[Amps] W8ZR Autotune Amplifier Progress Report

[MU 4CX250B]

Good question, Jim.  Yes, you will definitely need to remove the
detent assembly. You also will need to gear down the stepper motor a
bit. Here’s why: Stepper motors are typically 1.8 degrees per step, or
200 steps for a full 360 degree revolution. Radio Switch Model 86
bandswitches  typically have 30 degree indexing (12 steps for a full
revolution.) A 3:1 reduction of the stepper shaft reduces the rotation
to 1.8/3=0.6 degrees per step, or 50 steps per bandswitch position. In
their MRI amplifiers, ETO used nylon gears with 32 teeth on the
stepper shaft and a larger 96 tooth gear on the bandswitch shaft. This
gear reduction not only made the bandswitch detent unnecessary, it
also tripled the applied torque. Alternately, commercial gear-reduced
stepper motors are available at very reasonable cost. I’ve even seen
them with 30:1 reduction. Of course the bigger the gear reduction, the
slower the shaft turns.  That’s not a big problem because you don’t
the mechanical movement of switch to turn too quickly.

Another option completely is to use a motorized rotary solenoid on the
bandswitch, instead of a stepper motor.   Ledex has made these for
years. Every pulse of the solenoid winding advances the shaft one
position. A minor software tweak can implement this feature into the
Propeller code. The only disadvantage of a rotary solenoid is that
they turn clockwise only. That means that changing bands from, e.g.,
20m to 30m would require  12 pulses, while going from 30m to 20m would
require only one pulse. Again, a minor software tweak.
73,
Jim W8ZR

Sent from my iPhone

> On Dec 25, 2020, at 1:47 PM, Jim <jimw7ry at gmail.com> wrote:
>
> Question Jim..
>
> Do you (would you) remove the detent from a Radio Switch band switch? Or will a large enough stepper turn them with the detent in place?
>
>
> Thanks
> 73
> Jim W7RY
>
>> On 12/19/2020 3:33 PM, Jim Garland wrote:
>> 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
>>
>>
>>
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