KSA-150 and KSA-250

It is unclear whether the KSA-150 and KSA-250 used a managed variable bias system.  Reading the manuals carefully, it sounds as if there may be some kind of bias management, but not as sophisticated as Sustained Plateau Bias which officially appeared with the KSA "S" models.

If KSA-250 has multiple bias levels, John Atkinson's famous calculation of the Class A power of a KSA-250 is only true for the lowest bias level, not the higher ones.   (In the first version of this post, I was sure KSA-250 had multiple bias levels.  Now not at all sure.)  I'm guessing the relatively cool running 250 had variable bias whereas the much hotter idling 150 might not have.

The Mark Levinson amplifiers like 331, 332, and 333 definitely use some kind of managed bias system to provide more Class A power on demand.  It says clearly says so in their manuals but only calls it "proprietary" and "developed by Madrigal Labs" and doesn't give it a fancy marketing name.

The KSA-250 and KSA-300S were much larger than the FPB 300 and approaching the FPB 600 in transformer capacity and weight.  The FBP 300 chassis actually seems like the successor to the KSA-200S chassis.  So the successive models appear to be in two groups (though within each group, models do get slightly heavier and certainly more sophisticated over time):

KSA-150

KSA-200S

FPB 300

FPB 400cx

(this group always had something like 3kW transformer)

and

KSA-250

KSA-300S

FPB 600

FPB 700cx

(this group always had something like 6kW transformer)

As time went on, Krell was either getting or at least advertising more power from almost the same amount of iron and aluminum.  The old 150 could double down all the way to 1 ohm and get 1200W.  The FPB 300 doubles down to 2 ohms…where it gets 1200W.  Actually the FPB is said to get more power into 1 ohm, but the 1 ohm power is not rated and it is certainly not doubling down to 2400W into 1 ohm.  I think I've heard that something like 1600W may be available into 1 ohm.  Ultimately, the FBP 300 and FBP 400cx have the same transformer, output transistor assembly, and chassis and the 400cx is rated at 1600W into 2 ohms.

If Krell ever advertised Sustained Plateau Bias for the Evolution models..they don't seem to do so any more.  I thought they did at first, perhaps when Dan was still with the company.  It doesn't seem like they were doing full rail regulation anymore, just a fancy kind of filtering.

I think the sustained plateau bias on my FPB 300 is great, though it could be smarter still and I'd still like a manual indication and control as with the "S" models.  The S models lacked the sophisticated rail voltage regulation, and I think they had slightly higher levels of distortion than FPB, but I like the ability to control bias levels.  If I could preset/hold the lowest bias level, I wouldn't need a second "summer or background music" amplifier.