The 6AS7 is a pass tube in a DC power supply. Heres a link, one has to
scroll down a bit and read the application paragraph at the beginning. I
dont think brand is going to make a lot of difference as they are all
made for the same application as stated clearly here. .
I've certainly seen the page for the 6AS7 :) What I am telling you is the the 6H13C is a different tube (the Russian variant). Once preconditioned, they can hold up better than an American tube. Far less likely to see the cathode coating falling apart, at least until the tube gets weak.
Regarding bias stability, the amp has no need for a servo. If the DC Offset is unstable, its likely noise in the driver tube as the output tubes obtain their bias from the driver tube. Normally the DC Offset is the sort of thing that you set or at least check once every 6 months or so. IOW, its **very** stable!
Current is well defined by science. I didnt know there was a special audio definition.???
There is! I regard the audio versions as common myth, often bandied about inappropriately: http://www.atma-sphere.com/Resources/Common_Amplifier_Myths.php
Although we disagree on many things I do appreciate your Gentlemanly approach, unlike that other fellow who left us.
Thanks - let's hope he stays away. His approach produces so much noise, its impossible to have an actual conversation, not to mention his creation of an entirely new wing of physics (or at least alternate meanings to words to which no-one was previously aware)...
Just to be clear, I have a lot of respect for you as I do Nelson Pass, John Curl, David Berning and a number of others. There's a lot of snake oil in this business so its refreshing when we don't have to deal with that. Like you, I've been at this a long time but went down a different path a long time ago:http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php
and while I understand completely how the Voltage Paradigm works (IOW I don't make amps to be 'tone controls'), I'm not at all convinced that the Voltage Paradigm is the only way to achieve the most neutral presentation. For me, the reason was best expressed by Norman Crowhurst, who pointed out a good 60 years ago that while feedback of course suppresses distortion, it also introduces some of its own, which tends to be entirely higher ordered harmonics. Its not that I'm against feedback, but its inappropriate or inexpert application does bother me, and for that I'll use the current ARC amps as an example.
The problem is that the ear converts all forms of distortion into tonality (and the ear/brain system has tipping points where that tonality can be favored over actual frequency response), and the the most egregious problem in audio IMO/IME is brightness (and its twin brother, harshness), which in transistors is entirely caused by distortion; also in many tube amps that use feedback. This is because the ear uses higher ordered harmonics to sense sound pressure, so is really sensitive to them as a result (moreso than most test equipment due to the range that the ear has to cover)! The line I draw in the sand is I want it to sound like real music as opposed to a just a good stereo. To this end, I do my best to vet every customer's system and expectations in order to make a sale. This limits my sales for sure, but it also results in really excellent results both in sound and customer loyalty when everything is set up right.
Regarding the different gain in the M-60s, its entirely possible that the CCS is damaged. Clio9 was an early adopter of Mk3.3 and I suspect he has earlier CCS boards in his amps. IIRC that is... If you can send a photo of the CCS board to my email (found on the atma-sphere.com website) that will tell me a lot.