[AMPS] Automatic tuning of a VHF amp
km1h @ juno.com
km1h@juno.com
Mon, 10 Nov 1997 12:33:33 EST
Dave, how important is it to have to retune once the amp is set up and
running?
In my 2M 3CX1000A7 I have had no need to retune in over 7 years....since
I built it.
I also did away with the mechanical problems of flappers, etc by using a
small vacuum variable for Tune and NO control for Load. Load was
determined by moving the output ( a small piece of copper strap soldered
to the N Connector) along the split line until I was able to obtain rated
grid current. It was then silver soldered in place.
The amp runs an honest 60% efficiency and does 1500W with ease at 80W of
drive.
I suppose the input circuit could be measured and replicated inside with
no outside control and the Tune vacuum could be oriented also for a
completely blank panel.
The split line is coupled via 6 100pf 858 series doorknobs and there is
absolutely no tuning drift even after 3 minutes key down. The B+ has a
real HV fuse for safety along with a breaker on the 240VAC mains in the
PS portion.
Just a thought.
73 Carl KM1H
On Mon, 10 Nov 1997 12:26:26 +0000 Dave Kirkby <davek@medphys.ucl.ac.uk>
writes:
>Geurts, Gerard wrote:
>
>> In theory it is easy. If the triode is in grounded grid, the tube
>has
>> a
>> purely resistive load when the cathode and plate voltages are
>exactly
>> 180 degrees out of phase. This takes care of output capacitance etc.
>> as
>> well. You need a way to detect the phase difference between the
>> cathode
>> and the plate voltages and use this to drive a servo controlling
>> 'tune'.
>>
>> Problem is that you now measure one parameter (phase difference) and
>> need to control two variables ('tune' and 'load'), so you will need
>a
>> second parameter you can measure to control the 'load'. I guess
>output
>>
>> power (or RF voltage at the output) could be used. It might be
>> possible
>> to do something with grid current as well.
>>
>> This is all theoretical. I have never tried to implement this in
>real
>> life...........
>> I would go for the eeprom option.
>
>> Gerard, MoAIU / PA3DQW / AA3ES
>
>Thanks for your comments on automatic tuning. I assume a computer
>could
>be programed to do it from just the RF output, grid current and anode
>current - after all, thats all we use. However, I did not intend using
>computer control. I did at one time consider using an old 386 computer
>motherboard to look after things, but gave up the idea, for fear of RF
>noise, computer crashes that might be expensive. One gets the feeling
>it
>will be more complex than necessary, even though I have the computer
>and
>necessary A/D cards sitting around doing nothing useful.
>
>I had in mind a simple feedback loop, but perhaps that is not possible
>in practice.
>--
>Dave Kirkby G8WRB
>Department of Medical Physics,
>University College London,
>11-20 Capper Street,
>London,
>WC1E 6JA.
>
>Tel: 0171 209 6406 (International +44 171 209 6406)
>Fax: 0171 209 6269 (International +44 171 209 6269)
>
>
>
>--
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