300b lovers

The interesting thing is that "conventional" amps like PP KT88’s can be substantially improved as well. For example, floating paraphase phase inverters instead of split-load inverters or Mullard long-tail pairs. The phase division isn’t as precise, which is why they dropped out of favor, but the drive capability is much stronger than the other two types. And they really sound better. That’s one thing.

Another is dual B+ power supplies per channel, and is especially worthwhile in any PP amp, more so than SE. Isolating the noise and distortion of the final section from the driver + input is very worthwhile and lifts the amp into another class. This applies to solid-state as well. That’s another thing.

The third thing is scaling up driver current two or three times, and choosing the most linear driver tube that is available. Typically, this will be a medium-power tube, not a video amplifier out of a 1958 color TV. Video amps are powerful but linearity was not a major consideration in the tube design. You might get a linear one, but you might not. And you really want matched pairs for equal and symmetric distortion spectra.

Combine these three things, and the PP KT88 is now on a different level, but still has the same power as before. The slew rate goes up, and the noise and distortion go down.

Am I going to design this? Nope. But folks in the medium to high-power PP KT88 arena should check it out. You can’t keep making copies of Marantz amplifiers forever.

So, Lynn, given your comments, would an experienced technician know where to start to make these kinds of modifications? Or are they too obscure for the average tech to understand and execute...

Unfortunately, revising the driver away from the usual single 12AU7 means another hole in the chassis, and a pair of octal sockets at that. And split power supplies means another power transformer and rectifiers, although it relaxes the endless search for Holy Grail rectifiers. When an amp is that sensitive to rectifier choice, frankly, there is a design error lurking in there somewhere.

Somewhat counter-intuitively, splitting the power supplies front to back sounds better than isolated stereo power supplies, where the gain is fairly minor. The front to back isolation is not minor. Another counter-intuitive thing is PP amps benefit more from power supply isolation than SE amps.

As for inverters, the split-load inverter plus input tube uses two sections, while Mullard and the floating paraphase use three sections. And the last two sections can be octal medium-power tubes, not a single 12AU7.

This will change the forward gain of the amplifier, not by by much, but some. Probably a few dB less gain thanks to different and lower-mu drivers. Maybe a small trim in the feedback network, but less gain is easy to adjust for (unlike more gain).

In practical terms, a new chassis. Sorry. Those old amps were engineered down to the last inch, with no room to spare, and not one part wasted. Flip one upside-down and the parts in there are really, really tight. Too tight, and prone to overheating, which does no favors to the capacitors.

Most of the engineering effort in the Golden Age, even for Marantz and McIntosh, was simply watts-per-dollar, and keeping chassis size down as a secondary goal. In the late Sixties, the Crown DC300 and Phase Linear 700 blew the watts-per-dollar tube amps out of the water, and forced them into the audiophile market, where they remain today.

As for the tech, none of this is difficult. They will know how to connect and bias the floating paraphase phase inverter ... plenty of old schematics out there ... and the rest is a matter of chassis space and heater current for the new drivers.

"Floating Paraphase" sounds more scary than it is. The secret of any phase inverter is finding what the lower grid is connected to.

In a Mullard circuit, the lower grid is grounded through a coupling cap to ground, but is connected to the other grid through a high-value resistor. This sets the bias point the same as the other side, but the grid does nothing and is simply a reference. The common cathode does the phase splitting. The long-tail is either a high-value resistor or a current source .., yes, I know, transistors. The main drawback of the Mullard circuit is restricted peak current delivery to the power tubes. (But the split-load inverter is much worse in that respect. Split-load inverters do not like to deliver current ... they go out of balance. Ideally, they should be buffered with cathode followers.)

The floating paraphase has the upper tube drive the upper power tube through a totally normal RC coupling. Nothing to see here, sir, move along. The lower tube is often drawn in an opaque way, but what’s going on is the lower grid is connected to a pair of resistors midway between the power tube grids, after the RC coupling, What this weird-looking connection does is local feedback that forces the lower, phase-inverting tube to act as a unity-gain inverter, or plate follower. Phase inversion isn’t quite as pretty as the other two methods, but ... more power is available to drive the power tubes, which is what really counts. And you can ditch the 12AU7 and use real power tubes, because, why not? Just a matter of another socket and heater power.

I have a question to @lynn_olson 
I have 300B SET 6sn7 input, 6f6g in triod driver with RC coupling. One friend of mine who reads DIY forums and tries all this thing by himself, recommended me to enlarge cathode capacitors up to 100,000uF in input and driver tubes.
So I did 10,000uf in input first and because I like the results I did 100,000uf on input and 50,000uf on driver. So I get even more improvement - deeper and more controlled bass, bigger soundstage, more low level details, transparency, bigger more dynamic sound.
Then I increased driver capacity to 87,000 uF and it gave a big improvement in dynamics and sound quality. Despite intuitively it shouldn't do too much!
In all cases I use a bunch of 10000uf-15000uf (CDE, Nichicon KA) medium quality capacitors bypassed by one 50uf AN Kaisei NP.
Can you explain why these huge value cathode capacitors work?