300b lovers

Don is being modest. The last year, going through the present, really made Don pursue every obscure byway of amp design, building and listening as he went, every step of the way.

Don started with an obscure version that I called the Symmetric Reichert, which was literally a Reichert 300B done twice, with a phase-splitter transformer at the input. All RC-coupled. He built that and called me out of the blue, about a year ago.

Don then tried separate B+ supplies for the input+driver and output section, and an interstage transformer between the driver and 300B’s. A few months later, Don used a triode-connected 6V6 instead of the hard-to-find 45 driver. Thom Mackris and Don independently tried this at just about the same time, pretty much on the same day. Don (but not Thom) then used active current-source loads for the input 6SN7, instead of resistor loads. That was the Stereo version Don built and shared with the Spatial team and the first customers.

Next, replacing the active current source loads with custom Cinemag inductors designed for the purpose, and using the shoebox-format monoblocks that became the show amps. My Colorado neighbor, Thom Mackris of Galibier Design, has been following along in a parallel project, with a SET architecture, but with passive CLC B+ supplies and damper diodes for rectification.

That’s where all of us were a month ago ... Don Sacks, the team at Spatial, and Thom Mackris. The latest from Don is an IT between the 6SN7 and the 6V6, replacing six other parts with a much simpler approach ... provided the IT was up to the task, which it is. The IT has turned out to be superbly designed, exceeding expectation, and also making our lives simpler. Don and I have gone full circle, and re-invented the Karna (after trying every alternative), with far more advanced power supplies that were not available in 2003.

Don really has tried every topology, one after another, and carefully measured and auditioned each one. RC coupling, active loads, LC coupling, and now, IT coupling. By lucky coincidence, Thom has been walking a parallel path with his SE topology. All four groups ... Don, Thom, Spatial, and myself, have been exploring this zero-feedback approach for several years now.

If other folks want to build transistor Class A, Class AB, or Class D, more power to them. Those designs have an entirely different set of challenges that have nothing to do with triode amplifiers. In triode amplifiers, the devices themselves are exceptionally linear, and the appropriate circuits take advantage of that.

Hi @atmasphere , 

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?  

I’m no expert on the MC30, but it is very unusual. The cathode feedback (from a special tertiary winding in the output transformer) results in very low gain for the 6L6 power tubes, so the driver has to swing 100 volts, putting extreme demands on linearity. And I think it operates in nearly pure Class B, with a very small Class A region. This requires substantial feedback (which it has) to linearize the output section. The Class B operation requires very close coupling between the tertiary and primary windings, otherwise the circuit will have tube cutoff glitches with every zero crossing. So the whole thing is very much a package ... multiple feedback loops, a unique output transformer, a wide voltage swing from the driver, and cathode feedback for the power tubes.

Almost the polar opposite of the Brook 2A3 amplifier, which relied on the linearity of the 2A3 power tubes instead of massive feedback. The high-power (30 watts!) Brook amplifier used sliding bias to keep the output section in quasi-Class A.

It should be mentioned there was no awareness of slewing distortion at this time, because signal sources had very limited HF bandwidth (12 kHz) and limited peak energy. Phono cartridges were very primitive and could barely track LP’s at 5 grams.

When the first writings about slewing distortion appeared in the late Seventies (25 years later), things were very different: moving-coil cartridges with exotic styli were flat out to 50 kHz, and cutterheads could put down tremendous levels on the disc. Tweeters were much better as well.

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.