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As for the performance difference between the circlotron bridge amplifier output stage and the totem pole output stage, there is none, as long as the same tubes, the same idle current, and the same drive voltage is provided. I was distressed the first time I saw an electronic textbook treat the two circuits identically. "Wait a minute, these circuits are totally different?" I thought to myself. And they are in terms of ease of setting up bias points and living within the heater-to-cathode voltage limits, but not in electrical terms. The electrons do not know that they are in a long-named circuit: they just flow and their flow is governed by the voltage relationships and impedances in the circuit. To the electron, the both circuits are identical. Our eyes disagree. But then our eyes do not have to move the loudspeaker's diaphragm back and forth.
Subject: Dynamic Bias for SE Amp
Many like the sound of SE amplifiers, but most of us think the output power is not enough unless high efficiency speakers are used.
In Feb. 1990 issue of MJ, there's an article regarding Dynamic Bias for SE Amp. It looks quite promising, the bias point keeps shifting when signal level changed. It uses a power tube with Pa = 85W. At idle, the average current is about 40mA (30W dissipation) and about 90 mA (30W output and 35W dissipation) at full power.
Is it possible to apply same idea to 300B in order to have 16 to 20 W in Class A1 output or even more in A2?
Eric
California, USA
I do not have that issue, so I cannot comment on the circuit. But I can say that I leery of trying to milk too much out of an amplifier. It seems to me that we seldom get to have our cake and to eat it the same time.
My worry is that the auto-bias adjustment will come too late, after the crescendo has
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passed. I do think, however, that a two idle current states would be nice in a tube amplifier, one low current mode for background listening and one high current mode for serious listening and showing off to our audiophile friends.
A simple toggle switch on the front panel is all that is needed. In one position, the bias voltage is say -100 volts and in the other say -80 volts. This two mode feature would greatly extend the life of some very expensive tubes.
Subject: April/May issue errors
First of all, thank you very much for Tube CAD Journal. I can't tell you how much your magazine has taught me about tube circuit design.
Anyway, I have a comment a question about the latest issue. First the comment: I believe that several of the schematics have a small error in them. In most of the schematics, you have drawn n-channel MOSFETs with the arrow pointing out. The correct symbol for an n-channel MOSFET has the arrow pointed in. It's confusingly the exact opposite of bipolar transistors, where the arrow always points out for NPN devices.
Second, could you tell me your sources for several of the MOSFETs mentioned in the issue. I haven't been able to track down either the BUZ900/905 series or the Toshiba 2SK2013.
Thanks, and thanks again for such a great service.
Jon
Berkeley, California
Thanks for the kind words. I assume that (early on) you downloaded the PDF version of the issue, as the PN mistake was caught and the issue reposted with the correct symbols. Try the website today and see if the correct schematics made it. (Having 4000 readers re-download the issue is going to financially hurt the webzine, as the web hosting service charges a bandwidth penalty, which the journal's increasing
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