300b lovers

Once and always, Charles sets the bar for thoughtful and positive responses to topics on this forum, which in 10 months regarding this thread, have always been very upbeat and positive.  When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph.  It has been a real education for me. 

@whitestix 

When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph.  It has been a real education for me

Very well put.👍

This thread you initiated has miraculously avoided the all too prevalent trolling and disruptive deterioration. Bravo!!!

Charles

Ralph brings up a very good point about feedback: the underlying theory assumes a distortionless summing point. (The summing point is the comparator input between signal input and the sampled output.) Any distortion introduced at this point of the circuit will be amplified without correction, and there is a real possibility of introducing new, higher-order terms that are not present in the forward path of the physical amplifier. Norman Crowhurst mentions this in passing in his Audio magazine articles in the late Fifties.

Don and I go to some trouble to avoid even local feedback in the Raven and Blackbird ... all cathodes are bypassed in every stage, we do not use the Ultralinear connection in the triode-connected KT88, nor global feedback. Any distortion that is present is the result of first-order effects in the tubes themselves, not anything else. It also mandates selecting tubes that have a minimum of high-order distortion terms in the intended operating range.

This overlaps with the philosophy of some SET amplifier designers, but we prefer to use complementary pairs to cancel most of the distortion without recourse to feedback (local or global). The transformers, if correctly balanced, cancel out most of the distortion before passing the signal to the following stage, thus, each grid-pair is fed a cleaner signal.

This comes in particularly useful for the 300B grids, which can be a difficult nonlinear load for the preceding stage. Unlike RC coupling, when one grid demands current, both phases of the driver pair are available to deliver current to the 300B grid that needs it. This enables seamless and jump-free transition to A2 (positive grid drive) when the signal demands it.

In essence, the signal flow is: Complementary Pair of 6SN7 sections -> Signal is re-summed in the transformer -> Complementary Pair of triode-connected KT88 drivers -> Signal is re-summed in the transformer -> Complementary Pair of 300B outputs -> Signal is re-summed in the transformer -> Loudspeaker Output.

To minimize interactions between output section and the rest of the amplifier, there are separate and isolated high-voltage regulators for input + driver section and the output section. The only signal flow, either from the audio path or the power supplies, is from input to output. There are no secondary paths, at least to the 130 dB limit of filter isolation offered by the power supplies.

A few SET amplifiers have a roughly similar signal flow, but only offer a few dB of RC filter stage-to-stage isolation between sections, so there is a signal backflow pathway between sections. Isolated power supplies resolves that problem. In a SET amplifier, 2nd-harmonic distortion builds up from stage to stage, but there can also be unpredictable level-dependent cancellation effects since signal polarity is inverted with each stage.

If all these transformers disgust you, or you want to use local or global feedback, you can re-visit the Acrosound schematic from the late Fifties. Three stages, all differential, each RC-coupled to the next. It can be easily updated with modern current sources on the cathodes, giving precise differential action and almost no even-order distortion. The Acrosound circuit also offers some interesting options with cross-feed feedback between sections.

If you are bold, you can DC-couple some of the stages, but you will need good servo circuits to maintain DC balance as the tubes warm up and then gradually age. The servo circuits also need to be failure-proof, since a runaway servo circuit can destroy the amplifier.