[Amps] Mobile Amps

[Will Matney]

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 at 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 at verizon.net>
>To: <amps at 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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