to make units cheaper with better profits.
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There are some theoretical problems with parallel SET topology. Some designers claim that it is hard to avoid one tube in a pair from becoming a "current hog" and seemingly end up doing most of the work while the other tube puts out much less output. There is also the claim that having multiple tube tends to make the sound less pure and a bit more "muddled" because the sound of the tubes is never perfectly matched (this would be an argument against ANY design with multiple devices providing the output).
I own a parallel SET amp (Audio Note Kageki). I like the sound so I cannot say whether these theoretical problems actually do affect performance. But, I will note that the output tubes do tend to become unbalanced over time, with one testing much stronger than the other in the pair for each channel.
The advantages of parallel output tubes in a SET are not only higher output. Using parallel tubes will also lower output impedance which means that damping factor will be increased or the designer can reduce the turn ratio between the primary and secondary windings of the output transformer.
I am guessing that there must be some challenges to making good parallel SET amps; my guess is based on the dearth of such designs on the market.
I would also dispute the notion that any particular design is inherently better than another. I've heard very fine examples of pushpull, SET and output transformerless amps (OTL). The amp I currently favor in my system is a pushpull amp (I still do like the Kageki, which I have loaned to a friend). One of my all time favorite amp is an OTL amp.
I built OTL 6AS7 GE & ECC81 and it is my favorit. In OT amps sound and the bass are defined by quality of OT. How sound the amp deppends how sound the OT /the best OT is missing/.About PP and SE. Crossing of the signal 0 between upper and down sholders of PP cause sharp and metal sound, that people define like detail sound.
No matter the topology of an amplifier each will have it’s advocates and invariably detractors, no exceptions. SET has been the most satisfying choice for me, yet certainty OTL or class A solid state for others. There’s variation and a hierarchy within each specific topology. I could never proclaim or bestow "best " for any of them. You listen and eventually you’ll find what suits personal preference and objectives. So many variables, all SETs don’t sound the same. Listen to for example OTLs, Atma-sphere, Einstein Audio, Joule, Tenor each has it’s own sonic signature. This is true for any and all amplifier genres/topologies.
Id like to change the discussion....What it should it not contain the tube amp? IMO, NFB that press the scene of the sound sending the part of signal back into the previouse stage. Unique amp must be built lineary without NFB. IMO, never use tone control that change the sound. If You need more bass, put big bass speaker, if You need more high freeq. - put sensitivity tweeters, if You need meedle fr. - put sensitivity fullrange not less 8 inch. IMO, Hi End is back proportional to the number of composite parts - use simple decisions with not cheep parts. Every passive part change the sound. IMO, in SE iron for OT must be 10-20 time more than output power, /for 5 W never use 5 W OT/. IMO, in power supply use kenotrones, that provide soft start and soft amazing sound. Every time use iron chock and hmmm will be far from Your ears. IMO, do not use iron enclosure that collect noises....But your opinions?
If you have a higher impedance speakers whose impedance doesnt dip much below nominal I dont think you can beat an OTL amp. To me efficiency is not as important as impedance. In fact 5 of use recently did a shootout using 4 different amps on a pair of DeVore 0/96s and the Atma-sphere 60s crushed a very highly regarded ($ 12K) P.P. KT-88 amp, an older Mastersound 32B SET integrated and a pair of Audio Note Parallel 300B amps. Funny thing is that when we bypassed the preamp section of the Mastersound it went from last place to 2nd. We used an older CAT Ultimate preamp for all comparisons.