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In the Forum: Melquiades Amplifier
In the Thread: Driver tubes, grid leak biasing, and fidelity
Post Subject: I am not an electrician.Posted by Romy the Cat on: 2/17/2006

 peter foster wrote:
My understanding had been that biasing method is a method of setting up a given tube so it works with the wanted anode current at idle with the wanted anode voltage and these conditions are met by somehow applying a correct grid DC voltage and that properly implemented biasing could always be done several different ways and the tube would have exactly the same harmonics and sound during normal AC signal operation.

Peter, I disagree with many assumptions you make. You view a circuit only from a perspective of propagation of currents but there is a lot more to it.  Defiantly to set the different parts of the circuit to operate within the wanted conditions is a noble task but is it sufficient enough for shaping Sound? No all aspect of Sound might be described by harmonic patterns, measured by distortion analyzer and extrapolated inot voltages and hertzes. If it would be so then probably any person who has a multi-meter and scope would have good sound in his/her listening room: as you know it is not the case..  Furthermore the most hard-core "electricians" in audio usually end up with most disable-sounding amplifiers.

Anyhow, there are many biasing method and despite that all of them in the end set a given tube to work with a wanted anode current at idle with the wanted anode voltage but Sound that this tube /stage produce will very different with different biasing method. It would vary form tube to tube, from application to application and we can’t not blindly look at it using a targeted plate current/voltage as an abstract reference. To understand that it requires embracing very different thinking about the entire mechanism of amplification, it’s goal, the definition of success, and to clear recognize the differences between the essential foundation of process and a language (superstructure) that exists juts to support the survival of the process’s foundation.
 
 peter foster wrote:
My understanding had been that there would be no engineering explanation for why well implemented biasing methods in class A signal tubes should change the AC operation and distortion character of the tube.

In case of Milq’s first stage, there are explanations (I have writhen about it) but I do not believe to those explanations and I have many contra- arguments against those explanations. Once again, if you look tan the biasing of the Melquiades first stage ONLY from a perspective of maintaining a necessary negative potential on grid then it would not make any sense to you.
 peter foster wrote:
My understanding had been that when connected to a preamp with DC at its output, this DC could be amplified, and many speakers do not like DC. 

Peter, first off all Milq has no DC it inputs (the is what the positive gas tube is for). Second, if it has then what would happen? Absolutely nothing. I played Milq in past with many-many volts at input and it only affect sound when you switching volume control…
 peter foster wrote:
  My understanding had been that tube aging does not rest well with grid leak biasing and that grid leak biasing was confined to AM radio makers who used it in preference to other methods primarily for cost purposes.

The gas regulators tube when they age they pick some nose and nothing else. There are no even theoretical issuers with the ratability of gas tubes in my case. It used in many many many circuits as voltage reference… how many of them you have seen to went down? If the gas tube do go down then it juts stop regulate, nothing else. Also, I do not care about  “AM radio makers who used it in preference to other methods” as they had totally different objectives in thier design and used gas tubes for totally different purpose. Peter, if you stop to look at the Milq as at a “voltage arrangement devises” then you might discover that the gas tubes in Melquiades used not for a voltage regulation but as the current decoupling devises.,,
 peter foster wrote:
Okay, about 600 to 1,200 ohms for 1/2 a 6C33C will do, so about 400 ohms for a whole tube, and that's not far away from a guess of 320 ohms.  400 to 1,000 ohms would give the better fidelity….

Of course all configurations (more loading and less loading) were heard and evaluated. Frankly speaking, Peter: how you might propose that something might have a “better fidelity” without knowing the compliance of the specific drivers and without performing the actual listening?
 peter foster wrote:
My understanding had been that OPT ratios for triodes RL should exceed 4 x Ra and for pentodes and tetrodes which have loads = 0.9 Ea / Ia, where Ea is the idle anode voltage, Ia is the idle current, RL << Ra.

Perhaps, but I do not follow those rules. Dima, proposed initially to start loading the plate with 10X but then, playing with plate voltages, current and combining the section of the secondary we end up with whatever we and up.

Rgs,
Romy

 

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