Favorite 300B: Sophia vs KR

Hello Al,
The VAC REN amplifiers have auto bias, would`nt that compensate(at least somewhat) to the various ma levels?

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

Charles1dad,

The new Takatsuki TA300B tubes are still breaking in. I've got 31 hours on them so far. They are truely a game changer in the Frankenstien amps. The highs are chrisp and well pronounced and the base seems much more controlled. It is uncanny how these tubes change the personality of these amps. You were right on with your observations and I want to thank you for this tip!!! I do notice improvement from session to session. I have not tried the BT's in the VAC 30/30's yet. Hopefully before weeks end I will have a chance. One set of my BT's is marked with 69ma and the other is not marked at all and worries me. The ones marked with the 69ma came from Coincident and the second unmarked pair came from an Audiogon ad and perportedly have 100 hours on them. I'm a little afraid of screwing something up. I'll keep you posted

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.