Reader Paul's stock AQ-2004 line stage amplifier

      Either way the 1500 ohm cathode resistor shared by both halves of the 12AU7 is suboptimal. It implies a miniscule idle current for both triodes and a very nonlinear operation. No doubt its value was chosen to increase the voltage drop across its leads rather than pay the price of using a negative power supply or not to tax a wimpy power transformer.      Additionally, the 12AX7 output tube makes little sense, as the tube's miniscule current draw cannot overcome long lengths of capacitive interconnect or drive amplifier with low impedance inputs. Oddly enough, I have seen other similar designs. Strange.
     The easiest rework of the existing circuit would be to connect the positive portion of the heater power supply to ground and thus create a negative power supply of -6.3 volts. This negative power supply would then feed the common cathode resistor. The value for this resistor is still too low for an adequate current draw for the 12AU7! A 12AU7 should have an idle current of at least 5 mA per triode, which would require a common cathode resistor value of 1k. This brings up a concern I have with the power supply: just how strong is it? Can it support any increase in current draw? I suspect it cannot and thus the wimpy idle current and tube selection. The potted power transformer looks good, but it also hides the actual size of transformer inside.
      If the power supply is at its current limit, then the best move would be to use the 12AX7 as the input tube and the 12AU7 as the output tube. The common cathode topology is retained. And as the 12AX7 will provide far too much gain, a feedback loop is used to lower the final gain of the line stage.

    First of all, thanks for providing the schematic. I get requests like yours but without schematics. Am I really expected to hunt down one? Second, I commend you for moving up out of the mere part-exchanger mentality into the higher realm of tube circuit understanding; very few audiophiles do. An  acquaintance's single-ended tube amplifier disappointed him. So he bought the most expensive tubes he could find. It still disappoints. Its bandwidth begins to fall off at 8 kHz. And no amount of part exchanging is going to extend it. When told this, his reply was that was not what the people who sell expensive parts tell him (amazing, considering that those part sellers have not seen the circuit or even know how to use a scope.)
    As for the line stage, you are right: it looks very cute. So cute that it would tempting to buy one and then gut all the original circuitry, as the circuit is about as unfortunate a design as I could imagine. A 12AX7 output stage??? Usually, we need only to look at the parts quality to see where an amplifier went wrong. But in this case, the part quality is relatively high. What went wrong was a poor circuit design. Just about any other circuit would be preferable and I can think of about five possibilities that would retain the same amount of tubes and many of the parts of the original. But first let's look into what is already there.
     After a volume control, the signal passes to a cathode follower that then cascades into a grounded grid amplifier, which in turn, cascades into a cathode follower output stage. Another way of looking at the circuit is that it is made up of a common cathode amplifier cascading into a cathode follower.

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