[AMPS] RE:autotune

[Peter Chadwick]

The autotune business is of interest - my next amp is going to have that.
First I need my 'utility room' finishing - the builders are on it now when
it stops raining. Various mishaps have led to the foundations going down 2
metres and absorbing nearly $10K (!) of concrete..........'Utilities' will
be a lathe, vertical mill, sheet metal brake, guillotine and a drill press,
so I can make a tool to make a fly cutter to cut involute teeth gears for
motorising the vacuum variable. Yes, I could have a linear custom built for
the price, but it's not as much fun. 

I'm torn between using phase detectors on plate and grid, or using a PIC to
tune up in the 'classical' way. Never having done any PIC programming, that
would be interesting - you need to run a loop which tunes the plate
capacitor for max plate RF volts and another to ratio plate RF volts to grid
RF volts for loading. These loops need to 'hill climb' to find the peak.
That suggests to me that with the +/- 1 bit possible error in the A/D
converters, the comparison has to look for a larger difference - possibly
digitise to 8 bits with the internal A/D C, and compare on the four most
significant. Comments welcome.

I also remember Dick (W4ETO -sri, can't remember the new call) saying that
the big problem is to keep the RF out of the controller. That will need
dealing with.

Using IC phase detectors on plate and grid will depend on the available
IC's. It probably really needs a pair of D types in ECL, but fast CMOS may
do. There's also a synthesiser IC made by Mitel that has a phase detector in
it that works at 50MHz. The big difficulty will be in attenuating the input
signals sufficiently without awkward phase shifts. I guess that can be
compensated on set up.

The tube is going to be a 4CX1000, as I was given a brand new socket and
chimney, 2 tubes, and the vacuum variables (500pF, 40 amps, 10Kv, and 1000pf
3Kv) came as wedding presents....the PSU will be a tuned choke filter.

Comments on autotune methods?

73

Peter G3RZP

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