6c33 Single End Class A

20W+20W Integrated Valve Amplifier

In the case of pure Class A valve amplifiers, we at Indeed Hi-Fi Lab, have a quite different concept towards it.

In the pursuance of achieving single-end, high-powered output, many producers tend to use 211, 805,845 tubes, with the expectation of achieving an output of 20w or above. But the fact is that these high-powered triode vacuum tubes requires a direct current of >1000v in order for them to operate properly and fully, reaching ití»s maximum potential. Therefore, in using these tubes results in expected output unable to be achieved, moreover, under such high pressure conditions, the high voltage of >1000 volts tends to í«elevateí» the circuits, and putting enormous strain on the current wave, hence unstable output. It also gives the user a psychological pressure, because they would feel dangerous in using these amplifiers. To our opinion, these kinds of amplifiers are high cost with low efficiency.

Putting all the above problems in mind, we are proud to have designed our new 6C33-B single-end pure Class A, 20w + 20w integrated amplifier, which overcomes all the above problems above, achieving the criteria of low cost and high efficiency.


L x W x H:  43cm x 44cm x 22cm

Weight:  38 Kg

Output:  20W (R.M.S) 4ohm/8ohm

Input Sen:  600mV

Distortion:  15W>1% (T.H.M)

Noise ration:  >92dB

Frequency response:  5 Hz---40 kHz (+/- 0.5dB)

220VAC or 100VAC (B.T.O) 


1. (i)  Massive 750W power transformer.

(ii)  Purpose built 150W single-End output transformer with 4 independent inductance coil, (choke), enabling various grades of power supply.

2. (i)  New conceptual design of the Class A circuit- reaching a power output of 20W each side before paring peak.

  (ii)  Sound perfection unmatched by other products such as the 211, 805, 845.

3. (i)  Chassis made in 1.2 mm thick high quality stainless steel.

  (ii)  Front panel is 8mm thick top quality alloy in one piece, giving an attractive modern look.

4. (i)  Using two 274B diode tubes, achieving electric current output of >1100mA.

  (ii)  Sound quality is extra-ordinary, rich and artful. 

The high precision ALPS 24step volume control (click picture)

STORY OF ORIGIN- 6C33-B power valve tube                      

6C33-B power valve tube, originally made in Russia more than 20 years ago, but not until recent years that it had been adopted and used by manufacturers from the U.S., U.K., and France.

So why was the tube past so many years being un-noticed? Well, it was not known to the world until the news of an aircraft incident 20 years ago broke out, which at that time attracted international concern.

It was said that, a Russian pilot flew a top secret fighter aircraft, the MIG 25 ( NATO refers it as Fox-bat), into Japanese territories, and landed at Hokkaido airport without being detected by Japanese forces. This incident brought serious concern to the Japanese and American forces.

The American forces dissected the fighter plane and putting it back together before handing it back to the Russians. Later, western experts described the MIG 25 as coarsely made but with exceptional fighting qualities, extremely efficient regarding the outdated equipments used. It was only then that the 6C33-B vacuum valve tube first came into light, because it was used on the M.I.G. 25.

This reminds me of another vacuum valve tube, the fu-29, which was found on the high frequency radio station of the fighter plane-MIG 15. This tube was later on being used in the music industry and with great praise.

I use it myself on my Barker 560 vacuum valve amplifier, made in Zhuhai, China.

Since the disintegration of the U.S.S.R in the 90s, products like these (once used for top secret military purposes, and have now became commercial products), became easily available on the Russian market, and relatively inexpensive compared to western standards.

It is heaven and earth when the 6C33-B is compared with similar tubes often used, like the EL34, 6550, 6L6, 300B etc.

In general, it has several characteristics, for example, hindrance is especially low; a professional welder in Hong Kong specified that it is 72 ohms, typical listed specification is between 60 ohms to 100 ohms. Therefore, with such low hindrance, to the fan of OTL electron tube, it is like a dream come true.

In recent years, because of this low hindrance of the 6C33-B, top welders of Japan, and famous makes from Italy like Graaf, also use it for the back portion of itsí» OTL electron tube amplifiers. The reason is, it makes the production of the transformer much simpler, therefore, reaching more ideal replay results. Moreover, with a pair of triode tubes, the flow of hindrance can reach 200mA and above, with a possible peak of 1A, it is a natural born super power when in comparison to other tubes like the EL34 or the 6550, which at maximum can only reach a mere 60-70mA. This clearly separates the 6C33-B from the other tubes in terms of its shear power output.

But its most special characteristic is that it can operate smoothly under very low voltage, usually between 150v~200v. Under this low voltage, the working requirement for the other internal parts of the amplifier is reduced, especially the part of electrolytic electric capacity. The factory make, VK-60 uses four 6C33-Bs with ease under a restrictive voltage of 185v and a flow of hindrance of 200mA.

Also, the 6C33-B is an extremely solid, stable, and durable tube, its operating qualities can only be compared with the American make of Bandix in the 1950s, 1960s, which makes tubes for military use. The glass of these tubes are extremely hard, because it has to be able to withstand the extreme gravity load of 7G in high altitudes of tens of thousands of feet, and being able to operate normally. In the military forces, military expert verifies that these tubes have a working life of 10,000 hours, but in real life situations, it is much more than that. An example is that, the famous make, BAT, sold over a thousand of the VK-60 which uses these tubes since 1995, but only a fraction of their users say that the tubes needs replacement. So you can see, how durable that they really are!

Nevertheless, the 6C33-B is quite a demanding tube when in use. Ití»s electric current flow of its filament is very large. It has two sets of 6.3V filaments, each set requiring 3.3AV electric supply, just like the VK-60 which only uses four, and each requiring two sets of 3.3A electric current flow of the filaments. Moreover, it also needs a certain amount of í░high pressure groupí▒ electric supply. Therefore, the requirement of the voltage transformer cannot be overlooked either.

Giving that it needs such high electric current flow, it inevitably becomes extremely heated up, hotter than the í░back portioní▒ of the 8K, EL34. Although the 6C33-B is a triode tube, because of its huge demand of electric current flow, if not well managed, it would become quite course in the final outcome. Hence, it really does quite depend on the quality of design of the amplifier to get brilliant results from this kind of tube. 

                                       Return to product list                                                     Peter Cheung / Indeed Hi Fi lab