300b lovers

McIntosh MC30 has the similar cathode follower driver 12ax7 to drive 6L6 directly. The output tube works in fixed bias in this schematics. Isn't it? Can you explain how the output tube bias is self adjusted?  

@alexberger 

Yes. R21 (120K; refer to schematic) sets the bias point of the 12AX7 (which to me seems a terrible choice for this application- a 12AU7 or 12AT7 would have suited better), which in turn sets the bias point of the power tubes.

The operating point it based on the idea that as the tubes weaken, at some point you just replace them, rather than readjusting the bias as the tubes age. The output power is not a function of the bias- its affected by the condition of the tubes! That is why the operating point was chosen to be class AB2 so the actual operational point is not critical and the power tubes will run cool with long life, high power and low distortion (due to the various feedback means). A good driver tube will present the output tubes with a very consistent bias voltage over time- that circuit is quite stable and has some ability to handle some grid current in the output section.

The bias rectifier and its power supply play a role in this. The original rectifier was a selenium device which has a larger voltage drop across it so if you renovate the amp and replace the rectifier with a new device (recommended- those old seleniums were terrible) this is a minor thing to pay attention to if you want to set the same operating point, although as I said its not critical.

Interesting circuit ... thanks for posting it, Ralph.

It’s kind of weird the actual gain/driver tube is a powerful 12BH7, while the cathode follower is limited to 1mA or so from the 12AX7, which is about the worst possible choice for a driver. Very strange. Maybe an intentional current-limit for the power tubes?

Also was not aware the global feedback network has its own winding, making four secondaries on the output transformer. Most everybody else samples the 16-ohm tap, which captures the entire secondary that powers the speaker. I kind of wonder if McIntosh did this to make the circuit hard to copy.

From this distance, I wonder about circuit stability. I see at least two feedback networks, one nested inside the other, and the outer loop not actually sensing the voltage on the speaker terminals, but a separate winding (which will never be exactly the same, especially at high frequencies). The circuit has a massive amount of forward gain (12AX7 -> 12AU7 -> 12BH7), so the feedback networks are definitely active.

I rebuilt a number of classic mac tube amps when I was restoring vintage gear.  They all sound very nice, but the mac house sound is a bit soft to my ear.  Sort of on the warm side.  Pleasant sounding, but not the absolute last word in detail.  The preamps sounded like that too.  Very good gear if you like that sound and very reliable, but warm....  CJ gear was also a warm sound.

They all had a different "house sound". Marantz was crisper and more resolved, a more hi-fi sound, and Fisher sounded like a really good console. Not sure if the H.H. Scotts had a consistent sound or not ... they were mostly known for their FM tuners.

Dynaco was always the "value for money" brand, like a VW Beetle.

The HK Citation gear was the most razor flat sounding, and the Fisher receivers were actually quite good.   The Scott gear was also quite good, and my favorite small integrateds were the Sherwoods.   Their little EL84 based integrated was quite good if rebuilt.  None of these are as good as the really good modern tube amps, but only the really good gear.  The transformers in the much of the old gear were really good, but they didn't have the quality signal path parts we have today.  Also, they had all sorts of features that robbed signal quality, like miles of wire in the signal path to support things like stereo/mono and phase reverse switches, and of course tone control circuitry.   Very long signal paths in many of the old integrated amps and preamps.

The Dynaco ST 70 had good transformers, but the driver section was barely adequate and if you replace it with one of the more modern boards, especially the all octal tube ones, the amp is greatly improved.  The small chassis limited the quality of the power supply you could fit in there though.  Still, if modded, the ST70 is a great little amp for the money.