I don't have a clue as to what your say'n. This is from a Dick Olsher review. Maybe there is some information here that might explain things for you.
A Few Technical Details
Push-pull Class AB operation is essential to maintaining good efficiency in a high-power amp otherwise you end up with a space heater and poor power tube life. And within this context, Bob Carver seems to have made all of the right design decisions. Let's start with the KT88 plate voltage. At 725VDC, Bob has entered a HV frontier where no designer has gone before, and it's a choice that is bound to raise a few eyebrows. High plate voltage coupled with a cold bias setting near cutoff allows for huge voltage swings without encountering tube saturation. But one traditional argument against use of a high plate voltage has been the need to operate the output stage at low idle currents in order to stay safely within the plate dissipation rating. The end result is of course only a few watts output in Class A and the potential for increased distortion when each half of the output stage is driven into cutoff. This would appear to be a classic conundrum of efficiency vs. distortion. Can you really have your cake and eat it too? The fact is that the Cherry 180's total harmonic distortion remains less than 0.5% at any power level. How's that possible, you ask? Well, it's partly a function of some global feedback, but mainly this is due to Bob's secret weapon his DC restorer circuit, to be discussed later.
Just how cool running is the Cherry 180? The front panel bias meter displays the combined idle currents (plate plus screen) for all six KT88s. To obtain the idle current for a single KT88, the meter reading needs to be divided by six. Bob sent me measurements of the plate and screen currents for one of the KT88s. At idle, the plate current measured 11.6 mA at 715V, while the screen current measured 0.3 mA at 375V for a total dissipation of about 8.4 watts. To quote Bob: "Cool enough so we can't injure ourselves if we hold on to a tube. Still, and just the same, I would not tempt fate by holding on for an extended time. I'm sure it would feel quite uncomfortable." The idle current may be adjusted via the bias pot to a maximum of about 150 mA.
And what about tube reliability at high plate voltages? Bob reminded me that all push-pull designs approach twice their plate voltage during dynamic conditions due to the coupling between the two halves of the transformer's primary winding. For example, a 200V drop in one half of the Cherry 180's output stage (from 725V to 525V) would give rise to a voltage of 1250V in the other half. Furthermore, Bob says that "the only worry would be arc-over inside the tube. I did some experiments to find out what voltage on the plate was required to make it arc-over inside. No problem at three thousand volts." Note that the tubes are warranted for an unprecedented period of one year!
Fixed bias is used rather than cathode bias, which is the right choice for overdrive conditions. To begin with, cathode bias is wasteful of B+ voltage. An even more important advantage is that since fixed bias is not affected by large plate current fluctuations, the operating point maybe set closer to Class B operation without the penalty of increased distortion.
The DC Restorer
Carver says he copied the DC restorer from old TV circuits with tube video amplifiers. "Those sets needed to deliver the DC component of the video signal to the CRT all the way down to DC. As used in my amp, this circuit reduces distortion by a factor of three as well as the tube dissipation by approximately the same amount. It works by keeping the DC component on the control grids the exact correct value over the whole signal swing, getting rid of the need to operate the tubes at very high currents (even at low level operating conditions just to get them to work right). The main thing that causes non-linearity in tube amps is that the screen voltage drops when the amp is driven, not so much because of low idle current to begin with (though higher idle current does make for lower distortion, but the trade is a bad one). It takes lots of idle current to drop the distortion a substantial amount, so the DC restorer is a better choice than lots of idle current. I get lower distortion at eight watts idle per tube than at 28 watts idle per tube."
The restorer is simple yet ingenious. It uses a 6AL5 dual diode. Each diode section services one bank of power tubes. The 6AL5 cathodes are connected to the control grids of the push-pull power tubes which are held at a nominal fixed bias of -47VDC. The diode plates are at an acquisition threshold of -56VDC. Bob admits that some aspects of the DC restorer operation are somewhat mysterious to him, as they are to me as well. My take is that the circuit aids significantly in recovery from hard cutoff conditions. Under those conditions the KT88 grids act as rectifiers and shift the effective DC level below -56V, to the point of causing the 6AL5 to conduct momentarily and pull the DC bias back to its nominal value. Bob estimates the lifetime of the 6AL5 as about 50 years. "I know that seems wrong, but 6AL5's are ubiquitous in tuners, and 50-year old tuners always have these tubes and they still check as new. There are so many of them in this world that a guy on eBay sells a string of them for three cents (each tube) to be used as Christmas tree lights."
