S.E.T. amp output tubes have you determined seem to persist in their particular DNA?


I’m asking for input from those SET tube devotees to lend their experiences and knowledge on the subject of what tube sounds like what irrespective of the SET application, generally speaking.


My goal is to try getting a better feel for which SET Tube amp design, if any, I’d want to pursue and possibly invest heavily into going forward as the soul of a new system.


Tremendous thanks to all!

blindjim
atmasphere's avatar
atmasphere
11,942 posts
 
@inna 

Ralph, would you like to tell us about disadvantages of OTL amp design? 0.5% distortion at full power is quite an achievement.
One thing is certain - more powerful SET amps done right are very expensive.
We get that figure without using feedback. With feedback the THD can be as low as 0.05% but we don't like feedback for the same reason that SET designers don't: it adds brightness.

OTLs have several disadvantages- speaker choice is an issue, as the smaller OTLs can't do 4 ohms very well. Generally speaking, if the speaker works with a low power SET it will work with a low power OTL too- the difference being that the 'low power' OTL will likely have several times the power of the low power SET.

The next issue is the number of power tubes, for example we use the 6AS7G which is good for about 10 watts. So you need a few of them if you want to make any power. Some OTLs use the EL509 or EL519, which are pentodes but the most common alternate is the 6C33. Four 6C33s will make about 60 watt in an OTL.

Finally, the smaller you make an OTL, the less efficient it becomes. So while our M-60 is perfectly comfortable on 8 ohm speakers, our S-30 is a lot more finicky about that- it can do well on some 8 ohm speakers but if there is a wild phase angle or the amp has to work hard it may not like it. Conversely, the bigger the OTL the more efficient it becomes so even though the class of operation has not changed, the bigger amps tend to have lower temperatures on each tube because more of the power is dissipated in the speaker rather than in the output section. 
blindjim OP
3,092 posts
 
leotis > Thank you for the very interesting answer!

Blindjim > No problem. I’d keep them on the higher taps for a while, listen, closely. That is of course once the amp has settled in on that portion of the circuit. A few days more or less should suffice. .

Another analogy for incorrect IMP matching of amps to spkrs could be like driving a 5 speed vehicle only in 2nd gear. It will take off and it will run but overall performance will suffer.

Another will likely be able to definitely say this more accurately, but I tend to think the taps on tformer output amps were designed to run in a range, not just one specific imp load. So I’m figuring the 8 ohm tap means about 8 ohms, and from above 4 to 8.
This is unless ther is a 6 ohm tap on the amp too. Then the range for the 8 tap diminishes to between 6 & 8 or above.
The 4 ohm tap is for that section of IMPs at or below 4 ohm loads.
Follow?

The only real worry is as said, clipping. Its when a driver is starved for power predominately. Usually it’s a tweeter. Ordinarily clipping occurs when running the spkrs at higher levels and the power a driver asks for can’t be provided appropriately or comes intermittedly. That’s when actual . damage to the loudspeaker can take place. It happened in a pr I had bought preowned. His INT did not have the power to satisfy those speaker’s demands and one of the four blew out. Quit working. The factory fixed it eventually. Once I paid for shipping and so forth.

inna
7,346 posts
 
Ralph, thank you, now I got it.  Yeah, relatively limited speakers choice. Number of tubes would not be a problem for me unless each cost $1k and lasted only for a year. It's a bit more difficult to do amp first approach than speakers first approach. But with Atma-Sphere and LAMM I would still do exactly that.
thom_at_galibier_design's avatar
thom_at_galibier_design
729 posts
 
Directly heated triodes are quite linear devices, and they tend to take on the characteristics of the driver circuit.

The larger transmitting triodes (211, 845, GM70) are particularly difficult to drive, which is why you see some of the esoteric Japanese designs employing power tubes for the job.

The 300B is an interesting animal. It has a stereotype of being lush to the point of sounding bloated and slow. Again, this is a driver tube issue (let’s not forget about adequate power supplies, of course). You’d be surprised how "quick" a 300B amplifier can sound when driven correctly and I’ve heard very few 300B SETs that get this right.

Everyone has their opinion about various architectures (DHTs vs. pentodes, OTL vs. transformers, etc.).  My stance has always been that there's a convergence in architectures as the designs get better and better.  While I'm a big DHT/transformer coupled fan, I recognize that this is personal preference (no accounting for taste), and not an expression of superiority of one architecture over another.

Cheers,
Thom @ Galibier Design
atmasphere's avatar
atmasphere
11,942 posts
 
Number of tubes would not be a problem for me unless each cost $1k and lasted only for a year. It's a bit more difficult to do amp first approach than speakers first approach. But with Atma-Sphere and LAMM I would still do exactly that.
We warrant the tubes in our gear for a year and they are relatively inexpensive.

The only real worry is as said, clipping. Its when a driver is starved for power predominately. Usually it’s a tweeter. Ordinarily clipping occurs when running the spkrs at higher levels and the power a driver asks for can’t be provided appropriately or comes intermittedly. That’s when actual . damage to the loudspeaker can take place. It happened in a pr I had bought preowned. His INT did not have the power to satisfy those speaker’s demands and one of the four blew out. Quit working. The factory fixed it eventually. Once I paid for shipping and so forth.
The mechanism for this is when the amplifier clips, the distortion generated is high enough in frequency that the crossover allows it through to the tweeter(s), which usually only handle maybe 2 watts or so. With solid state amps in particular, overloading the amp is a good way to toast the tweeters.

This is considerably more rare with tube amps because they generate less higher ordered harmonics when overloaded (which is usually caused by bass notes) but still can happen.

If you see tweeters blown and nothing else, its a sure sign that the speaker was exposed to an amplifier that was clipping. Most manufacturers would not consider that as a warranty claim as it is clearly abuse of the speaker.


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