[Amps] W8ZR Autotune Amplifier Progress Report

[Jim Garland]

Hi all,

     As you probably know, commercial automatic vacuum tube amplifiers 
have been available for more than decade, but they are expensive; their 
pricetag adds about $2000 to the cost of a comparable manual-tuned 
amplifier. For homebrewers, autotune capability is complex and difficult 
to duplicate, at least for most of us who are not professional design 
engineers with access to sophisticated workshops. For the past year, 
I've been working on an autotune control circuit intended to overcome 
these barriers. The purpose is to make it possible for amateurs with 
average technical skills to add autotune capability to almost any vacuum 
tube linear amplifier, whether homebrewed or commercial.
     Here are the design goals for my controller:
     (1) The performance should rival that of top-of-the-line commercial 
autotune amplifiers, (My benchmark is the Alpha 9500.)
     (2) The controller should be easy to duplicate for amplifier 
builders with average homebrew experience.
     (3) The controller should be affordable, costing no more than 
$100-$200.

It has taken me a year to realize these objectives, most of which was 
spent learning to use an advanced, yet inexpensive, microcontroller 
called the Propeller PX32A. (The PX32A was designed in California by the 
Parallax Corporation, maker of the popular Basic Stamp controllers) The 
PX32A is a sophisticated device containing eight fully independent 
32-bit microprocessors that share 31 read/write IO ports and a common 
memory for storing variables, computation results and data. These 
features make it possible to construct a complete amplifier autotune 
circuit on an uncrowded file card-sized  printed circuit board using 
ordinary through-hole components that can be wired up in an evening. The 
circuit board intelligently operates inexpensive stepper motors and 
motorized switches to adjust the tank circuits of almost any h.f. vacuum 
tube amplifier. An ordinary PC or laptop computer programs the device,  
but once programmed, no external computer is required. Because of its 
power, the PX32A implements numerous advanced features while requiring 
only a handful of additional components.

Although it has taken me almost a year to write and debug the program 
code for this controller, I 'm finally ready to move the project out of 
the breadboard stage. Here's a link to a YouTube video that demonstrates 
the controller's user features. (My eventual amplifier will use an 8877 
triode in a conventional grounded grid circuit, but that's a topic for 
another day.) I apologize for the crudeness of the video, but hope you 
find it interesting and will let me know your comments and suggestions. 
(If the below link doesn't work, just Google "W8ZR YouTube Prototype 
Controller" )

    https://www.youtube.com/watch?v=1qDGoEElKcU

    Thanks and 73,
    Jim W8ZR