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But if you truly feel bold, you might want to address the real problem with this amplifier: the tubes need more current and more voltage: 29 volts and 2.5 mA is a ridiculous operating point for the 6922. At least 70 volts and 5 mA would work much better. I once built a tube distortion box (fuzz box) for a guitarist friend of mine. The box held a 24 volt power supply and used 12AX7s! One grounded-cathode gain stage cascading into another. It distorted. One strange result was that different brands of 12AX7 sounded radically different. My friend was overjoyed and labeled the tube boxes with the type of guitar (or famous guitarist) sound the tube inside produced. I was puzzled, as tubes were new and I tested each tube carefully before giving them to him; besides, I had never heard such stark differences between these tubes before. Experiments were in order. My phono preamp ran its 12AX7s with 250 volts on the plate and at a fairly high current. Rewiring the preamp to half the plate voltages and half the currents made the tube differences much more noticeable and made the overall sound much worse. An analogy might be that while most of your friends are law-abiding easygoing people, should all them be placed in a life raft with no food for a week, each personality would twist in its own strange way. Some will say that this situation reveals the true nature of your friends; but really, as the brilliant modern day Socrates, John Searle, points out, this situation only reveals what they are like in a boat with no food for a week. So it is with tubes: deny them voltage and current and who knows what they will sound like, but run them up from their gooey bottom of their plate curves and they will function much more cleanly. |
But what can you do if the power supply only holds 35 volts and you do not want to add an extra power transformer? The remaking of the power supply shown below gives an additional 105 volt rail voltage. It uses a full-wave voltage doubler (in this case it triples because of where it finds its "ground" to boost the voltage up the tube's preferred range. (It also provides some safety enhancement in that all the extra capacitors are protected from too great a reverse polarity voltage by the extra diodes.)
Doesn't a higher voltage mean that the direct coupling must be eliminated? First of all, the safety enhancement garnered from using a coupling capacitor may easily outweigh any potential feedback instability or even any sonic liability the capacitor might bring to the circuit. As the amplifier is now configured, a blown fuse will probably blow your speakers as well. Removing the tube while the unit is use might do the same. Coupling capacitors break the DC domino effect's path of destruction and thus are highly recommended. But a coupling capacitor is not necessarily necessary and DC coupling can be retained even with the higher voltages. Resorting to our friend, the transistor, we can implement Lender's circuit, which combines the differential amplifier's two outputs into one. While the output of this circuit is single-ended, it is linear and the slewing ability we lost buy forgoing the push-pull arrangement is more than made up the sevenfold increase in idle current through this stage. Erno Borbely referenced this circuit in his 60 watt MOSFET amplifier article in the second issue of the 1982 set of The Audio Amateur. The circuit works better than most transistor circuits because it tends to cancel out much of the transistor's squirrellines in AC terms, but does require some tweaking in DC terms, i.e. matching and tight tolerance resistors. |
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