Calling all SET fans

The 6BM8 combines a triode section and a pentode section in a single tube. The triode section is much more limited in power capability than the pentode section, so the triode section is normally used to drive the pentode section, which in turn provides the amp's output power. But the pentode section is itself very limited in power capability, such that to achieve 8 watts I believe the EE integrated must be using two of them in push-pull.

So as might be expected there are many variables that distinguish the EE integrated from a 300B or other comparably powered SET. One being single-ended operation of the power tube vs. what is probably push-pull operation in the EE. Single-ended operation avoids "crossover distortion," which affects the zero-crossing of the signal and therefore becomes increasingly significant at low power levels. Another being use of a triode power stage in a 300B amp vs. a pentode power stage in the EE, with good triodes generally being considered to be more linear (at least when not pushed in terms of power delivery). Another being that the 300B, as well as the lower powered 2A3 and 45 tubes, being especially well regarded for their sonics in comparison to most other triodes. Another being that since the 300B, 2A3, and 45 tubes are relatively expensive, they tend to be used in amp designs that are less constrained by cost considerations than amps that would be designed around less expensive tubes.

So as might be expected many factors are involved, undoubtedly including others that I haven't thought of.

Best regards,
-- Al

Al, you know the EE uses 4 6BM8, 2 a side ?
Thanks for the lesson , I really appreciate it .

I was just reading about the cool EE amp. It uses one of the triode sections for the input gain stage which then goes to the other triode section of the other tube to split the phase of the signal. The signal is then recombined in typical push pull fashion in the pentode sections of each tube. At least that's how I think I am reading it.

This phase splitting is the most crucial part of a push pull amp and if not done perfectly can diminish the sound quality of a push pull amp. i think in one of Shindos amps they split phase with an interstage tranny. Al might be able to better comment on the strengths and weaknesses of plitting phase with a tranny vs tube. Jet

Thanks, Jet. Yes, implementation of the phase splitter in a push-pull amp is certainly a critical aspect of the design. I can't offer any particular insight, though, regarding the tradeoffs between use of a transformer and use of a tube for that purpose.

I'd imagine, though, that as with many things in audio either approach can provide results that are either better or worse or similar to the other, depending on the specific implementation and the other aspects of the design.

Best regards,
-- Al

My understanding is use of one output tube versus multiple tubes that must be "integrated" well as discussed above is the inherent advantage of a SET.

The downside is the power limitation and the distortion that comes into play if the amp is pushed hard at all.

So its a very clean pure and simple circuit design that is the benefit but the "slack" must be picked up elsewhere, by the speakers which correspondingly have to be very efficient and a very easy load otherwise you cross into high distortion territory a lot faster than with other more "scaled up" designs.