Subject: Circlotron
   John I think you need to offer us one more article on the circlotron amp, this time addressing offset adjustment. Like the Atma-sphere boys I plan to use one stage of gain, but cascoded with MOSFET to eliminate the need for a ridiculously high driver stage B+ to obtain the necessary current draw through the triodes and also solving the heater to cathode voltage concern.
   Also a pot tying the cathodes together with it's wiper arm connected to a resistor to ground in the first stage, followed by a cathode follower driving twelve 6AS7G's. Global cross coupled feedback will be introduced into the fully differential drive stage. Thanks to you I will be making use of the abundant and cheap supply of 12SN7's or 8SN7's. I find the circlotron to be absolutely simply and straight forward. I implore you to compile all your work into a book, we will buy it in honor of the magnificent tutor you are. You are not of the average tube specie, your articles are modern classics, I'll be on your ass until I see the book.

Rowan
   
   First of al thanks for the compliments. I don't think this circuit is anywhere near being exhausted. Now that I own a pair of Atmo-Sphere MA-1s, definitely expect some more articles.
   MOSFET cascoding of a triode does allow for big voltage swings. But do not forget that like all cascode circuits, the cascoded differential amplifier has virtually no PSRR. So either a supremely well filtered or regulated power supply or some of the noise reduction tricks this journal expounds will be needed.
    Feedback can be placed at the cathodes of the differential pair or at the top of the plate resistors by cross coupling, which may help preserve the gain of the driver stage.
   As for the book, I am asked monthly to come out with a book and I should consider it.

Subject: Ideas for articles
    I enjoy you magazine very much, it's just a pity that it is a bit irregular at present, though we all have problems.......
    I have an idea for an article which may or may not appeal to you (it doesn't to me, but seems necessary) is one of tube (valve) substitution, particularly in push pull output stages.
   What I want to be able to do is replace the KT66's which are becoming far too expensive for me in my Leak TL25+'s, with another valve which has nearly the same characteristics, but need re-biasing to work properly. Perhaps also, I may need to alter the voltage to the screens, but cannot alter the taps on the transformer, for example. So, in effect I need to be able to work backwards in designing an output stage, having certain givens, for example a-a impedance, supply voltage etc. screen taps at 43% etc.
    In view of the increasing rarity of some valves, and the almost plentiful supply of similar valves, it seems that an article like this is becoming more and more necessary to keep old amps alive. Maybe the performance won't be as good as original, but that's better than none in my case.
    Keep up the good work! (as we say here in England)
 
Julian

   The issue of tube replacement has always been a headache. Amplifiers designed around a certain tube that is no longer made or made well are particularly problematic. The supremely fine EL34s made by Telefunken could handle huge plate voltage that no current EL34 can match. The same holds true of the 5AR4 rectifiers being made today.
   What to do? I would give Eric Barbour's kind grunt work on this topic at the Svetlana web site a look. In your particular case I would hunt down one of the better 6L6 type tubes being made today by Sovtek or Svetlana.