Rerurn to Romy the Cat's Site

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…..

Rerurn to Romy the Cat's Site