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.

That was very interesting post, here are some thoughts of my about it.

KOTriode wrote:

Romy,  No, the DAC was not set at maximum level (for best sound) while Cyrille listening to the system. It was set at normal volume for the whole system.

Ok, it was not at it optimum sonic level but at least it was at identical level of digital degradation for no preamp test and with preamp in the signal path. If so then it was fine.

KOTriode wrote:

What we both discover is that with the preamp the soundstage is much larger (by about 25%) and extend way beyond the speaker edges, there is also more depth when listening to Aznavour live track. The tonal balance is not that much different except that Tempation track, the bass was  deeper, the midrange tonal balance remain the same, but there is so much more detail and speed, the high was smooth and much more extended through the ribbons.

From what you describe I can conclude that your DAC is not good. You said you use DIY wireless DAC with tube output. I do not know what wireless mean in this context, but it is not important. I also know do not like any tube output stage in DACs, I have written about it extensively. Anyhow, my rational to blame your DAC is following: the sonic changes you describe shall not take place if your DAC is able to drive your power amp properly. It is not only about output impedance. Something in that DAC’s out stage does not work properly as a good preamp much not make a difference as you witnessed. With very good preamp you might have a little better dynamics and slightly better bass but it shall be all. If you witnessed so dramatic soundstage change, change in detail and speed and higher extension of HF then you are just way beyond of definition of preamp transparency. Are you sure that what you observe was not change in cables or something like this?

KOTriode wrote:

This is for me an unexpected result, since I was expecting tonal balance to change with this 6C33C preamp or any additional amplification stage, it wasn't in this case.

As I said above, if I got the result you did then I would revise what my front end outputs. Find another DAC, of phonostage of CD player with DC-coupled SS output stage, it shall not be any good devise but any consumer unit would do. Do the same experiment with insertion of your preamp. You most like will not has the same difference.

KOTriode wrote:

The preamp was not without fault. On this 100db system with 8ft away from the speaker there is a faint 60Hz hum, on the bench the hum was measured at about -58db which is quite high. My guess is that the hum came from the transformer and was picked up from the chassis, since like most power tube when using in a preamp, there is much higher microphonic than small 9 pin or octal tube. There is more work to be done to eliminate the hum. I will probably try to mount the tube sockets on some kind of rubber minimize vibration from transformers.

Oh, this is nothing. I am sure you will find out the problem and address the hum. The hum must not be there, the preamp shall be dead silent and with indirect heated 6C33C it is very simple to do, particularly with regulated circuitry. Beside the standard ground loop procedures and dealing with ripples you might want do not let the undirected heaters to hang without reference to gerund…

Romy the Cat wrote:

From what you describe I can conclude that your DAC is not good. You said you use DIY wireless DAC with tube output. I do not know what wireless mean in this context, but it is not important. I also know do not like any tube output stage in DACs, I have written about it extensively. Anyhow, my rational to blame your DAC is following: the sonic changes you describe shall not take place if your DAC is able to drive your power amp properly. It is not only about output impedance. Something in that DAC’s out stage does not work properly as a good preamp much not make a difference as you witnessed. With very good preamp you might have a little better dynamics and slightly better bass but it shall be all. If you witnessed so dramatic soundstage change, change in detail and speed and higher extension of HF then you are just way beyond of definition of preamp transparency. Are you sure that what you observe was not change in cables or something like this? As I said above, if I got the result you did then I would revise what my front end outputs. Find another DAC, of phonostage of CD player with DC-coupled SS output stage, it shall not be any good devise but any consumer unit would do. Do the same experiment with insertion of your preamp. You most like will not has the same difference. Rgs, The Cat