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KOTriode wrote:

Thank you for the comment on this amp, this is my first time of using the 6C33C tube, I decided to build this amp after reading about the Melquiades, this amp is just a quick experiment on the 6C33C sound, since I have a single chassis intended for a phono/preamp laying around for year now, normally I would build a mono block. I picked up 4 these tubes for less than $50 on my trip to Asia last year, they are harder to find now over there, why? nobody care of these cheap tubes, so nobody stock them any more.

I am surprised to hear it. The 6C33C is incredibly widely available tube and it is VERY cheap. In the Milq time I was getting them from Russians for under $5 shipped to Boston. I think nowadays they are a bit more expensive but still then are in the range if 12-15 bucks for tube in you buy a quantity of them. They shall be widely available on eBay and everywhere else.

KOTriode wrote:

Vaccum tube rectification/regulator in amplifier: Yes,  there is quite a few debate on these subject. Two years ago after building a 845/WE212E amplifier with tube rectification/regulator (yes, with 1200V regulator), and compare the sound with previous rectification/unregulated 845 amplifier, I would never go back. With this 6C33C amp, I have made an extra step in this experiment, using the same tube in the amplifier for the regulator, so that the regulator tube  do not add it's own tube sonic signature (I normally used 6AS7 in other regulator). Also, with tube regulator, I did not have to use too much capacitance, and do without choke, saving amplifier real estate and weight. Another strong point for regulator: I can adjust B+ anywhere from 160-220V, and with adjustable bias on the tube from 100ma-300ma, I should be able to find its sweet spot.

I am not convinced by “I would never go back” comment. In addition I think that each type of tube, each type of biasing and each specific application might response differently to the benefits of regulation. Your experience with tungsten-coated direct heated tune might be very different with indirect heated tubes. What I am trying to say is that I would find your experience more educational if you use the same 6C33C in the same application with and without regulation. For instants I was able to get much more interesting sound regulating the screen supply of phonostage input pentode (7788). Still, if I regulated with the same regulator the 6C33C grid supply then it was disasters to sound. So figure…

Another very important reason why I do not regulate my 6C33C is that you have mentioned – the regulation does not go with input chokes and large caps. Regulator does not drive them well and mostly I saw after regulator very flimsy filtration. This all make the stage to sound pretty much like the “sound of regulator”. I do not like it. I tend to stabilize current with very big and heavy PS along with strictly input chokes. The adjustable current on the tube from 100ma-300ma and 160-220V? Why do you need it? The experiments with tube operational parameter people do during the design phase and in most of the case the tune is drive from external power supply. I use Fluke 409, half-ampere, 600V PS with two dozen tubes in it. In the operational amp there is no need to change current so much. Also, if you change the current from 100ma-300ma then you change the plate impedance too much to keep the same transformer ratio…

KOTriode wrote:

It's still too early to tell how this amp sound, as of last night the meter showed only 6.6 hours of break in time (you can barely see the hrs above the "Curtis" hour meter. So far I learned that one of the brand new tube failed with properly heater burn in, it needed -125V bias instead of around -70V. I also learned from the tube seller that you need burn-in 24 hrs just for the heater on these virgin tubes before applying any current. This amp run really hot, the whole chassis get hot, I installed an fan under each power tubes and and it helped a lot. I should able to tell the sound of this tube after about 100hrs on the meter.

The hrs meter was a great move. I would like to have it on my amp but it is too late. Post your observation about sound when you feel you are ready. Also, be advised that 6C33C is indirect heated it means it will not work in A2. With your former 845/WE212E if your driver was able to drive them with plate currents then these DHT more or less were operational on the “negative side” the 6C33C will clip VERY sharply. I do not know what you drive with this amp and what the size of your room but make sure that it does not send the 6C33C to A2.  H

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