Most desirable vacuum tubes for SET amps

^^ At very low power levels the distortion of the amp is very low- hence that 'inner detail' you hear so much about. As the power increases distortion rises with it. If you are nearfield yous speakers may be efficient enough that you're not getting into trouble with distortion, but usually a 94dB speaker isn't enough.

Unless you're clipping the amp you won't hear outright distortion. At power levels less than clipping distortion imparts a tonality and at levels above about 20% of full power, a sense of 'dynamics' due to higher ordered harmonics present on transients.

 What do you think about OTL SET amps/integrateds, and the most desirable triode tube in that instance?  (I have a Vinnie Rossi L2iSE).

When you eliminate the output transformer, you eliminate with it the reason to have an SET. Its the way the SET makes distortion that makes them interesting to listen to; at low power their distortion is so low as to be unmeasurable- and this is the source of their 'inner detail'.

One argument that has been made for the last 30 years is that SETs sound better since there is no 'phase splitter'. That argument is hard to prove out in practice since there are so many variables involved! How many people have heard a push pull amplifier using exactly the same tube complement as their SET?? One thing I can state here is that they certainly won't be able to make that comparison if the amp is transformerless! DHTs have far too high a plate resistance value to be successfully used in an OTL, unless a number are used. We made a 300b-based OTL about 25 years ago that used four 6300b graphite-plate power tubes to make about 7 watts. Four 6AS7Gs (which are indirectly heated triodes) did a better job.

Recently and just for fun I've been designing a push-pull ultralinear tube amp that makes 5 watts/channel (ostensibly for my bedroom system, although it would make a good desktop or headphone amp too). Often when SETs are compared to push-pull amps the P-P amp makes a lot more power. I am interested to see what happens when the power levels are the same.

I also have a Dynaco ST-70, also running the EL34 in ultra linear. I love the sound of this amp as well but if I listen to the EL34 in SET triode and then listen to the Dynaco, the Dynaco then sounds very mushy.

Is the Dynaco sorted out properly? Older ST70s will need all the filter capacitors in the power supply replaced and everything spec'ced out to really assess that.

The best sounding SETs are the ones with the least amount of power. This is due entirely to the output transformer- the differences people talk about between the various tubes has more to do with the output transformer than the tubes themselves. Once you get over about 7-8 watts of power it gets extremely difficult to build an output transformer that has full frequency bandwidth- and as a result many have compromised bass response on that account.

So back in 1990 there was a mystique around the 300b; by 2000 the 2A3 had ascended the throne and these days its the type 45.


If you really want to hear what any SET does, that will best be done with a set of loudspeakers that are efficient enough that the amplifier is never asked to make more than about 20-25% of full power. This is to prevent distortion, in particular the higher ordered harmonics, from showing up on the transients present in all music. The ear uses the higher ordered harmonics to sense sound pressure, so when they show up on transients it tends to make the system sound 'dynamic' but its really distortion masquerading as dynamics.