Still, I think you are definitely on the right tract. Forgoing a feedback loop is worth pursuing in its own right. My own twist would be to add a switch that would allow switching the output stage on or off, so that I could double the usefulness of the power amplifier by including a 6 pin jack for electrostatic headphones.
   A good test bench for this idea is the Dynaco ST-70, as the 6SL7 and 6SN7 could fill the two front 8 pin sockets and two 6550s could fill in the two back sockets. True, this would render the stereo amplifier a mono block amplifier, but the power supply was somewhat overly tasked before anyway; now, it would loaf. The negative power supply rail could be derived from completing a full-wave bridge center-tapped circuit  by adding to solid-state diodes and one high voltage capacitor to the power supply. This arrangement would still allow for using the GZ34 rectifier tube.

Subject: Great webzine...
  I just found your webzine, and really enjoyed it. It must be really a labour of love. It is, as you say, very rare to find anyone looked behind the hype and claims at the theory and electronics of tube amplification. This is what interests me (along with listening to music of course). A few comments...
  I have a pair of M60s, by now very heavily modified, and have followed numerous arguments about whether they are really class A, their output impedance, and so on. Sometimes I contribute (mainly on the owners group) but I have got tired of being flamed when I do so. The truth is that they have an output impedance of around 11 ohms, and they cannot possibly be Class A all the way. They have a meaningful power output of around 25W into 8 ohms. Also they never go into A2, because the 6SN7 driver circuit cannot provide the huge currents involved. Most importantly, they sound absolutely wonderful, with a

Subject: ES headphone amplifier
   Your November issue contained an article on tube amplifiers for driving electrostatic headphones. While I don't own any headphones, I was impressed with the simplicity, i.e. elegance, of the final design. (Beautiful chassis by the way.)
  You see I have been tinkering with the idea of building a push-pull cathode follower output stage. This topology has only been tried twice commercially, Heath Kit and Electro-Voice, I believe. Apparently, while the output performed handsomely, the input and driver stages suffered from an inability to perform as well.
  My thought is that your ES headphone amplifier circuit would be perfect for driving this type of output stage. In fact, the whole last half of the amplifier could be DC coupled and with the naturally low output impedance afforded by the cathode follower topology, NO FEEDBACK would be needed!
  Thank you for such an excellent magazine and so many great ideas.
William J.

  The amplifier outlined in that article would work quite well in a push-pull, Cathode Follower output amplifier.  I would scale up all the parts and voltages to allow for even greater voltage swings.  For example, +/- 400 volt power supply rails and a 6SL7 driving a 6SN7.  But I would think twice about trying to directly couple the grids from the output tubes to the plates of the 6SN7. When the amplifier is cold but B+ voltage has developed, the grids of the output tubes will see +400 volts; this equals catastrophe. Ditto removing or jiggling the 6SN7 while the amplifier is in use. It is much safer to use a pair of coupling capacitors. Besides, I do not see how the output tube's bias could easily be set with direct coupling.


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