Quote:
Originally Posted by MaloS /img/forum/go_quote.gif
Interesting. Although, 900 Vpp does not mean it needs to be 900 V peaks, it can swing -450 to 450. Is that still outside of limitations of 6sn7?
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What follows is general information and my own opinion. I don't know anything about the prototype of a commercial amp you're speculating about above. I suspect your information or interpretation is mistaken, or perhaps the advertising copy was written by someone with more enthusiasm than knowledge. No criticism of an existing or future design by anyone is made or implied. I'm just giving information to illuminate the discussion, okay?
There are several classes of 6SN7, their electrical distinctions being far, far more important to their capabilities and thus their sound in an amp than any mickey mouse differences tube rolling within any class can achieve. The "standard" 6SN7GT has a maximum plate voltage rating of 300V. The 6SN7GTB (or older, now less common GTA) has a max plate voltage rating of 450V. Thus a GTB can be operated in theory up to 900Vptp with a split rail power supply. (1)
However, a maximum rating is not a suggested operating rating. Far from it. The makers of the tube tell you that if you approach any of the maximum ratings, you should derate the other operating parameters of the tube. You may not use two or more maximum parameters at once.
There are further limits, called maximum dissipation, that is heat management. The plain jane 6SN7GT is good for 3W5 on either plate, or 5W with both plates operating (i.e. 2W5 per plate). The GTB is good for 5W on only one plate or 3W75 per plate with both plates operating.
That in turn limits the current you can apply to the tube. A certain minimum of current is required to drive the earphones, and a different minimum of current is required on tubes driving output tubes to overcome Miller (parasitic) capacitance and to ensure proper bandwidth in both the bass and the high frequency.
Thus, if you hog out the voltage on a 6SN7GTB to 450V with both halves operating, current on each plate can be only 3W75/450V = 8.33mA. That's the theory. It is good practice not to run tubes over 80% of Pdmax, so now we're down to 6.6mA. That's not enough, so one reduces the plate voltage to get the current up.
There are other problems, relating to noise (distortion), when too high a voltage causes the current to be reduced so as not to overheat the tube. The amount of current on the plate determines how high above the illinear portion of the tube's transfer curve the quiescent (zero signal) operating point lies. This point is the electrical centre of a sloping line. At one end of the line, assuming a Class A1 amp (for an electrostatic headphone anything else would simply be too noisy), grid current will be drawn and thus drive the amp into Class A2 (noisy!), at the other end the line will dip into distortion because the tube can no longer amplify the signal faithfully (linearly): big noise. Beyond that lies current cutoff and 100 per cent distortion. We thus want to put the design centre (zero signal) of the transfer slope as high as possible so that at its low end (max volume, lowest current) it still sits above the nasty bits of the tube's transfer. For that 10mA is better than 8 and 6mA is totally unacceptable.
There's an informative piccie on my netsite:
http://members.lycos.co.uk/fiultra/112KISScurves.jpg
This is a graphic illustration of the transfer function for a WE300B in my T39 Ultra-Fi design but the principle described above is the same. The illustration shows that everything is interrelated: voltage, current, power, distortion, etc.
More pics of transfer curves here:
http://members.lycos.co.uk/fiultra/KISS%20190.htm
Description of the pics is found here:
http://members.lycos.co.uk/fiultra/T...mp%20INDEX.htm
My general tube amp site:
http://members.lycos.co.uk/fiultra/JUTE%20ON%20AMPS.htm
This is where I shall publish the Stax earspeaker drivers I designed last year when I get around to writing the accompanying text.
Andre Jute
Visit Andre's books and hobbies at
http://members.lycos.co.uk/fiultra/
"wonderfully well written and reasoned information
for the tube audio constructor"
John Broskie TubeCAD & GlassWare
"an unbelievably comprehensive web site
containing vital gems of wisdom"
Stuart Perry Hi-Fi News & Record Review
(1) These maxima apply to NOS American and British manufactures and direct equivalents. Chinese and Russians so-called equivalents to 6SN7 do not necessarily have the same ratings or quality of manufacture; what they are capable of may depend on who manufactured them, the batch, or, most commonly, who tested and selected them for rebranding.
