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Sine I informed public about Melquiades amplifiers I receive many emails from people asking me about the operation points of 6C33C. For whatever reasons people mistakably took me for some kind of 6C33C guru, which is am not. Moreover I could be hardly be a consultant to any electronic engendering subjects, probably with an exception of the very narrow fields where I personally happened did something and was able (mostly accidentally) to reach some conclusive results. This post is my sharing with public about my experience regarding the 6C33C’s operation where I today do feel comfortable with my findings and with my result.

I do use 6C33C for years and I experienced it in many amplifiers, I like this tube. However, I begun to actually drive it on my own since the Melquiades project started last year. There were countless experiments done during that time to optimize the 6C33 used and more or less optimum scenario was found. However, with the Super Melquiades (3 channels of Melquiades) the task was to push the envelop of Single Ended’s capacity. The Super Melquiades’ MF and HF use ½ of the 6C33 and the LF channel uses one full 6C33.

My primary attention of this post is to share my finding of how the LF channel of Super Melquiades was resolved as the solution turned out to be not that I was expecting.

Forget the paper’s operation data on the 6C33. The 125V and the current up to .5A perhaps goes a job in a voltage regulator of MIG-29 but it has no relation to sound. Certainly, in my case the 6C33 was used and tested with fixed bias, no other biasing methods considered civilized for this tube (sound-wise).

My initial sentiment was to replicate what Vladimir Lamm did with this tube - 175V/300A. He suggested that he used this tube for his designs sine 1971 and that he was the person who opened this tube to public. Obviously he knows what he does with 6C33, however, he uses 6C33 regulated but I do not, and do not practically like anode voltage regulation. Secondary Lamm is highly conditional in his visions and this restricts his output. So, when I made Melquiades I had in there 200mA transformer and it was no surprise that I had the very best bass result at 180mA/200V. Any further increase of current worsened sound whish obviously was die to the core saturation, or at least it was what I though… the ironic part was that Melquiades with it’s little transformer ad way better bass then 310mA Lamm ML2 drive at full throttle. I attributed it to many Melquiades’s specifics but not to the lower plate current.

Last few days I spent a LOT of time playing with the Super Melquiades. The Super Melquiades has 450mA 1/8 output transformer with inductance of primary of 15H.  It is non-sectional little-monster that specifically made for LF operation and that roles off at 630Hz. It powered by 1A power transformer with multiple secondary 275V, 300V and 330V. After a full wave rectifier the channel has input 10H/500mA chock, followed with 100uF, then 50R resistor and 350/10.000 capacitor, sitting directly next to the primary of the transformer.

After I set it up I spent quite a time to driving my LF line array and to listen the thing under the different operation, voltage and current. Then, armed with scope and function generator I begin to measure the thing trying to find out some correlations between the auditable and measurable. Dima Kereev (“deemon” within this site) pretty much ran the show with the “measurable” and he was the one who interpreted the results of all my measurements. We spent countless amount of time on telephone and now I learned how to measure with a oscilloscope the pH factor of my coffee. :-)

Anyhow, to my big surprise the fashionable believe that the 6C33 loves high voltage turn out to be incorrect. With increase of voltage and dropping current the tube certainly give up more power but bass become contrived and pretentious. The channel kind of demonstrates a willingness to push bass even if there is not need for it. With the specious recording there was bass but there was no Space – the primary evidence that the sub-audible bass was not there.  In ALL scenarios I find that 45-50W on plate is more suitable power dissipation for this tube. So, the measurement-wise at 275V and 170mA the channel pushed at full power down to 23Hz with 5.2V at input,  then with decrease of input voltage to 4.8V the channel was able to care this power down to 14Hz. The rise of current helped a little, however driven by continues 20Hz  at 5V input signal the tube (any tube that I tried) had a tendency to heat the plate.

At 250V the total out was also strong but the behavior was in some way identical with 275V on plate. The exception was that the lowest non-distorted max power was at 21Hz and with reduction of input voltage the channel begin to distort at the same 14Hz. The heating up of plate was way less aggressive and sonically the bass had way less nervousness and anxiety then when it was driver at 275V

I liked the most when I drove the 6C33 plate with 230V. I run all possible currents and… surprise, surprise I ended up at 150mA.  At this current I had in somewhat reduced total output power (34W on plate ~12W at aoutput) but the tubes measured very nicely. The most important it has a phenomenally interesting sound bass- very reach, very low, very articulates but at the same time very… VERY… “loaded”  At full power the channel ran down to 16.5Hz and with a reduction of input voltage from 5.2V to 4.8V the amp holds the perfect shape of wave down to ~7Hz. The anode at 20Hz and 5V of input hold the temperature very nicely.

Before the closing up the amp I thought to dump those finding on my site. I do not know how generic my result might be but I hope some of you would find it useful. At least it might be a collective reply to a dozen of guys who contacted me asking how I used the 6C33C, I never said to them anything defiantly before….

Rgs,
Romy the Cat

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