Favorite 300B: Sophia vs KR

03-28-12: Charles1dad
The VAC REN amplifiers have auto bias, would`nt that compensate(at least somewhat) to the various ma levels?

Yes, to a considerable degree. Also, the bias conditions that the amp is designed to automatically establish are most likely different than the conditions under which the 48 and 80 ma numbers were obtained.

03-28-12: Maxmad
I believe there are differ auto-bias circuits around, also how and what exactly they do, is not the same.

The Renaissance amps are cathode biased. This paper provides a good explanation of that.

03-28-12: Pdreher
Al - yes, the readings on the bases of the VAC / Chinese tubes read 45 & 5.4.

I guess the fact that VAC supplied those tubes provides at least a little bit of confidence that the 48 ma tubes would perform well, as the 45 ma number is significantly less than the 60 or so that is marked on the tubes they supplied for my amp. Of course, chances are that the test conditions VAC uses are not the same as the test conditions the vendor of the 48 ma tubes uses.

BTW, the other number on my tubes ranges from 5.7 to 6.1. I suspect that number is the tube's transconductance, aka mutual conductance, in "millimhos" (ma per volt, referring to the change in plate current that results from a change in grid voltage, relative to the bias point). 5.7 millimhos corresponds to 5700 micromhos. The original Western Electric 300B spec indicates 5500 micromhos under typical conditions.

Best regards,
-- Al

Markus1299

Always test tubes before using them in your amp. Ones, I tried to use 300B TJ Full globe/mesh, what ever they called, and this cost me an USD60 fuse and I am lucky that it was fuse only. Never again.

My VAC 300bs are marked 48 or 47, so that gives me SOME better confidence but not much if that can vary so much depending on the test conditions.

Swampwalker,
Al makes a very good point concerning the 'ma' value can very under different test conditions or machine used.
My Takatsuki spec sheet said my tubes ma is 66.6 for both tubes. My friend has a digital tube tester and measured my tubes at 75.4 ma.

Markus I`m really happy the Takatsuki worked out for you. They`re a lot of money but I felt the level of your system would exploit this brilliant tube`s quality.

Has anyone been able to compare the Takatsuki with 1950 WE's?

I've just managed to purchase some 1950 WEs. After they arrive and they have run for a couple of weeks I can try and do a comparison between my Takatsuki that I have had since September.