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Old 17th March 2005, 06:22 PM   #11
percy is offline percy  United States
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I now realize after reading richwalters post that the tubes should be transconductance matched AND the DC bias point should also be same for both tubes.

9am53, what this means for you is that get the matched tubes and make sure their DC bias point same. It depends on your amplifier circuit, and how comfortable you are doing it(lethal voltages). It wont help if the tubes have only one shared cathode resistor. Basically measure the voltage across the cathode resistance for each tube and adjust either resistance until both voltages are same and as per spec of the design.

To go even further you should assure that the opposite phase AC signal swings coming from the driver and feeding each tube in the P-P pair is also of equal amplitude - thats where the matched anode loads for the driver stage comes into picture.
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Old 17th March 2005, 06:42 PM   #12
9am53 is offline 9am53  Canada
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Thanks for the breakdown Percy...but I am still not getting this stuff...there is a lot to learn when it comes to the theory of these beasts and I have only but read the table of contents in the text. I will keep researching, but for the meantime I will use the tubes I (will) have, till they fry (which from what I understand will happen) then I will get a matched pair. thansk for the input though
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Old 17th March 2005, 09:01 PM   #13
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Hi there. I'll throw in another variable...to what already has been discussed....All tube testing is done with the heater emission at 6.3V (I've forgotton why this weird 6.3V was got at) but I've come across power tubes where most characteristics seem matched at this nominal voltage........then change heater volts +/- 10% and gm and Icath loose track of each other. In general there is considerable running tolerance.......an example; the 6550WB Svet is very emission sensitive, a 0.3V drop in heater volts can see Icath sag whereas the effect on gm won't be so dramatic.
Do the same on an orig mullard EL34 and GEC 88's both sit steady like a duck...I'm implying that there is probably some merit in running tube heaters at somewhat higher voltage but within the spec allowed for. I've always argued that 5.7V (at the lower spec end) is really too low for power tubes and low emission can do more harm than running too high. I may stand to be corrected....

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