It may strike some readers as weird that Don and I are taking a minimalist approach to a PP amplifier, more like a SET than a typical PP. The signal path is simple:
Optional SE/Balanced input transformer -> Balanced 6SN7 -> Interstage #1 -> Balanced KT88’s in triode in Class A mode -> Interstage #2 -> Balanced 300B’s in Class A mode -> Monolith Output Transformer -> Loudspeaker.
Similar to a fancy Japanese-style SET done twice. The hard part are the interstage transformers, which are not easy to find off-the-shelf, and ideally should be designed for the specific tubes in the circuit.
Don and I tried many variants to get rid of Interstage #1, since that operates at the highest impedances (thanks to the 6SN7) and is hardest (almost impossible) to design. The first version was simple RC coupling to the driver stage, with 6V6’s as driver running at 24 mA each. It sounded pretty decent and measured quite well, as you would expect from RC coupling. But Don wanted more ... so we tried paired dynamic loads for the 6SN7, which reduced its distortion about three times and was noticeably clearer sounding. But ... and there’s always a but, isn’t there ... there was just a faint trace of solid-state coloration from the current sources. Not much, but there. This was a version the folks at Spatial really liked, and we built some of the early "shoebox" format amplifiers in this format.
My grumble was the insanely long "burn-in" time for the super-deluxe coupling caps between the input and driver tubes. 50~100 hours. I am wary of burn-in times this long, since I suspect the part might be chemically unstable and never actually settle down, always changing its sonic presentation over time.
We tried another version, replacing the current sources with 100 Hy custom inductors. Was it any better? I’d say different, with deeper tone colors, no solid-state coloration at all, but losing a bit of snap and attack compared to the current sources. All expected ... no transistor sound, but unwanted stray capacitance in the high-value inductors, and question marks about linearity in the bass region.
Our transformer designer saw the high plate impedance of a balanced 6SN7 as a personal challenge, and insisted he could design an interstage transformer just for us. I was skeptical it could be done ... I only knew of one other transformer that could do that, from Tribute in Europe, and that thing was quite large and a one-off project done for the Karna amplifier. It might still be available from Tribute, for all I know. Tribute transformers are pretty special and the equal of any Japanese confection.
But ... a couple months went by, and Don got a special care package from our transformer designer. It was a pair of special custom Interstage #1 transformers, made just for us. Very simple wiring, compared to all the other variants we had tried, just six wires from the primary and secondary, as simple as it gets. In it went.
And that was the winner. Burn-in time was much faster, an hour or two, no super-exotic caps with their temperamental burn-in times, the circuit was way simpler, and the depth of tone colors was much deeper than any of the other versions. Obviously better than the inductor or current-source loads for the 6SN7, and no stinking coupling cap.
In fact, the IT coupling revealed the pretty obvious "cap coloration" of all the many coupling caps we had tried. The brutal fact is all caps have a sound, even though they measure extremely well. Worse, there is zero subjective correlation between sonics and the standard Df and Da measurements, not to mention they really do change sonics quite a lot over the first hundred hours. And nothing about that shift over the first hundred hours is measurable ... at all!
You hear the cap coloration by its absence. Go to true direct coupling, or IT coupling, and it is gone. After you’ve evaluated the sonics of twenty or more different brands and types of caps, you realize they all share a common coloration, with some much worse than others, but they all have something going on that is veiling the sound. And whatever it is, it can’t be measured with the instruments we have now.
I have no idea what it is. And I am taking about a coloration much more obvious than a cable swap between components. I’ll go out on a limb and say caps are the dominant sound of most tube electronics, whether preamps or power amps. Swap the coupling caps, and you have a brand-new amplifier, with brand-new colorations of its own. It makes you realize why there is a whole industry of guitar-amp tuning ... the tone coloration combinations are limitless.
But ... get rid of the caps, all of them in the signal path, and you are in a different sonic world. Do transformers have a sound? Well, if they are cheap transformers, yes they do. Murky and dull. The best, though, are very clear and have no cap sound at all.
That snap and clarity is what solid-state enthusiasts crave. Hey, I get it! The best solid state is very good and is free of cap coloration, as it should be. But ... solid state has its own sound, too. Unfortunately. Traces of it intrude on tube circuits if they have dynamic loads, or regulators that are not well designed. Don and I have gone to some trouble to use B+ regulators that are well behaved and have very high noise rejection (130 dB) and very low stray capacitance (using cascoded MOSFETs).
So I’ll go out on a limb (again) and say most audiophiles have never heard tube electronics without coupling caps. Ever. They think tube electronics just "sound that way" and tolerate burn-in times of hundreds of hours, with the sound changing hour by hour, sometimes better, sometimes worse, and sometimes just weird.
This plagues exhibitors at hifi shows, too, because the transport process results in some of the caps needing a re-set at the show, so the sound on Friday and Saturday can be remarkably different. And the culprit? Not the cables. Not the speakers. The coupling caps buried deep in the circuit, temperature cycling up and down, and going through unknown electrochemical changes deep inside.
And guess what? Solid-state electronics have caps too, not in the direct signal path, but as filter caps, typically large-value electrolytics. And you can bet they have a sound, too.