David,
Ok I see what you mean. No, the way I built a few was to use the air variable
but mount it inside the case where you had to take the top off to tune it. Now
that guy who built the one in question mounted it on the back panel. The
problem is that they can get to it there and tune it or really mis-tune it to
death. See the problem is with that cap, when you run the numbers to calculate
that output, you can never find the exact value of capacitance I've found using
fixed caps. On smaller amps where you can use those Arco compression trimmers
on the input and output, that's a big step up mounting them inside the cabinet
where the user cant get to them. At that, I always dropped a little paint on
the screws to show if they had been tinkered with. But, when you go up in
power, the caps size goes up current wise as in any amp. It would be the same
as using a bandswitch to drop in and out doorknob caps in paralell with Load C.
I have seen some use doorknob caps there, but never get t
he exact capacitance, so a variable is needed in my opinion. Just put it where
they can't readilly get to it to turn it.
Best,
Will
*********** REPLY SEPARATOR ***********
On 7/31/05 at 1:34 PM Dr. William J. Schmidt, II wrote:
>Will... I think you missed the point. If designed correctly, there is
>absolutely no reason for an adjustable capacitor in the output circuit.
>It
>must be a comfort thing... some folks can't seem to get away from knobs to
>play with.
>
>
>Sincerely,
>
>Dr. William J. Schmidt, II K9HZ
>Trustee of the North American QRO - Central Division Club - K9ZC
>
>Email: bill@wjschmidt.com
>WebPage: www.wjschmidt.com
>
>"If computers get too powerful, we can organize them into a committee;
>that
>will do them in." -- Bradley's Bromide
>
>
>----- Original Message -----
>From: "Will Matney" <craxd1@verizon.net>
>To: <amps@contesting.com>
>Sent: Sunday, July 31, 2005 10:16 AM
>Subject: Re: [Amps] Mobile Amps
>
>
>> David,
>>
>> The air variable is used in the output tuning. I've seen some use fixed
>> capacitance here and a step up using a mica compression cap. However, in
>> these large ones, I've seen many burn out. What some of the guys started
>> doing was making a piston type cap out of two pieces of brass tubing
>with
>> a piece of teflon sheet wrapped around the inner tube to insulate it.
>They
>> then slide it in and out to tune it. The ones they do this way are
>really
>> made kind of rough plus take up a lot of room. I always liked using an
>air
>> variable here and never had the problems. The smaller amps used either
>> fixed caps, which your limited on being able to tune, and or compression
>> mica trimmers where you can tune them. They just wont hold up on an amp
>> this size. This would be the cap in paralell with the secondary of the
>> output transformer, or on this amp, right after the combiner. The rest
>of
>> that amp looked like the others that's being built though.
>>
>> It's hard to see on the pic, but the RF keying circuit is wired together
>> by the parts leads in mid air and soldered. That's what I refered to
>> earlier as a rats nest. It's located right at the antenna relay. The
>best
>> I could see from the pic, it didn't have any bias circuit, so it's a
>class
>> C amp. $900 is a little steep in price, but they are 16 transistors
>there
>> or 8 sets of modules if one wants to call them that. At say 150 watts
>min.
>> each set, that's 1200 watts, or probably a little more. I don't know of
>> any mobile amateur amps available that size. That wasn't why I posted
>> about this amp. It was really so folks to see how they were being made
>by
>> the pics.
>>
>> Best,
>>
>> Will
>>
>> *********** REPLY SEPARATOR ***********
>>
>> On 7/31/05 at 9:55 AM Dr. William J. Schmidt, II wrote:
>>
>>>Will,
>>>
>>>I seen you discuss adding "air variables", etc. to the output circuits of
>>>transistor amps before... and can't quite figure out why one would do
>>>that.
>>>I've built and converted many transistor amps in the past, and they all
>>>used
>>>a nice set of high quality wide-band transformers... and a good gain
>>>leveling circuit that would cost just a few dollars. Anyone that reads
>>>the
>>>handbook or Schoneke's book ("Single Sideband Systems")... should be able
>>>to
>>>design and build adequate transformers. If its the antenna that's
>causing
>>>an SWR problem... its probably better to fix the antenna than build a
>>>pseudo
>>>antenna match into the amp.
>>>
>>>I see the mentioned amp lacks any harmonic filters. I *suppose* one
>could
>>>argue that the antenna is the filter network in a mobile situation.
>>>
>>>Sincerely,
>>>
>>>Dr. William J. Schmidt, II K9HZ
>>>Trustee of the North American QRO - Central Division Club - K9ZC
>>>
>>>Email: bill@wjschmidt.com
>>>WebPage: www.wjschmidt.com
>>>
>>>"If computers get too powerful, we can organize them into a committee;
>>>that
>>>will do them in." -- Bradley's Bromide
>>>
>>>
>>>----- Original Message -----
>>>From: "Will Matney" <craxd1@verizon.net>
>>>To: <amps@contesting.com>
>>>Sent: Sunday, July 31, 2005 8:28 AM
>>>Subject: [Amps] Mobile Amps
>>>
>>>
>>>> All,
>>>>
>>>> I ran across this solid state amp on ebay and thought it might be a
>good
>>>> one to convert over to amateur service. I about bet now it's class C
>but
>>>a
>>>> bias circuit can be added. It doesn't have a good RF keyer in it
>either.
>>>> However, the builder did add an air variable capacitor for output
>tuning
>>>> which is a step up for most. This "Pac Man" brand is a name I've never
>>>> heard of. From the pictures though, it looks like a good candidate
>>>inside
>>>> and that it may have enough room for a bandswitched filter board. If
>>>your
>>>> not interested in buying it, the pics give a good idea of what they
>look
>>>> like on the inside. Also, this amp uses 8 sets of push-pull amps using
>>>16
>>>> transistors. In other words, it's a big amp.
>>>>
>>>> Link;
>>>>
>>>>
>>>http://cgi.ebay.com/PAC-MAN-16-PILL-10-METER-AMP_W0QQitemZ5794298630QQcategoryZ48700QQrdZ1QQcmdZViewItem
>>>>
>>>> Best,
>>>>
>>>> Will
>>>>
>>>> _______________________________________________
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>>>>
>>>
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>>
>>
>>
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