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In Reply to: RE: PX25 max dissipation?? posted by twystd on July 13, 2007 at 14:38:08
Interesting question
There is not many info of these tubes
I play abouth a half year now with these tubes
but i am not shure what the best operation point is
after many try outs i stuck with 390 B+
cathode resistor of 535 ohm and measure 29,8 volt across them
thats around 20 watt.
Tried also 22 watt but find it a litte to hard sound
but i still experimenting.
There is not many advice on the net about this tube
i use them with a 3k5 load and sounds many times better than a 300b
also the load lines that are suplyed with the tubes are not correct
the tubes that i use are very different with a little math i came to a internal resistance of 760ohm and a mu of 3.94 and a steepness of 5,2
I hope that more people give a post on this question
to learn from them
Best regards Robert
Thanks Horny (great moniker !;-), that's helpful information. When you say 390B+ are you stating the voltage on the anode to ground, or the voltage across the tube (anode to cathode)? I assume it is anode to ground.
If the actual measured results don't match PX25 plate curves, it makes it pretty tough to design an amp. I realize it will only be in the ball park on paper, and that measurements and listening will determine the actual op points, but at least it will be close. Perhaps Brian might take an average transductance PX25, and run some curves on his Sophia curve tracer, and make the information available.
Your measured results sound a lot closer to a 300B than a PX25, but I really don't care about that, what I care about is the sound. It is encouraging that you say it sounds many times better than a 300B
My concern for accurate plate curves is because I'm designing a differential amp from scratch. Without reasonably accurate curves I can't design a proper driver stage, and play with different load lines and op points, to see how it effects distortion and output power.
I really don't care if the actual curves match the original PX25 or not. Even Osram put out two different spec sheets, that I could find. The curves weren't the same, and one sheet said max dissipation was 20 watts, and another claimed a max of 30. What I do care about is the sound, and reasonably accurate curves (I realize there will always be some variation).
twystd
I have done the curves. Email for a copy.
Brian
Take control! DIY.
O yes my moniker i need a new one, have no horns anymore
and it is a little weird indeed :-)
The 390 is measured on the anode of the px25
so with 30 volts tru the cathode resistor its 360volt over the plate.
my amp is like this:
12a4/5k:5k interstage/ux245/5k:5k interstage/px25/3k5 opt
tried also a 300b on this amplifier but the px25 is more delicate like a
ux245 same character but more power.
i like it a lot but still finding the best operationpoints.
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