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| Chirag wrote:|
|The 6E5P is an interesting device (I did some research after you dropped the "tetrode" hint and bought a few :-)), but the bias scheme with the OA2's is still incomprehensible to me. |
Ok, I will try to explain.
The cathodes must sit on ground, this it given. You can apply fix bias to the tube but it would require a transformer or a cup to decouple the DC from input. Defiantly you do not want to do it. Dima came up with an idea to milk voltage on a grid resistor, that works very nice but it require an opposite polarity voltage compensation at the input. So, the schema of the “resistor bias” was born. I built the amps I listened it. The problem is that the amp sounded too rough and too unbalanced. It was a very good enough to get the “Best sound at CES” of to thrill the idiots DIY-enthusiasts but my objectives were much more evolve and I found that this was not as musical enough as I would like to. Dima and I initially thought that the problem was due to the minute drift of the voltages across the bias. Dima, come up with a number of quite elegant automated ideas how to get the reference voltage from AC, not really the regulation but rather the active monitoring…. I implemted them but unfortunately they did not sound good.
We were thinning about a solution and the probable reasons and eventually we found it. I had a guy (Geoff Cook) who managed to make microphone amplifiers and phonocorrectors using input penthods that did not sound as the usual input penthods. The Input penthods all sound “hard” but his amps were “soft”. One of his secrets was a very specific type of screen supplies. So, we decided to apply the similar principle to our bias supply, feeding the 6E5P grid and to treat the 6E5P’s grid as the penthod screen. Dima, disagreed with this semantic substation but I personally, operating in the conceptual language rather then via the language of the mathematical approximation still call the 6E5P’s screen and grid interchangeably.
You see, it is very fascinating to feed screen of a penthod with different type of voltages and to see how the penthods sound changes. The penthod screen, or in Milq’s case the grid, pick come currents/violates that travel to the source - the screen/grid voltage supply. The whole story is how the power supply would react to those “returned alien voltages”. The PS is basically a termination of a transition line from the grid. It should very gently absorb those “bounced voltages” and completely decuple the “bounced voltages” from the plate supply. I build two, negative and positive regulators, to do the job. They were ultra fast regulator unsung 7788 as an amplifier and 6C19P as at regulator. It still did not produce the sound that I needed. Then, and I do not remember who and how the ideas come to us, we decided to put as simple well sounding gas regulator tube in a negate supply of the bias. The rational was that a gas tube essentially should be beautiful buffer that should very softly react to the grid currents and would be perfect isolation from the plate supply
I remember as I very first time when I connected a separate gas regulator of my Fluke 409 tube power supply to the Milq’s bias. It was 10PM and it was like an instant bliss. It immediately reimbursed me with the exact sound that I was visualizing for quite along time. It slowed down the brutal and unreasonably fast dynamics, it harmonized and coordinated everything and set the correct relation and reasoning between the pitches, it created completely different phenomenal (!) bass and it basically made up the Milq sound in a way in which it is know today. After that to put a positive regulator juts to balance out the DC at the amps input was a simple mater.
I know electrically it completely has no meaning and it all was done juts get a correct sound out of the driver stage. All together it took a month and a half or so to discover all of this and to optimize the driver’s stage performance. This is why I fully inherited the Milq’s driver stage for the “Baby Melquiades” project- because it works. If you wish I might post the complete schematic of the “Baby Melquiades” in a way in which is currently assembled. However, looking at the compile Melquiades schema is very simple to figure out what the “Baby Melquiades” is. Juts replace the plate resistor with a transformer and drop the plate supple voltage. In my case I feed the 6E5P ‘s plate with 178V…
Romy the Cat
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche