• Very high plate voltage of 1000 - 1250V. It is very difficult to make a good power supply for this voltage range. There are no large enough capacitors for these voltages so it is necessary to use two (as in the Ongaku) or three (as in my Triode) high-voltage power supplies connected in series. The output impedance of high voltage supplies, their hum and reliability are worse than for power supplies of lower voltages. Many sonic aspects such as impact, control, space and focus are intricately connected to the power supply quality.
• Another problem exists with the output transformer. As peak voltages on the primary exceed 2500V, all the insulation must be thicker than usual. The distance between the wire windings increases and magnetic coupling diminishes.
• Because this output transformer must have a high primary inductance, it limits bandwidth. High inductance combined with high capacitance between the turns creates a self-resonance close to the audio band.
• The 211 is difficult to drive. To obtain full power from a 211 or other transmitter tube, you need a positive grid voltage relative to the cathode. In this mode, the power tube needs not only voltage but also current. Drive power can be a few watts as in the Cary CAD-805 but the bigger problem is with the DC drive signal. It is not possible to drive a 211 with capacitor coupling. You need another transformer for driving it as in the new Qvortrup Ongaku; or a DC-coupled cathode follower as in the original Kondo Ongaku.

"So, my next selection was the 300B in the Silver 300 with silver wiring and silver output transformers. The 300B was a better selection because it works with lower plate voltages from 380 to 450V. The 420V point was the most popular because it accepts industrial-grade 450V electrolytic capacitors. It is easier to make a good power supply. Also, the power supply can be fully regulated using semiconductors. This was a revolution in 300B amplifier designs. The impedance of a solid-state regulated power supply is 100 times lower than using tubes. This power supply can deliver a lot of instantaneous power, a few kilowatts on peaks. This became the base for designing my Silver Grand Mono, my flagship and the best amplifiers we have auditioned to date.

"The 300B is very linear but doesn't need drive current. AC coupling with a capacitor is sufficient. A good part like a V-cap produces a very transparent sound compared to a cathode follower or interstage transformer. So the combination of these advantages made the 300B a very popular tube. It was easy to create a good-sounding amplifier. Also, due to the big competition between tube manufacturers, a few new excellent 300Bs are made - the TJ Full Music Mesh plate or carbon (graphite) plate is just one current example.

"Is the 300B perfect then? Of course it too has its limits:

• Low power. Some excellent mesh-plate 300Bs have a maximum dissipated power of 25 watts. That limits single-ended output power to 7-8 watts. Greater power requires parallel tubes.
• The cathode wire has a double function. It also heats. So the power heating current is mixed with the audio current. The heater power supply must be of very high quality because it influences the sound. This is the limitation of all direct-heated triodes.
• 300Bs are very delicate and expensive. Long delicate cathode wires and delicate girds are difficult to manufacture and sensitive to mechanical shock.
  

"My Silver Grand Mono is an excellent amplifier, one of the best ever made. It is fast and delicate, spacious and precise, soft as a feather and taut as a string. Like James Bond - perfect in every situation. But due its no-compromise nature, it is also very expensive (€30.000/pr). My customers and importers clamored for a more affordable amplifier to go with my Lektor Prime CD player, my current bestseller


"So as a first step, I considered a single-tube version of my flagship. But it too would have been expensive due its hand-made silver output transformer, each of which takes 2 - 3 weeks of hand labor to wind. So I arrived at the Soviet 6C33S. The original symbol uses the Russian alphabet where the letter "C" is spelled "S". The letter for "C" has no equivalent in the Latin alphabet. The 6C33S was designed by the Soviet army for regulators in MIG fighters. Russia's army today still uses tube radios because tubes are resistant to the electromagnetic impact of a nuclear explosion which would destroy semiconductors. This particular tube is very modern because only the Soviets continued to develop tube technology when the rest of the world abandoned it in the 50s and 60s.

"The 6C33S is attractive because:

• It has a plenty of power - 60 watts max. This allows for 18 watts in single-ended mode.
• It is easy to drive - it needs only voltage, no current.
• It works with low plate voltages between 100 - 250V . It is easy to make a good, fast, low-impedance HT power supply for it.
• It can deliver very high current up to 2 - 3 amperes! This inspired the creation of a few OTLs which unfortunately need very high negative feedback to destroy the music.
• It has very low input impedance - nine times lower than a 300B. So it is easier to design a wide-bandwidth output transformer (7Hz - 80kHz) . Also bass control is better due to a higher damping factor.
• It uses an indirectly heated cathode. This cathode needs 3 minutes to fully warm up (compared to the 10 seconds of a 300B) but filament heater and audio circuits are separate. There is no audio current flowing across the heater power supply. So the power supply can be simpler and smaller to integrate into a monoblock chassis.
• It has a very strong internal build that is resistant to transport shocks.
• It is cheap now, 20 euros. Of course we must remember that manufacture has long since been terminated and there always was only one manufacturer.
  

"So the 6C33S was a better selection for a more affordable SET amplifier that would provide enough real-world power and control. As stated earlier, the Single Six applies a lot of design aspects from the Silver Grand Mono: the HT power supply, the flat ribbon silver wiring, V-Caps etc. But is it as good as the flagship ? No, and that is not only due to the copper output transformer. The biggest disadvantage of the 6C33S is:

• Non-linearity. The tube's high current and high gain are made possible with very small distances between the electrodes - less than 1mm. The side effect is that this tube is less linear than a 211, 300B, 2A3, 845 or 805. So such an amplifier needs feedback. Ours is slight, 6dB compared to 20dB in pentode tube amps or 40dB in transistor amps.
• Another problem is sensitivity to internal failures. Any small particle (for example cathode oxide coating) can create a connection between adjacent electrodes for a short. Because this tube can deliver a lot of current, this would be lethal to the amp. The same low distance of the electrodes also creates big changes in bias current depending on the individual tube and the warm-up cycle. So special design addresses are required to protect the amplifier circuit from potential tube failure and stabilize the bias. The Single Six was designed accordingly."