Well, I would say that “something is wrong” as what you report it is not how it shall be. I had my doubts about the rational to use 6C33C in direct heating coupling but the “danger”
 in direct coupling uselessly coming from the plate of the driver tube and from breaking of PS sequencing. Abyhow, I think the problems you are experiencing now are normal for new amp and exposure to new tubes. As both of them mature you shall not have those problems. Let look at what we have in the hands.

KOTriode wrote:

Totally disappointed today, while I was listening to music, the left channel make a small pop and quit. The bias meter showed about 25ma bias instead of 200ma. Test the tube on bench showed that the tube current stay about 25-50ma max. This tube is gone abrubtly instead of slow death after 31 hours of playing.

I would call it unlikely. Sure any tube can catastrophically fail and you might be unlucky in this case but I would say that it is most likely not what happened. I would rather blame your regulators and PS then the tube. From the mid 90s I use 6C33C. I use a few BAT amps, few Lamms and few Milqs, in all cases it was multi-amping and I went over many of 6C33C. I have perhaps 2-3 tubes that failed with what I call “catastrophe”. So, I fell the reliability is not the factor for 6C33C at all, in fact it is VERY reliable tube. The stability of the parameters is totally other mater – it might be anything you can imagine but still the tubes with ran way paramagnets in my view are perfectly operational.

Make sure that your PS are reliable and that you do not loose bias supply “at the mid of the night”. Then the sequencing. Here is how it must be: 1) heat the filaments for at least 2 min 2) Apply bias and lock it value. 3) Apply plate voltage. I do not know how bias your 6C33C but I presume that it might have some kind of bug that kills your 6C33C.

KOTriode wrote:

Took another brand new tube and plug into the amp with just heater power on, nothing! Turn out a brand new tube has dead heater, amazing!

Never had it I think. What I has however was that the new 6C33C have the pins covered with some kind of protective anti-corrosive shit that does not conduct electricity. It is not to mention that if you use bad tube sockets then the heater female pins might burn up very fast (not after 31 hours of playing of course). So, you need to make sure that you clean/scrub the 6C33C pins what you put need tube in. If there is no ignition on the filament then jerk the tube in the socket vaoalsntly and it will scrub the pink and you will have your filaments back. It is not DHT tube and the filaments are not fragile in this tube. It is impossible to shake this tube to the point that filaments will be damaged.

KOTriode wrote:

Take out another tube, heat the filament for an hour, and check the bias, this time it is less than 100ma, another goner.

KOTriode wrote:

Check reliabilty of 6C33C on the net and found this: I can also sell you a carton of 50 for $900, including ground shipping within the continental US. At this price they are "as is" - I will not replace any which are broken, out of spec or dead  (they're a bargain anyway). Reports suggest you can expect 10-20% to be significantly out of spec. So, it looked like that these tubes left the factory untested and it is up to the buyer to sort them out. I order some " Ulyanov " tubes on Ebay, anybody have tried them yet? What label of these tubes proven to be reliable?

KOTriode wrote:

Romy, I have good confidence that it was the tubes and not the circuit. I have checked the cathode driver that bias the 6C33C and there was not any problem. Sure if the cathode bias failed, the 6C33C current will shoot way up and burned up the fuse, it did not happen in this case. The 6C33C tube went with a small pop, such as you cue the cartridge onto a record, it did not die, but the bias current is way low. I have a total of 8 tubes, after burning in the heater for a day and test the rest of the tubes, now I have only 4 that bias is within 10V of each other. The other 3 is about 30V off, and one did not have conitnuity on 1 filament.  Like you said, I probably bought some garbage tubes.I just order some 6C33C tubes on Ebay: Edited: link removed. because these looked interesting with gold grid wire, none of my 8 tubes I had it. I will test these as soon as I receive them.So far nothing was damaged except the tubes, that is good news. I remember reading somewhere that other tubes like 6AS7 sometimes short the plates, one time at the show, I was listening to an OTL that has about a dozen of these 6AS7 tubes, and one of them sparked with a flash and make a very loud noise on the speaker, I hope the same thing does not happen with the 6C33C in the future. Anybody out there felt that there is one brand of 6C33C better than other? What about the Svetlana?

way I am not the best person who ask about the reliability of 6C33C as I do not acknowledge the reliability problem for this tube.  I have seen this tube failed on OTL applications. I have seen it gone down in PP application as the operational parameters for 6C33C very difficult to match for positive and negative halves. However, I did not see the 6C33C behaved badly in SET applications with output transformer. I did heard the people complain about the reliability of the tube, primary the thermal  run out of plate but I did publish the preventive measures for it. So, what I personally experienced is that the 6C33C with preheated filaments and stable fix bias demonstrate absolutely no problems that you describe. BTW, I use to know a number of 6C33C users and I never hear from them any complains about reliability of your type. Again, the reliability is NOT the same as the values of parameters – they might be anything you can imagine with 6C33C. As I told: since I do not recognize the bias voltage as important operational parameter and can accommodate any voltage with my bas type then I do not particularly care about the parameters run of on the new 6C33C. I care only about plate current and gain and they are dead stable year after year. Below is the picture of my amp with currents right in front of it and they run pretty much dally for a few hours.

I understand that you do not want to hear my assurances that the tube is reliable if you have problems but since I did not have any problems and do not know the specifics of your circuit then all that I can do is to share my experience. I would tell you even more. I personally, because of some my stupid accident in experiential stags was running the tube at 900mA and 150V for prolong time, dissipating 135W on plate. The tube not only did not go down but I in fact still have this specific tube and the tune is still perfectly operational. If you want more stories then I’ll tell you that I know people in Russia who stupidly believe the 6C33C need to be run at high voltages. Those fools run 6C33C at 650V – twice max permutable for this tube. Reportedly the use it for years and do not complain…

About the different version of 6C33C. I wrote about it in past. I did experiment with many different types of 6C33C but my search was not for reliability (never had problems with that) but for sonic differences. My conclusion is that it was not worthy. I did not see any specific differences between early or late productions, between different manufacturing plans, between different markings, between different “K” marks (Russian equivalent of QA acceptance). They all more of the less were identical in my view. The only reliability problem that I had were the 6C18C – the precursor of the 6C33C. The 6C18C are good sonically but 50% of them I trashed right the way.  But if you read Russian specification for 6C33C then you will see that 6C33C was developed to replace the 6C18C and to eliminate the know mechanical problems with 6C18C assembly.

Regarding the tube that you pointed at eBay with gold plate – I thought that I have all 6C33C ever made in my collection but I never had this one. Russians did make the gold plate 6C33C. The gold plates were made for military and in fact only gold plate tubes were use in Mig’s radar power supply (the regular tubes that we use in audio were not used in Migs). It is VERY hard to find the military gold plate tubes nowadays, no one was able to find them yet as far as I know. It was very limited production and they were reportedly given to air force technicians by signature, like bricks of gold in Fort Knox.  The advantage of the gold plated grid is that they are able to handle full power right from beginning, with no tendency to overheat, as gold-plating reportedly restricts the secondary emission from the grid of the tube. Will it make any difference in Sound? I do not think so but it needed to be tried. The only obvious advantage is that you shell be able to test this tube at full power and full LF right from a freezer… I still would not do it…..

Paul S wrote:

To be clear, I do NOT recommend anyone else leaving any 6C33C in 50+ Watt boost position this long….

The 6C33C preamp? Hm, this is bold. As I see you use it as a single stage buffer with only one 6C33C and couple output with a transformer? I head the Germans built something like this but I never heard it or saw it. I do not feel a need for such a preamp, even though I drive very complex load.  Still, it is for sure interesting. When you fill conformable then writhe how it sound, particularly in context of a mandatory for any preamp bypass test. BTW, I can see in the back of the amp two tubes, looks like regulators. One of the tubes has getter all eaten up. I usually do not use the tubes with so dramatically eaten getter.

KOTriode wrote:

From what I remember from a tube dealer who said that these 6C33C need to have the heater burn for many hours before apply any current, if not the tube will have shorter life.

What your dealer advised is fine:  you can burn 6C33C heaters for many hours before apply any current.  I disagree that it will prolong the tube live but it will minimize the current swing during the tube break in. I do not recognize the current swing in the new 6C33C as a problem and I have given dental instructions how to deal with it:

http://www.goodsoundclub.com/Forums/ShowPost.aspx?postID=6140

Let look at in the process deeper. Reportedly when the 6C33C were made and the oxide-coated cathodes was activated some of the oxide contaminated grid that in a new tube begin to act as second emission devise in new tubes. As the tubes begin operation the oxides on the grid begin to burn out producing gases that are eaten by getter.  It is not know if the problem with 6C33C is due to the contamination of grid or die to the large amount oft left over gas in the balloon. In one way or another it looks like the heating up the tube for prolong time before running it at full power do help. It burns oxides from grid if they have any and it heat up getter, letting it to absorb gas from the tube.

What I do not feel comfortable, purely from practical perspective, is a recommendation to run the tube for a long time with heaters only. If you have a separate chassis to burn the 6C33C filaments then it would do but to do it on your main amp it is bit too irrational in my view. You need to put a separate swatch in the amp to hold the high voltage and you need to take the amp out of operation for long time – how you will listen music if your amp is busy with tube burning? With the same success you can get a party of your new 6C33C and dump them into kitchen oven at 320 degree for few hours. This will activate getter to do it’s duty and will help with some grid pre-burn.  To put the tube into the kitchen oven? Ok, not we are talking about the pure cooking with 6C33C. BTW, do not forget to put in the over some Italian herbs and paprika….

My methodology is much simpler in my view and do not take amp out of operation. Put the new tube at 20-30mA and in 30-45min you will have the max current swing the give tube will be able to demonstrate. You need to lower current constantly during the first 30-45min as it will/might go up and up. In my estimation what you start from sub 50mA you will not have during the first turn on more then 250-300mA. Eventually you will reach the point where currant does not grow. It will be it. This is the max swing. From now start to use the tube as useless. With the time, 12-70 hours of initial operation you will see that the current will be dropping and dropping, drop bias and give to tube more current.  The point is that if you use your amp let say 3 hours a day then it will let you to have your amp for 1-2 weeks perfectly operational while the 6C33C will be breaking in.

KOTriode wrote:

On my next mono block,  I will push them hard to get at least 25W with less than 5% distortion, that means run 2 tubes in parallel, but only one triode in each tube.

Why would you need it? I do not see any rational behind it.

KOTriode wrote:

The preamp has the classic tube regulated power supply using 5R4, 6AS7, 6SJ7. It is transformer coupled output and has no capacitor in the signal chain, the gain is about 8db, so it is not a buffer. For convenience, it does has a remote control. It is still in the work, but will be able to sing pretty soon. The 6C33C preamp and amp has only one power switch, both has two transformers, one for just the heaters, the other for B+ and regulated supply. When plugged in, the heater will light up first, when flipping the power switch, the B+ will start slowly with the regulated power supply. Without hitting the power switch, I can burn in the new tube filament over night . The rational behind using two 6C33C with one triode in each tube is that it will allow you to have a total of up to 90W plate dissipation for two tubes instead of 60W using single tube or for higher reliability (according to the spec) 3000h with 70W versus 45W two tubes. Another feature is that I will be able to use the amp with 1 tube and adjust the bias for 45W dissipation for less demanding speaker ( Quad ESL 57).  Also my output transformer is rated with 350ma/30W so it should be interesting to squeeze more power out of it. For me it is a very convenient feature. By the way the preamp only use one triode instead of both. I always like using output tube in preamp, to me  it is sonically superior to the small 9 pins and octal tubes, the only downside is the microphonic. I will report the sonic quality of the preamp/amp in the future.

The microphonic 6C33C in your preamp? This is interesting. I never use the 6C33C in the line level situations and therefore I never had the situation that it might behave microphonic. Try to put a small rector before grid and put some ferrite bids before the grid pin. Also, try to shut down temporarily your fans that cool down the 6C33C from the bottoms (it would be noise to have a picture of it).

I also like preamps with very powerful drive but I am with the world of SS preamps with direct coupling. I found that it would be very interesting to listen the 6C33C preamp. What kind transformer you get for it. Also, I do not know where you live but if you in US’s East cost and ever come to Boston then I would very much do not mind if you bring that 6C33C preamp for a few hours to let me to hear the beast. I have no idea how it might sound but the notion to have 6C33C in preamp sound too absurd to pass the opportunity to hear it. Too absurd? Well, this is very much in my alley…

The biggest question would be if the half of 6C33C is as good at line level buffer as let say 2-4 of small 9 pin tubes. You also have that output transformer that is controversial aspect. I kind of do not feel at ease with line level output transformers, but it is me….

KOTriode wrote:

Romy, The normal bias for the Golden Grid tube for 200V Plate at 200ma is about 70V-80V for the good tubes, the bad tube read 0ma at this bias, when drop to -30V it start reading some current, but can't get to 150-200ma current for the SE amplifier. The people who make the OTL amp that I talked to also see abnormally higher number of new 6C33C tube out of spec and unusable than other Russian tube such as GM70, pentode family. He also added that he has seen an OTL amp used in a store played 18 hours a day every day and the tubes lasted for 3 years.