[AMRadio] Ongoing Quest: Legal Limit AM Amp

Tue Jan 24 10:09:30 EST 2012

I had a 3-500 pair amp and it did not do 375 carrier without the  
plates getting very hot even with lots of air.  My current amp with 4  
572b tubes does better on am but the power supply transformer hums  
pretty loudly on voice peaks but does not seem to overheat.

Quoting Bernie Doran <qedconsultants at embarqmail.com>:

> nowhere does it say a 375W carrier
> ----- Original Message -----
> From: "Warren Elly" <w1gud4 at gmail.com>
> To: "Discussion of AM Radio in the Amateur Service"
> <amradio at mailman.qth.net>
> Sent: Tuesday, January 24, 2012 8:45 AM
> Subject: Re: [AMRadio] Ongoing Quest: Legal Limit AM Amp
>
>
>> Me thinks the gentleman needs something less than a dx-100...
>> You're not going to see much difference between a barefoot 100 and 375
>> watt carrier allowed by FCC
>>
>> 73, Warren
>> W1GUD
>>
>> On Jan 23, 2012, at 11:07 PM, Rick Poole wrote:
>>
>>>
>>> Looking for a good design for a legal limit amplifier for my DX-100
>>> (looks like I'll be homebrewing it).
>>>
>>> I've pretty much narrowed down the choice to a linear amp capable of
>>> 375 watts carrier output or 1500 watts PEP output at 100 percent
>>> modulation, vs a Class C amp with high level modulator driven from
>>> the DX-100's audio output.
>>> .
>>> I was thinking about a pair of 3-500's in Class B push-pull.
>>>
>>> Everybody keeps telling me that I'll get maybe 30 percent efficiency
>>> at best during dead carrier times.  At 100 percent modulation and
>>> class B I should get 60 percent at worst.  I really, Really don't
>>> understand that at all... seems like if I can get 60 percent at 1500
>>> watts PEP I should be able to get 60 percent at 375 watts dead
>>> carrier.  But, I'm accepting it for the moment, and anyway, I digress...
>>>
>>> 375 watts unmodulated carrier output at 30 percent efficiency means
>>> 1250 watts input, or 875 watts dissipation or 437.5 watts per tube.
>>>
>>> 1500 watts PEP at 60 percent efficiency means 2500 watts peak input
>>> or 1000 watts dissipation, 500 watts per tube peak.
>>>
>>> So it seems to me like a pair of 3-500's should be able to cut it,
>>> especially if I give them plenty of air.
>>>
>>> I want to stick with glass tubes because I'm planning to put them
>>> behind a window so I can "feel the glow" during operation.  Anyway,
>>> it'll impress the living crap out of the XYL :-).  Hence, the
>>> ceramics have been ruled out for now.
>>>
>>> So, a few questions...
>>>
>>> 1.  Will a pair of 3-500's in Class B push pull really handle the
>>> load with no problem, assuming adequate air?
>>>
>>> 2.  This may be a rookie question, but google hasn't given me a clear
>>> answer... is there such a thing as class B push pull grounded grid?
>>>
>>> 3.  There are some commercial class AB pair-of-3-500 amps (e.g.
>>> SB-220, TL-922) that are limited in what they can do by the power
>>> supply.  So, I plan to make sure the power supply can easily handle
>>> the load.  Any suggestions on the best places to get power
>>> transformers that are up to the job?  So far I haven't found one
>>> under about $600.
>>>
>>> 4.  I'm actually not that certain that I'm competent to design
>>> something like this from scratch.  Any plans available on the net
>>> that I can use for a starting point?
>>>
>>> 5.  Current tentative plans call for the power supply to be separate
>>> from the RF deck, and they may be separated by as much as 6 feet
>>> (there are compelling reasons for that that go way beyond the scope
>>> of this post).  Is that practical?  Where is a good source for high
>>> voltage (3500 VDC) cable and connectors?
>>>
>>> 6.  As noted above, I plan to put the tubes behind a window so I can
>>> see them.  Is there a glass that can be used for the window that will
>>> maintain shielding?
>>>
>>> Thanks...
>>>
>>> Rick WA1RKT
>>> Londonderry, NH
>>> Catch me on 14.300 MHz Intercon / MMSN most days
>>>
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