GRAAF GM20 OCL OTL Amplifier

 A. The Main Charactor 6C33C

The 6C33C has only one equivalent: 6C18C.

The 6C18C is an early version of 6C33C, might be used as drop-in replacement.

The 6C18C is very good tube, in some instances is better sounding then 6C33C but in may experience the 6C18C has a large percentage of faulty tubes.

Among fifteen new 6C18C that I used 5 of them were discarded immediately due to the pins shortage, 2 of the “good” tubes failed a few hours after they were heated and one of the surviving failed in a few month when it was subjected to shaking.

In contrary, the 6C33C is very reliable; among 60-70 tubes that I have burned I had only one that was defective. (1.42%)

Things you should know and do about 6C33C, it is getting more popular in 2023. Mainly because of the clonning of GRAAF GM-20 OTL/OCL Amplifier, I guess.

<6C33C Survival Guide> 

1) 

A new 6C33C has pins cover with something that I call “white shit”.

I have no idea what it is, perhaps some solidified oil or paraffin.

Anyhow it should be scrubbed out before you use 6C33C for a first time.

If you do not clean up the “white shit” from the pins then it will short the lifespan of your 6C33C tube socket

 

2) 

There are six 6C33C tube sockets (ordered from worst to the best)

A.        Chinese Ceramic

B.        Russian Plastic

C.        Russian Ceramic

D.        Russian Exotic (with side handles) 

E.         American Johnson

F.         Japanese Exotic

 

Each of them works fine but the best sockets will last longer.

The Chinese sockets will die after 1-1.5 year of heavy use and generally it is a good idea to replace a new 6C33C right along with replace of the tube socket.

The American sockets might last for years and the Japanese socket where pins hold by a full cylinder might last for a near a life-time.

 

3) 

The 6C33C tube sockets should be mounted at well ventilated location with nothing closer to the tube then ~ 2-3 inches. Out of chassis mount is highly advisable, but if the in-chassis mount use then punch necessary holes to crated natural air-convention or use forced cooling.

4) 

The Filaments should use AC.

The filaments wires should be 16ga or larger.

The wires to the socket’s leads should be soldered with high temperature solder.

 

5) 

If forces air use then do not cool down tube but cool down tube socket.

 

 

6)

6C33C should be use with fixed bios only.

If you feel that in your amplifier 6C33C sounds better with automatic of

Semi-automatic bias then ether you are in a strong need to perform a lobotomy over your reference point of your amplifier is a piece of crap to begin with.

7)

Biasing resistor that goes to the 6C33C grid should be the best possible quality and has value less than 100K.

 

8) 

The multi-turn precession attenuator that sets up you 6C33C’s bios should be also very high quality.

9) 

The 6C33C bias supply lines are very critical for 6C33C’s sound; even the proximity to the ground of the negative bias wire is auditable. Use transmission line techniques to set up that line.

 

10) 

Although the 6C33C marked for 60W plate dissipation and some of them will care even more with no problem still made for yours an absolute rule: the maximum plate dissipation of 6C33C is 50W, with recommended plate dissipation 40-45W for a full tube and 30-35W of a half tube.

 

11) 

The half 6C33C sound better then a whole 6C33C, the half 6C33C sound 30000 times better than one 6C41C.

12) 

Set up proper operational parameter for your 6C33C. The proper operational point will be the properly of your plate load, type of acoustic system you use and a few other factors. Still, with all things considered your 6C33C should experience power clipping by currant and by voltage at the SAME time.

13) 

Generally look for 180-210V on 6C33C’s plate and 180-230mA of plate current for a full 6C33C. still, the paragraph above (#12) should be denominating rule

14) 

The 6C33C has HUGE inconsistency of parameters when it is new, so huge that it is impossible to measure the tube in order to determine how much of cathode emission left or what mutual conductance would be is. The only possibility with such inconsistency of new parameters is to monitor how the parameters were changed for a given tube.

15)

 Make bias supply available from minus 50V to minus 120V

 

 

 

 

 

 

16)

 Generally the signs of the 6C33C dying are (in case the tube  still operates properly):

a)     Appearance from time to time mechanically sounding loud “ping” through loudspeaker.

b)     The tube’s getter loosing it’s gloss

c)      Change of bias voltage more than 50%

d)     Tube is used over one year in amplifier that works dally.

e)     Too small amplitude of “start up gap” (more about it later)

 

Since the tube is dirt cheap ($5 in Russia) it is advisable to replace the 6C33C each yeas or what the above mentioned effects show up.

 

17)

 All 6C33C have different gain.  Gain of 6C33C is the only parameters that should be matched. If you have 3 of those tubes on a row with the same gain then you are incredibly lucky and you should stop play with your amplifier and should become a stock broker.

 

18) 

When a well worked 6C33C starts in your amp the filaments should be pre-burn for =>2 minutes.

19)

Do not apply a signal that would exceed more than ¼ of class A1 to a “cold” 6C33C. The 6C33C stops become “cold” ONLY AFTER 30-40 minutes of burning by its cruise plate currant (I call it “Hot Mode”). Practically it means that you should not play your 6C33C amps too loud for the first half hour, particularly if you have low sensitively loudspeakers of too large listening room.

 

20) 

When 6C33C start with none-regulated power supply then it has the “start up gap”. The voltage is higher than cruise plate voltage and the currant is lower. With time, when the plate current hits up the tube the voltage drops and the current raises to it references cruise values. Usually a worked 6C33C reaches its 95% of cruise currant within 10 minutes.

21)

 Use the 6C33C “start up gap” to monitor the life cycle of 6C33C. If your cruise current is 220mA then a worn 6C33C might start at 200mA and then within 10 minutes reach 220mA. A newer tube will start at 150mA and then within 10 minutes reach 220mA.

22)

 When you take as brand new tube, then heat it’s filaments up for 2 minutes and start the amp with 20mA on plate. In 10-15 minutes the plate current will rise to 150mA – let it be this way.  Burn the tune for another 3-6 hours with 100mA-150mA and then consider it ready to be use at full power.

23) 

Violation of proper pre-heating rules and application excessive grid current to “cold” 6C33C lead to overheating of anode. Once anode was overheated it will have tendency to be more susceptible to overheating in this tube. A properly pre-burned tube, running “Hot Mode”, with fix bias and in class A1 is absolutely indestructible and perfectly thermo-stabilized.

 

24)

 It is advisable once a 6 moths to inspect the 6C33C’ filaments pins for the sights of corona, partially if your use not American trapezoid tube sockets buy Russian and Chaise triangular sockets. It worth also, if you have nothing else to do, once as 6 month to listed the balloon of cold 6C33C with juts applied filaments voltage with a stethoscope. A present of corona on the filament’ pins will, sound like a very light buzzing.

 

25)

Keep the power supply impedance that drives 6C33C as low and you can.

That is all that I have atop of my mind. Properly-used 6C33C is truly a problem-free tube with opportunity to have excellent sound if it is appropriately used in amplifier.

So, I was disappointed with moronic myths that audio people invent about this tube and to address that stupid audio-prejudicial folklore I have put together that “6C33C Survival Guide”.