300b lovers

Some fixed-bias amps have a separate servo circuit that monitors the bias of each tube, so user does not need to adjust the amplifier. This servo circuit needs to be very reliable, though, since a failure would destroy the output tubes, and possibly damage the bias circuit, as well.

Cathode bias, which is nothing more than a power resistor bypassed by a (very) high quality capacitor, acts like local feedback at DC, and more like fixed bias at audio frequencies. It is not suitable for Class AB amplifiers, though, since the total current going through the pair (or more) of output tubes varies with the power delivery (the efficiency actually goes up as output power increases). By contrast, Class A operation has more or less constant current draw from the output pair, but it is significantly less efficient than Class AB.

By the way, the traditional definition of "efficiency" is: (Max RMS output power at stated distortion level) / (Total power going into the plate circuit). Power consumed by heaters, filaments, input and driver tubes, or regulators is not usually considered.

For a pair of output tubes ... 6V6, EL84, 6L6, EL34, KT88, 6550, 2A3, 300B, 845, or similar ... they can be set up to run in either Class AB, or Class A. Class AB operation typically has a higher B+ voltage and a lower quiescent (steady-state) current, while Class A operation has a lower B+ voltage and a higher quiescent current. For the same average power draw, Class A usually puts out half to one-third the output power of Class AB, which is why it is less common than Class AB.

Most customers want more power if they have a choice. That was true in the Fifties and it is still true today ... partly because most loudspeakers are very inefficient (less than 1%) and need all the power they can get.

Hi @lynn_olson

I upgraded my 300b SET amplifier. I replaced 6f6g Torvac driver tube to 6v6 Psvane. 6f6 worked with idle current 26ma. 6v6 with the same cathode resistors gave 24ma. I changed cathode resistor to get 29ma. In result much better bass control and dynamics, better instruments separation and higher resolution.

The difference between 6v6 and 6f6 drivers is huge. It similar to changing RC coupling between driver and 300B to IT coupling!

Conclusion, the more powerful driver is better for 300b. I know some folks (including Sakuma) end by 300b driver for 300b.

Hi @lynn_olson ​​​​@donsachs 

When did you play with the driver tube to your amplifier? Which tubes did you try except 6v6 and kt88?

Did you try 5881, KT66, 6L6, EL34?

If yes can you compare the sound of these drivers?

I left the driver tuning up to Don Sachs, who’s been building PP pentode amps for decades ... and started out by restoring Citation I and II’s, which are notorious as the most complex amps and preamps of the Golden Age. By contrast, the Marantz amps and preamps were much simpler.

Don’s a big fan of the 6V6, 6L6, and KT88 (in triode mode). Kind of hard to argue with that ... some of the most famous amps ever made used those tubes. Anyway ... he tried about every well-known tube under the sun as drivers. I kind of thought something as petite as a 6V6 (which is equivalent to a 45 in ratings) would be optimal, but the KT88, running at fairly high bias, sounded best of all. Part of the reason this is relevant is the 300B is very, very sensitive to the driver tube, much more so than most tubes.

The 300B has a difficult combination of very low inherent distortion (bested only by a 45 triode), and a grid-drive requirement of 80 volts peak at very low distortion. In most commercial 300B amplifiers, all you hear is the distortion of the driver, especially if it is RC-connected. You never hear the 300B as it really is. All the usual complaints of dull, soggy sound are the result of a not-good-enough driver.

Get a powerful enough driver with enough current (30 mA or more) and transformer couple it to the 300B grid, and you hear a very different sound ... very fast thanks to the high slew rate, and very wide-open thanks to low inherent distortion. Yes, I know about the Sakuma-san 300B - 300B amplifier ... I met him and heard it at one the last VSAC shows in Silverdale.

The Sakuma sound is mostly about the Tamura interstage transformers he favors, along with unique tube combinations. Surprisingly, my own Amity and Karna amps sound nothing at all like Sakuma designs ... if anything, our designs kind of echo the Citation I and II ... a big, fast, American sound, like a track-ready V8 Corvette. Sakuma would be more like a Morgan, very vintage.

Sakuma is 100% right about interstage transformer coupling. All the flavor comes through ... this is the unique hallmark of any interstage coupled amplifier. You hear it instantly, as soon as you walk in the demo room. Other methods subtly degrade tone colors, and all capacitors have an annoying tone color that is always there. I wish I knew what causes it, but it is not there in the cap measurements, unfortunately. But is very obvious when it is gone, especially after you get the last cap out of the signal chain.

Don and I valiantly tried to use various types of RC coupling between the input section and driver, some better than others, but the cap coloration was always there, no matter how fancy the cap was. They were all colored sounding, in various degrees, and a little dynamically flattened. So we had our transformer designer come up with a one-of-a-kind interstage to couple the input to the driver tube, and boom, problem solved. Simpler circuit, too. No caps in the signal path any more, and all the tone colors coming through ... which is the whole point of any vacuum tube amplifier.