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Posts: 182
Location: Serbia
Joined: June 25, 2014
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"By hot I mean that the tube has far higher current than normal for a given bias. The bias must be adjusted 10 volts more negative with this tube than others for the same current (70mA). Maybe this is common for new pruduction tubes,but it is certainly different from the Czech produced tubes."
Hot operating point should refer to the anode dissipation. Depending on the single tube, it will draw more or less current for a given operating point (voltage across tube and bias voltage). The curves given in datasheets (NOS) are related (and should be related in current production as well) to a "bogus tube" or better, an average production tube. After all, that is why "tube matching" is necessary.
If you need to go 10V more negative with this tube than with another to get the same current draw at a given voltage across the tube, this means that the tube is "stronger", "newer" - actually it means that the tube has higher transconductance (which will gradually decrease with aging). While your tubes obviously have higher transconductance than "normal", this does not necessarily apply to all TJ tubes.
Actually, I have noticed a rather high spread of characteristics with Chinese tubes in general (for instance, 2A3 by Shuguang: you can get matched pairs that draw 50mA, 45mA, and 40mA for the same operating point).
Therefore I guess that the reason why there are no "official" curves for the TJ 300B is the fact that production shows too high variation from a given average (which is obviously due to manufacturing issues). Nevertheless, from the user's standpoint what matters is actual peformance in terms of correct operation (no arcing and similar issues), longevity, and constant characteristics across the life of the tube.
"By the way The TJ 300b "mesh" can only take 28W maximum. Check with Mr Google, and Jac Music. My amp operates at 27W with TJ "mesh" tubes, that are recommended to operate at 22 watt."
I would like to pass on "Mr Google", it will just find what can be found on the net, depending on words indexed...
But it is not true that the 300B "fake mesh" can only take 28W anode dissipation!!!
1) This should be declared by the manufacturer;
2) The anode should redplate or develop red dots after a given point that would evidently show the maximum anode dissipation has been exceeded - which does not happen at 36W dissipation neither with "globe fake mesh" nor with "ST fake mesh" Chinese 300B.
3) Unlike the EML tubes Mr Jac likes so much (and are extremely expensive) because he distributes them (sic!), which are "real mesh" and thus the anodes due to the intrinsic characteristics of the material used have lower anode dissipation (lower mass per surface) -- the Chinese "fake mesh" anodes are not made of wire, rather they have holes in the plates which allow for the translucency effect (and the passage of electrons). But the mass of the anode is just slightly reduced, which is amply compensated by the fact that the anodes are larger.
It is also important to note that the globe and ST version of fake mesh 300B are not the same in terms of mechanical construction and anode material, the ST version seems to me even sturdier than Russian 300B tubes, and even the filamentary cathode draws more current than the standard 300B tubes.
Mr. Jac is just a tube trader - and an interested party in this case. I would not take too seriously whatever he states about the Chinese mesh tubes, just like I take whatever he states about EML tubes with a large grain of salt, because he distributes those tubes...
"True, but that was done to match the measured output impedance from Stereophile. I have not measured or found the actual impedance yet."
I am not sure what you mean, but what matters to the operating point is the LOAD. The load is the primary resistance seen by the tube on the primary of the OPT, as a function of the turns ratio and the resistance (loudspeaker impedance) on the secondary. It you are using a nominal 5k into 8 ohm transformer, and your speakers are a nominal 4 ohm, than your tubes "see" 2.5k primary resistance, or LOAD. If 6 ohm, your tubes "see" 3750 ohms. Obviously, for calculation purposes you should take into account the nominal resistance of the loudspeakers.
"I do not need much power, I operate between 0.1 and 0.3 watts, and like to minimize output impedance effects on frequency response."
How come? Are you using 104dB/W/m speakers in a very small room? I need every single mW I can get from my amps... at least as a dynamic reserve.
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http://rh-amps.blogspot.com/
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