[Amps] W8ZR Autotune Amplifier Progress Report

[MU 4CX250B]

Having used both gear-reduced stepper motors and rotary solenoids on a
hard-to-turn band switch, I find the rotary solenoid is easier to
implement, but the stepper motor solution is the more elegant and quiet
solution. One challenge with both solutions, however, is the need for a
limit sensor to establish the starting position of the band switch when the
amplifier is powered up.  With stepper motors, an optical interrupter
 works very well and is easy to implement. WIth solenoids, just having a
mechanical switch close at the limiting position is probably the easiest
solution.

With stepper motor control of variable capacitors, one easy way to find the
limit position is just to put a mechanical stop on the capacitor. Then at
power up, program the stepper to turn CCW enough steps to always bump into
the mechanical stop. For example, if the stepper requires 200 steps to turn
a full revolution, and the variable cap has a mechanical stop at 0 degrees
(fully meshed plates), then program it to turn 200 steps CCW at power up to
ensure that it always bumps into the stop. When that happens, the limit
position is established. This trick depends on the fact that stepper motors
aren’t harmed if they bump into a mechanical impediment. SteppIR antennas
use this property on all their antennas. When reset, a SteppIR antenna
always retracts its tape fully by assuming the initial position of the tape
is at its fully extended length.
73,
Jim W8ZR

Sent from my iPad

On Dec 25, 2020, at 7:01 PM, Jim <jimw7ry at gmail.com> wrote:

 Thanks Jim.

Good points on the gear reduction of the stepper for the band switch.

I happen to have a Ledex hooked to 86 switch. It was part of a marine or
military automatic antenna tuner. The 86 has about 4 or 5 wafers on it. The
solonoid is either 12 or 24 volts. Its been a while since I've looked at
it, but I did use it for a remote balanced tuner with a single cap turned
by a 12 volt DC gear reduction motor.
The 86 switch did not have stops, nor did it have a detent.


And yes, it only went one way, which is not a big deal. Seems to me it took
less than 3-4 seconds to come back around full rotation. It had a small
switch wafer on it to remove the voltage from the solenoid when it got to
its wanted position.




Thanks
73
Jim W7RY

On 12/25/2020 3:32 PM, MU 4CX250B wrote:

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> <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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