Triode Audio Amplifier using Soviet 6C33C Valves

Based on a design by Tim Mellow for a 25 Watt OTL (Output Transformer Less) amplifier with direct connection to the loudspeaker. Orignally from "audioXpress" magazine Feb 2010 (see scans - below)

Some Background info'

In 1976 a Mig 25 fighter jet landed in Hokkaido, Japan; it was flown by a soviet military pilot that wanted to defect to the west.
This was a field day for the western military people as not much was known about the MIG25 and the plane was completely dismantled.
Two things startled the military analysts in the west, the first was that the plane was built using a lot of high tensile steel, it was earlier thought that the MIG25 was built in Titanium to withstand the heat that is generated by flying at mach 3 which the plane was capable of, but being built in steel made it heavy so the performance was actually less than predicted.

The second thing that surprised people in the west was that electronic tubes were used in some parts of the radio equipment.
After a while it was understood that the tubes were used so the plane should be able to withstand EMP.
An EMP (Electromagnetic Pulse) is generated by the detonation of a large nuclear bomb in the atmosphere, the electric field so generated destroys most solid state electronics and EMP is thought to be used as a possible electronic warfare measure.

In the power supply of the main radio unit of the MIG 25 a previously unknown triode tube was used, the Soviet military designation was 6C33C and it was a voltage regulator triode.
The data of this tube was much better than anything made before in the west and SanEi of Japan imported some 6C33C tubes for audio application in 1997, renaming them EC33C for the Japanese consumer.
Mr Dakesue in Japan was the first person to design a 6C33C-B OTL power amplifier circuit in 1997.
The main characteristics of the 6C33C is that it has very high transconductance, high current capability and low internal resistance and also as the tubes were intended for use in military aircraft they are mechanically very robust.
The nearest western equivalent is 6336 but data of 6C33C is better.

I've had no experience of a pure triode output audio amplifier (sound "purity" is supposed to be unequalled i.e. low THD)
I am utilizing valve types of Soviet manufacture.
Also, an obvious appeal in the design is that an (impedance-matching) output speaker transformer isn't required.
However; the need for a unique power transformer to supply all the various voltages and particularily the high current requirements for the 6C33C filaments does present a real challenge !
Although I have hand-wound many transformers to satisfy a projects requirements, it is a long/hard job that you'd best avoid if you can!
For the HT voltages (+150v & -150v) I have re-cycled two 1970/80's Colour TV power transformers.
(Lots are lying around now that analogue TV's are redundant)
They are most usually Mains to 120V plus a 6.3V filament supply winding.
I used a series-aiding connection the 120 volt secondaries, and each 6.3volt windings feed valves V1a V1b and V2 V3
I have reworked an ex PC switched mode power supply (SMPS) to obtain the necessary voltage and current for the 6C33C triodes filaments.
Each 6C33C valves pair of filaments requires at least 40 watts of power.
For a stereo amp this means 4 x 40 or 160 watt total.
The ATX type power supply I've utilised is rated at 250 watt, and has inbuilt fan.
In an earlier project (13.8 Volt 20Amp DC Supply - SMPS) I have used a cascaded pair of SMPS to obtain 13V8 at 20 amp. Please refer for further details about altering the output voltage setting etc.

SMPS modification summary.
Output Max Load After modification Expected Load
+5v 25A +6.3v 14 A
+12v 13A +15.2v 7 A
+3.3v 20A disconnect
-5V 0.5A disconnect
-12V 0.5A disconnect

The 5 volt o/p has been increased to 6.3 volt
Also the 12 volt O/P has now risen to 15.2 volt - because both outputs derive from one transformer secondary winding.
Therefore it is necessary to include a dropping resistor in the 15.2 volt line so as to reduce the filament supply to 12.6v (on load).
What I seem to recall as an S.O.T. (select-on-test) component.
Not good design I admit, but little else that I could do.
In Tim Mellow's design the heater supply is strapped to a voltage rail close to the cathode potential. This is to preclude any heater to cathode flashover.
This is good design practice, but with my re-worked SMPS one side of the filament supply is now earthed.
I will rely on the manufacturer's spec' of 300 volt(max) cathode-heater isolation to prevent any chance of flashover!

I have used minature wire ended (solder in) valves for the phase splitter and triode drivers. V1a V1b V2 V3
I could have used 12AX7 & EF86 but have saved chassis area and effort in metal-working holes for sockets etc. You can tag-board mount the Soviet valves just like caps/resistors etc.
My choice is the tubular wire-ended Soviet 6J10B-V pentodes (cheap and readily available).
I did successfully use them in the 1" CRO circuit
To get them to run as a triode, I've found you must strap the screen and suppressor grids to the Anode (then they work fine!)
Alternativley the 6N16B-V minature twin-triode could be used for V1a V1b
An example of a handbuilt 6C33C triode amplifier

Orignal article & further references:

Tim Mellow 25 Watt OTL amplifier Pg 1
Tim Mellow 25 Watt OTL amplifier Pg 2
Tim Mellow 25 Watt OTL amplifier Pg 3
Tim Mellow 25 Watt OTL amplifier Pg 4
Tim Mellow 25 Watt OTL amplifier Pg 5
Tim Mellow 25 Watt OTL amplifier Pg 6
OTL theory - Hans Beijner(Sweden)
The short "6C33C Survival Guide"
jogis-roehrenbude - Russian Triodes
jogis-roehrenbude - the 6C33C
Q-tron Audio Sweden
Q-tron PA-12 OTL Tube Amplifier
European Triode Festival
RU tubes (supplier)

Status: 2Q/2013 A work in progress