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You have given me my Rosetta stone. I have gone over all the schematics I have and now I understand how they work in general terms. I still have much to learn, but with the Tube CAD Journal I have a helping hand. Thanks again.
As for my great moment, this breakthrough in understanding is not it. My tube aficionado friends and I have a tube expert of sorts. Everything dealing with tubes must be passed through him for his approval. He is the Tube Man or at so I thought: I was convinced that I could never know more about the inner workings of a tube circuit than the Tube Man.
Recently, I had decided that I wanted to increase the idle current in my single-ended amplifier (300B based and cathode biased) and I pretty much knew what needed to be done. I mentioned my desire in front of my buddies and the guru. He explained that it was very difficult to change the idle current in a cathode biased amplifier. Puzzled why that should be so, I showed him the schematic to my amplifier and asked what needed to be changed. His reply was that the small valued power supply resistor needed to be changed! I asked if increasing its value would increase or decrease the idle current. His reply was that it was difficult to say. I then asked if decreasing the cathode resistor value would be a better choice. He acted as if a stupid child had asked a how to create atomic fusion. The cathode resistor is not changeable he told me. I was undaunted and asked if someone were to change it anyway, would decreasing its value increase or decrease the current flow through the tube. He didn't know. My tube expert was clothesless.
I will still listen to his advice on the sonic differences between tube brands (he has had a wide experience with different tubes and owns several hundred tubes), but not on the merits of the tubes parameters (he confuses mu for Gm) nor the circuits they could be used in. I have no desire to replace him as the group's expert or guru. This was my great moment, the moment I realized that I could understand tube circuits on my own.
Mark
Boston, Massachusetts, USA
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Your are welcome. Yes, current is the first thing to look into when trying to understand a new circuit. I have seen one tube circuit novice puzzle over the notion that one while one tube is completely turned off in a Class-B amplifier it is effectively not there, in spite of the voltage present on its plate. "But there's 700 volts on the plate!' he said to me. I finally offered him a thought experiment wherein a push-pull Class-A amplifier is transformed into a Class-B, manually. I told him to imagine that the amplifier is in use and whenever the output signal goes positive, the output tube that sees the decreasing grid voltage is immediately yanked out of its socket. And when the output signal goes negative, the is pushed back into its socket and the other tube is yanked out. After hearing this example the light bulb went on in his head. No current, no circuit.
Thanks for the great story about your guru. Tube gurus are one of things I dislike most about tube audio. It takes so little to be a tube guru. Just a passing knowledge of one or two circuits and a lot of shamefulness is all that is required.
I was once at a party and the hostess rushed over to me and told me that I had to meet this fellow who designed tube amplifiers! I met him and I was stunned to learn that he was completely clueless. Now ignorance is only ugly when it is accompanied by arrogance. He was ignorant. He didn't know that capacitors block DC current flow and he had no idea of how a transformer worked. He was arrogant. He was smug in his belief that he was supreme in his ability to design great tube amplifiers based on his ability to swap tubes and capacitors. What else could be possibly evolved in the design of an amplifier? I only asked questions and listened in horror to vastness of his ignorance.
This was a sad moment for me. A great moment, however, happened 15 years ago when a audiophile friend and I went to breakfast in Davenport California, a small sleepy coastal town 5 miles north of Santa Cruz.
We were eating our omelets when a young women sat 15 feet behind my friend and began
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