Living With A Class A Tube Amp

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 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.

@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.

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.

For your consideration

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