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popularity constantly incurs; in fact, it has pushed it into the porno-site level of access.)
As for the MOSFETs, I once found a source for the BUZ900/905s through the Google search engine, but I did not save it to either hard-drive or paper. I believe the price was about $9 each. If you or (anyone else) do find a source, please relay it back to me and I will in turn relay it on to the readers.
Subject: variable mu/remote cutoff tubes?
How about some commentary on the so-called "variable mu" aka remote cutoff circuit designs? I may have missed an article on this subject in the Tube CAD Journal, and if so, please point me in the correct direction. <g>
Older audio compressor/limiters used this design technique as an early sort of VCA, and as a "child" who grew up on the cusp of solid state and vacuum tubes, it's all a bit voodoo to me.
Thanks for a great webzine!
Bri
Variable mu triodes have their uses and Eric Barbour of the late Svetlana and the late Glass Audio is probably the most knowledgeable fellow when it comes to using these tubes in modern circuitry. (He has started a company that makes tube based music synthesizers.)
Subject: McIntosh amplifier
As a reader of your invaluable Journal, I am surprised to see than no article has appeared about the McIntosh circuit. The circlotron being a variant of it, it could be interesting to have a look into this structure for your readers enjoyment and their general education...
Charles R.
Paris, France.
The problem with the McIntosh circuit is that it requires a specially made output transformer, which limits its universality.
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Still, the amplifier is an intriguing one, as it effectively makes for a very clean, very lean Class-AB2 amplifier. And yes there is an echo of the circlotron about it.
The best explication for this circuit is found in Howard Tremaine's Audio Cyclopedia, section 12.231. Yet, I beg to differ with Tremaine's explanation for the merit of the amplifier. My take is that it is not the amplifier's ability to accept more feedback due to its output transformer's tight inductive circuit. Rather my guess is that because so much of the driver circuit finds its path to the B+ through the primary winding, the primary winding is never actually severed from a path to ground.
In contrast, in a transformer coupled Class-AB and Class-B push-pull amplifier, each output tube ceases conduction during part of large waveforms. This cessation of a current draw unloads one half of the primary, which can provoke huge voltage spikes and even momentary oscillations. An analogy can be made with a garage door spring snapping while it is under tension. Class-A amplifiers, either single-ended or push-pull do not release the winding during the entire waveform.
The McIntosh amplifier's driver stage, output tube's screens, and cathode followers cross couple to the opposing output tube's connection to the primary; thus always providing some "tension" to the winding. In other words, I believe that using a different driver stage power supply connection and triodes as output tubes would deprive this amplifier of much of its distinction. In contrast, the advantage of the original circlotron circuit is that used an output transformer. Thus the primary is always enclosed in a current path, even when one output tube ceases to conduct. So the main reason given for wanting to build an OTL amplifier might be misguided. Output transformers are too weak a link, so they must be eliminated we are told. But if the distortion the transformer gives rise to is born from misusing the transformer in a non-Class-A circuit, then why not use the transformer correctly instead of eliminating it.
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