Les dette, ardilla:
OTL-amps-pros-cons
Question. So just why are tubed OTL amps so technically flawed?
Answer. All power triodes, including "low impedance types" such as 6C33c and 6AS7/6080 the minimum load
for class B should never be less than twice the Ra as it given in the data sheets.
6AS7 with both its triodes paralleled has Ra = 140r, so the RLa load should be a minimum of 280r.
6C33c, with Ra at 80r, should not have a load less than 160r.
But most speakers are between 3 and 9 ohms, and where such loads are used with tubes without any OPT,
the tube is reduced to being a non linear current generator which is being force to work with what is a
virtual short circuit. So to get any useful power for the load there must be a pair of tubes at least and
they must only work in PP class B1 or B2. PP or SE Class A operation is not possible. Using multiple tubes
in parallel does little to improve operations.
The audio signal is grossly distorted when it passes through the very non linear Class B output stage.
A huge amount of GNFB is then required to clean up the mess to get similar measurements as for conventional
transformer loading, but it is impossible to fully achieve this. The output tubes are all too easily likely to overheat
and become damaged because the anode efficiency is extremely poor, and the heat generated in tubes due to
signals can all too easily exceed the rated maximum. Other parts or speakers may be damaged.
They say OTL amps give "very transparent sound" because no OPT is in the way of music in the signal path.
But the fact there is no OPT merely warps the minds of audiophiles who tend to irrationally believe in wacky theories
about stuff they just cannot and will not try to understand. And when you lead an audiophile over to where there
is a very good class AB PP amp or Class A SE amp - with OPT - they begin to question their belief on OTL.
They don't like being led anywhere though......
If anyone is very keen about tubed OTL amps, they should re-educate themselves starting NOW by un-learning all
they may have "learnt" by reading the huge amount of grossly erroneous information at numerous websites about OTL
on the Internet.
OTL amps became possible following the development of 6AS7 and 6C33c triodes and other tubes with high enough
Gm to allow grid drive voltages of under 50Vrms to generate an amp or more of peak anode current. Such tubes were
mainly used for pass elements in tubed voltage regulators for all sorts of tube gear so that a 6AS7 could spend its days
passing about 200mAdc, with Ea at 50V, and Pda at 10Watts. Such applications meant there was no need to have
multiple power pentodes wasting more power with a larger minimum Ea.
6AS7 and 6C33c can be used in audio amp applications with low B+ between about +80V and +200V.
Manufacturers of OTL have tried to convince the public that an expensive OPT is not needed to couple vacuum
tubes to a loudspeaker load. A minority of buyers were very easily fooled into agreeing.
Buyers hate paying for anything, especially if "something unnecessary" is included. But some ppl just cannot see why
tubed OTL amps did not conquer the world, and why they were almost completely forgotten, especially after solid
state was invented which became so much more reliable, after much trial and error, by about 1975.
After 1995, the Internet spread to the vast numbers of ignorant and non technically trained ordinary ppl, all somewhat
desperate to chat about stuff to others with an extreme shortage of real knowledge and experience. I have seen the
the discussion groups where someone posts the question," I'm thinking of making an OTL amp....." and the repliers
struggle to compose each tortured sentence and there is very little that can be learnt by anyone reading.
If anyone wants to understand OTL, and why they should not build one, then all answers are right here at this website.
The design proposed by the very dear Mr Tim Mellows uses 12AX7 at input, 2 x EF86 in a differential amp to drive a
pair of 6C33c with +/-150Vdc rails.
I've had a look at the chat group discussions and the Mellows schematic. IMHO, there is not enough detail
within the the sparse information to explain how it really works or to support any claim that the Mellows amp is worth building.
It was publicized in audioXpress.com magazine where many tube projects have been proposed. But I have never
wanted to build any single thing proposed by that Magazine, because it has always been so easy to build something
better if one uses one's brain just a little longer. I like a real amps for real men, not toys for boys.
Not many DIYer ppl build OTL amp kits. When they do, they soon learn about the smoke. "But I wasn't using much bias
current and the tubes just turned red hot..." Or, " I didn't have the volume up loud with 4 ohm speakers, but
the amp smoked..." > Ppl have spent large sums on brand name OTL amps with lots of tubes, and still they end up
being parked, after smoke and fused parts prevented further use.
Jeg har ikke hørt på W40 så jeg aner ikke om W60 er verdt ca 6000 kontra 3500 for W40 (iem + moms og fortolling). Men, siden jeg ikke hadde mulighet til å høre på før jeg kjøpte så var det bare å kjøre på med toppmodellen. 
