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Melquiades Amplifier
Topic: How to test 6C33C?

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Posted by Romy the Cat on 04-02-2006

It is sucks to live with Russians Tubes.  It is particular sucks to live with 6C33C. They are all deferent when they new, they age very ungracefully and differently.

The life with Super Melquiades, where I use 6x6C33C driving individual channels forced me to think traders about the testing the 6C33C as the lost of virility or gain if any of those six tubes could throw the Melq out of it normal performing level.

Over the years, using the BAT VK-60, Lamm ML2 and eventually the different versions of the Melquiades I was thinking about the different ways to test the 6C33C. Usually I did following:

1)  I had a log for each tube monitoring the amount of bias necessary to set the reference current. Since tube age the bias changes.

2)  Monitor the density and gloss of the tubes getter. Since tube age the getter’s appearance changes

3)  Monitor a tube gain by measure the amount of dB (with 1/4dB precision) that a channel outputs in it zone of transparency driver by a reference signal.

It kind of “works” so far but my approach has many major imperfections and points of instability, not to mention that with 6C33C the change in bias “might” means …. nothing. So, I always thought to find a stable and reliable ways to test the 6C33C. What I need is objective ways to measure gain and mutual conductance of each tube’s halfs at different frequencies and under different operation points

The tube testers are not good. Even if I added the 6C33C socket and slightly modify them then are not good for the 6C33C. Fist of all none of then can handle the current demands of this tube. For instance my Hickok TV-2 can run the 6C33C for not more then 55mA at 230V and at this operation the testers is ready to explode. Also, I do not need some kind of result but I need a perfect test result. The tubes testers do not drive the plates with DC (they use rectified but not filtered voltages) and I feel that it make the tubes in the testers and in the amps to behave differently. So, I am thinking now to build a “perfect” 6C33C tube tester that would be very precisely measure the mutual conductance and the tube gain.

It really should not be difficult to build it but I relay would like to avoid burning my fingers if the solution is already exist. Are anyone out there perhaps seen any ready to go testing solution for 6C33C? If not I will build the damn thing…

Romy the Cat

Posted by Romy the Cat on 04-03-2006

Well, it is getting interesting. I was able to make my Hillock TV-2 tester to test the 6C33C. I spend a few hours to filtering the plate voltage and replacing a couple resistors to 50W instead of the 5W. The plate coils in TV-2 is 100mA but it looks like it perfectly able to handle 200mA (burned some resistors the were replaced with more powerful). I was cooking a half of 6C33C for 30 minutes at 150mA and then inspected the testers. It looks like nothing got dramatically overheated. So, it looks cool so far.

The good part is the now I can measure worn of each half of the 6C33C. The bad part is that they all very deferent. This crappy damn Russkie’ss tubes, even when they new, very seldom measure the same cathode emission. Among the 12 new tubes that I have only one had the same mutual conductance between the half, the rest has 10%-25% of difference. The tubes that did measured identically were two tubes that were use with it’s bath hale in LF channel of Super Melquiades. Now I have to learn how to measure the gain of those 6C33C in this tester….

Romy the Cat

Posted by Romy the Cat on 04-15-2006

Somewhere in 1997-88 I bought a hundred 6C33C. I believe during those years the where more expensive then now and I paid around $7-8 per tube. Over the years burning the 6C33C in pair of VK-60 (8 tubes), two pair of Lamm ML2 (ML2), ML2 with Melquiades (6 tubes), in the Super Melquiades (6 tubes), conservatively giving the tubes away to some my friends (manse lending them but they never return) and scraping the objectively faulty 6C33C tubes (4 tubes among 100) I realized that I have a less then dozen tubes left. Yes, I admit that among those tubes that I have burned I did burn perhaps 10 of them purely intentionally. Still, how long the 6C33C lives? The last week I bought another 100 of 6C33C, so how long it will last in the Super Melquiades.

First of all, and many people might degree with me, but the life-span of the 6C33C dos not depends from the basing method. They use conservatively and sparingly in automated and semi-automated bias so what? If you use the 6C33C with fixed bias and overheat the plate then it is not problem with your amplifier but with the efficiency of your loudspeakers.

Also, if you stressed the “cold” 6C33C and overheated it one then it will be tend to be overheated in future, however it still will be perfectly functional and will sound with reasonable grid current perfectly normal.

So, how long the 6C33C lives. People out there claim that 6C33C will live for 5 year and more. If an amp works averagely 3 hour per day (an average number in average listing room) then it would make 5.500 hours. Good luck with those expectations!

Approximately 10-12 months of using 6C33C makes the tube loosing the getter’s gloss. This is a sign, as I undestend, that the tube’s getter worked up against the gas that built inside the tube. Sure, when it happens the tube should be replaces, but I feel that the tubes should be replaced sooner. When I do replace the 6C33C with another pre-burned 6C33C  after 10 months I do hear the sonic differences. Dose it mean that the last 2-3 month I used an inferior, worked up tube. Yes, it is more likely is the case.

The manufactures suggests that the tube is good for 750-1000 hours. We in audio do not burn the tube at 400-500mA (actually Lamm does at 310mA) and therefore the life-span we could get from the 6C33C might be much longer. With our burning rate of 3 hours per day 1000 hours is approximately one year but there is a catch. What would manifest the manufacture recommendation? Interestingly that the manufacture promises the 30% redaction of cathode emission in the end of 1000 hour. How does it reflect in Sound? I do not know how it reflects in Sound but I know that when I measure the mutual conductance of the 6C33C on my tube tester then a redaction of conductance for 15% is quite auditable.

So, I personally feel that 6C33C should be use very accurately after 7-8 moths. Of course it will sound 10 times longer but it will not have the necessary virility and color saturation. So, the 100 tubes would make 16 sets. If each set would live for ~8 month then would end up with 10-11 years?  Hm… not a lot….

Romy the Cat

Posted by morricab on 04-18-2006

Hi Romy,
I am very interested in what you have found out about the 6C33C tube lifetime because I am using a pair of Silvaweld OTL tube reference (their name for them not mine) amps that use 4 x 6C33C-b tubes per monoblock (so a total of 8).  These amps use a fixed bias scheme and so I check the bias more or less every other time I have them on after about 1 hour of warm up.  Also, as a push-pull OTL amp they are run in Class AB and therefore have significantly less current running through them than say for example the Class A Lamm ML2 (with 310mA).  With such conservative usage on the tube I would expect at least 2-3000 hours, wouldn't you?  Or are your comments only directed towards using the tube in Class A?

About the lifetime specification:  This is for their specified purpose inside a military aircraft radar power supply.  From what I understand they are driven quite hard in this purpose not to mention the constant subjugation to very high g forces most likely results in mechanical failure of some kind (also explains their extreme internal build quality).  My guess is that used in a more moderate environment that alone will extend their lifespan considerably (unless you have a habit of dropping them on the floor regularly).  Also, generally, when one runs a tube well below its rated max current or max. voltage the lifespan of the tube should increase accordingly as well.

Posted by Romy the Cat on 04-18-2006


I do not know the “truth” and what I said are pure my speculations based upon what I hear and what I measure.

Yes, Lamm drivers at 310mA quite hard but do not forget that his objectives, besides everything, were to get out of his SET max power in order to enable his customers to drive speakers with sensitively of 90x. Defiantly the lover plate current will prolog the tube life (and with appropriate transformer, in case of SET, will make bass better beside other things)

Regarding what you said.

First of all I think we should stop to refer to 6C33C as to some kind of crazy military tube used in aircraft radar power supplies. This is the stories that the marketing people drop to the head of literature-loving gullible audio people and they spread the myth. The 6C33C was designed as the powerful voltage regulator, the same as 6C19P, 6C18C, 6C41C and the rest. People in 1961 regulated voltages not only in the aircrafts. I remember Lars Fredel was blabbering that 6C33C was specifically made to use in Mig-29 radar power supply. The Mig-29 hit the design boards 20 year AFTER the 6C33C was mass produced. Also, how many those damn tubes is necessity to supply the aircraft demands? Victor Khomenko (from BAT) told that during the pick of the 6C33C production Russian made 100.000 tubes per month.  They did millions of them as still do them. Who the hell need them today in the aircrafts? Interesting that some of my Russian sources informed me that the 6C33C that were actually used in the aircraft radar power supplies were very much deferent (for instance golden plate and so on…) then the 6C33C that we stick into our amplifiers.

Second, let do not forget that the 6C33C used as a regulator dose not need a lot to regulate. The regulator tube in Lamm ML2 might be old and completely dead it still regulated perfectly fine and the amp sounded OK, even the regulated tube was completely unusable in the V2 position (the sound tube). In some other cases that regulator tube still fine measurement-wise but sonically it is dead (example is the 6C19P in Lamm’s L2 preamp, where the regulator sonically dies in 2-3 weeks) the point that I ma trying to make that the requirement that we have in audio are different than those that the engineers apply to the voltage regulators. I think we should consider that regulator tube is dead what it sounds bad not when it stopped regulate or when the tubes’ cathode emission reached a certain point. In most of the cases it does comply with some measurable of observable data, in some rare not really. Probably the different things should be analyzed with the different tubes, I do not know. In case of the 6C33C it is even more completed as they are very different to begin with and made according very not stable specification.  Some of them handle abuse very very nicely and some of the chock with 200 mA….

Generally if you do not stress them (play loud) when they still cold (15-20 min) then they behave much better in the long run….

Romy the cat

Posted by Romy the Cat on 12-09-2006

The more I deal with 6C33C the more this tube amuses me. Couple years ago when I started to use a half of 6C33C for my upper frequency channels I wired right and left my Super Milq with half of 6C33C connected to right and left side of the tubes correspondingly. The presumption was that if one of the cathodes used up (cathode poisoning etc…) then by flipping the tube to another cathode and anode side I would be able to use it longer.

I once in 6 month retest all my operational tubes and keep a log for each tube. So far I did not detect that a “used” half of the 6C33C   demonstrated any excessive wear relative to the idling half.  The used half do measure differently, sometimes 40% of transconductance was different (!) but it is not necessary in a favor of “used” or “new” side of the tube. Even more. When I take a brand new tube with identical transconductance AND gain of each half (hard to find among the 6C33C, though it is not necessary in most cases) and use it a as whole tube (two filament, two halfs of the plate and two cathodes) then after six month of operation each half still measure differently, despite of the fact that they were used identically. Tell me it is not strange? Perhaps I need to measure the cathode emission separately and the secondary grid’s emission…

Romy the Cat

Posted by carlosb on 04-10-2020
I have a TV2 too, please can you tell me wich is the configuration/settings to test the 6C33?. Almost I want to test filaments if it´s too much amp for the tester.

Posted by Romy the Cat on 04-13-2020
Carlosb, I do not remember. It should be written in my tester but… I need to find the tester. I do remember that I after I spend a hell of time to figure out how to test the tube with TV2 it I never actually used my finding and never tested 6C33 with TV2 in any practical sense. There is truly no needs to do it. The best tested of you 6C33 is your amplifier. You set the bias whatever your amp typically can handle and you measure the tube gain. Sometimes the sale of you bias will not allow you to do it. This would disqualify the tube but it will be very rare. Sometime you will not have the proper gain. This would disqualify the tube as well but it will be even rarer. Sometime the tube will not work at all. I have seen 3 cases like this in my life. Sometime the tube will have some odd light in it, even rarer. The 6C33 are very reliable tube and. Read about the new tubes bails run off at my site and you will be all prepared. You would need to know how a given tube ages. You can observe the gloss on the getter losing its gloss or you can assess how bias chases with time. Still, in my view all of it not necessary. Juts plug the tube, age it, confirm that you have proposer gain from you channel and then juts trash the tube in a year of a moderate use. They are cheap.

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