Issac.....certainly an amplifier is an active device. But we need to distinguish between active and passive tone controls; they are different. Active tone controls, by definition, feed a selected frequency range back to an prior amplification stage, therefore boosting it. Look at any active Baxandall circuit and you will see the feedback loop.
There is also a semantic issue with the word "boost". Some may perceive that an amp "boosts" a certain frequency range when the circuit actually creates a signal loss, but then allows more bass or treble of this reduced signal through to the next stage, thus affecting a "boost" of the selected frequency.
As most of us know, the original Model 60 amp was based on the Dynaco Mark III power amp and the Dynaco PAM pre-amp. The PAM preamp (as well as the PAS-2/3) used an active version of the Baxandall circuit. As Dynaco described the circuit, both the treble and bass controls are “flat” in the center position and cut in the counter-clockwise direction and boost in the clockwise direction. These controls truly boost because the output of the tone circuit is fed back to the prior stage thus amplifying the selected frequencies. This is the same circuit used in the Sunn Model 60. Conversely, the passive version of the Baxandall circuit affects a “boost” of a selected frequency by attenuating a different frequency, but without amplification, so there is a net signal loss. Here is a good discussion of this:
http://www.learnabout-electronics.org/Amplifiers/amplifiers42.phpThe later Sunn tube amps (2000S, 200S, Sceptre, Sonaro) all use variations of a different circuit that more closely resembles the typical Fender / Marshall / Vox tone stack. This is not a Baxandall circuit (either passive or active).
I agree with Greg that your comment on potentiometer taper could be an issue with Sledz perception of missing bass. There could also be some other malfunction with the circuit that is causing this issue.
Your comment on the Fender tone stack is relative to the fact that there is a significant signal loss with this circuit, so that the "boost" is actually less signal loss of a selected frequency range. I think you are mistaken that the "Sunn tone controls are almost identical to the Vox tone controls" .....similar, but not the same, even disregarding the component values. The layout of capacitors and resistors around the bass control is different.
I would love to see any of the Sunn tone circuits (they are different from model-to-model) modeled in Duncan's tone stack calculator so we could see how they perform.
In any case, the Sunn circuit is definitely passive (rather than active) and any perceived "boost" is the result of less signal loss of that selected frequency range. Again, I acknowledge the issue of semantics. If some people consider this a "boost", I can understand that.
I would also welcome any technical discussion that would shed more light on how the Sunn circuits function. I appreciate all the member contributions to this board! We learn by sharing. Thanks guys!
