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more reliable amp: tube or solid state class A

i got to reading this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1144724173&openfrom&1&4#1

i require no convincing that class A sounds better than AB or D or whatever else, but the efficiency is terrible, with the efficiency losses being reflected as heat.

and heat, as we know, causes thermal breakdown. this is a matter of engineering: the hotter a component runs, the shorter the mean time between failure. simple stuff.

but here's the question: if we took 2 equally hot-running amps, one tube and one SS, over the long haul, what would be more reliable? the tube amp, or the SS one?

i'm thinking the tube amp, solely b/c the tube is the hottest part, and its failure is accomodated for in the design (you simply plug in another tube). a hot running SS amp will eventually burn out resistors / transistors, and joe audiophile will be forced to send that to the factory for replacement.

(i am going to do some HVAC work on my room, and if i can keep in cool in mid July, i will be moving to the winner of this argument)

thx
rhyno

Showing 2 responses by atmasphere

atmasphere's avatar
atmasphere
12,088 posts
 
We've been making class A amps for nearly thirty years. In fact, other than a few prototypes, that's been the only class that we have indulged in.

So I can tell you this, and it is not a matter of opinion but is simple fact. It does not matter whether it is tube or solid state, what *does* matter is how conservatively the amplifier is designed to survive operating class A day in and day out.

Tubes of course have a service life that is different from transistors and if set up properly, will be just as reliable in class A as any other class of operation. The same can be said of transistors as well. Its all in the design- nothing with the device.
atmasphere's avatar
atmasphere
12,088 posts
 
Hi David, Your comment that the 6AS7 is not that linear is not really true. The curve has a lot in common with a 300b and also 2A3s; in that regard the 6AS7G is in good company. On that point the idea that the amp is class A due to the non-linearity of the tube does not hold up. BTW if you study the RCA documents that they released on the tube back in the early 50s, RCA called the tube an audio amplifier *and* VR (Voltage Regulation) tube. FWIW for best VR operation linearity in the pass element (in this case the 6AS7G) tube is important!

While power supply stability is important, I don't think I would say that is why you do class A. IMO, you do it because of the increased linearity of the device. Particularly in push-pull, very low zero feedback distortion levels are possible. Achieving low distortion without feedback allows for lower amounts of higher-ordered harmonics, which are loudness cues for the human ear. Sort of a have your cake and eat it too thing.

Our power supply voltages are not as high as you describe, and it is a fact that the tubes operate class A2, which is something that I am happy to point out should anyone ask; like, right now :)

In A2 there is substantial grid current during part of the waveform, similar to class AB2. Our driver circuit is designed to handle the current, similar to the way Fisher did their class A2 amp back in the 50s. The 6AS7G is remarkably linear in the A2 window, like a lot of power triodes are (we built a 300b OTL once just to see if there was any advantage; the 300b has a similar A2 window BTW).

If the bias and B+ points were a bit different (IOW if the tubes went into cutoff before clipping; right now they cut off only *after* the amp clips), the amp would be class AB2, not A2.

I am if the opinion that class AB is harder to design for and get right since the driver waveforms have to be larger amplitude and the driver power supply has to be more stable. If you are trying to set things up without feedback, which is what I would do, class AB gives you less opportunity for distortion cancellation so you really have to have your ducks in a row to make it work.
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