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Re: [Amps] Quad Tube 4cx800 hf amp

To: "'Phil Clements'" <philc@texascellnet.com>, <amps@contesting.com>
Subject: Re: [Amps] Quad Tube 4cx800 hf amp
From: "Gary Schafer" <garyschafer@comcast.net>
Reply-to: garyschafer@comcast.net
Date: Fri, 26 May 2006 16:00:33 -0400
List-post: <mailto:amps@contesting.com>

> -----Original Message-----
> From: Phil Clements [mailto:philc@texascellnet.com]
> Sent: Friday, May 26, 2006 12:18 PM
> To: garyschafer@comcast.net; amps@contesting.com
> Subject: RE: [Amps] Quad Tube 4cx800 hf amp
> 
> > Moral of the story: Build the amp for the capability you are going to
> use
> > it> at. Not for what it could be operated at.
> >
> 
> I strongly disagree with that advice.
> Harris had a large contract with the Navy to build an amp that was to be
> mounted deep into the bowels of ships and operated in a no-time-limit,
> 1000
> watts output on RTTY, and all other modes. Another challenge was that it
> was
> to be "no-tune" from 2-30 mhz with no gaps in that range. Intermodulation
> products needed to be down 40 db below full rated output in all modes. It
> needed to be as idiot-proof as possible, and easy to repair. (.5 hours
> mean
> time to repair) It needed to operate in ambient temperatures up to 50
> degrees C.
> 
> The product was the RF-110A, which exceeded all the expectations, and then
> some. Hundreds, if not thousands were produced. I have two of them in
> operation here. They are over 20 years old, and have all the original
> parts
> and tubes. Two 8660 tubes, with an anode voltage of 500 v boost the RF
> input
> power of 100 mw to 10 watts to drive a pair of 4CX1500B's. If you look up
> "headroom" in the dictionary, there should be a picture of the 110A there!
> 
> (((73))))
> Phil Clements, K5PC
> 
> 

This is exactly what I am talking about. At what power level do you tune the
amp up at? If you tune it up at 1500 watts output and you get 55.5%
efficiency then you do not have any headroom above 1500 watts. It will
flattop if you push it any harder.

If you tune the amp up at 4 or 5 kw out and operate it at 1500 watts out you
have lots of headroom but you no longer have 55.5% efficiency at 1500 watts
out. The efficiency will be on the order of 35%.

You get the same efficiency as you would if you were operating the amp on AM
tuned to 4 times the carrier power to handle the modulation peaks.

Intermod will not be substantially better. But with the Harris amp I am sure
that it has negative feedback to reach the 40 db IM spec.

73
Gary  K4FMX


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