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From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re:amplifiers
Date: Thu, 7 Dec 2000 14:21:57 -0000
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Peter et al - 
I guess the main reason for the three transmitter module design of the
Decca Txs is to allow a large element of redundancy or soft degradation
to be built into the design, in that PA units could fail and still leave
the system working at reduced power.  I don't really see the advantage
of duplicating this concept in an amateur capacity, as we don't mind
blowing the occasional device.  If modules were to be combined, this
would more likely be used as a way to get a lot more power.  My main
driver for using a 340V rail was the need to not have to have a separate
PA.  A 50V 25A PSU is fine if you can get one - I have an old telephone
exchange PSU - but normally it is a big beast if you have to make one
up, so multiple bridged lower voltage PAs is a complex way to go if
building from scratch, even if a lot safer.

My breadboard PA at 650W with a pair of IRF840s is approaching the limit
of what ought to be done with these particular cheap devices - using
them at about half of their rated Idmax, in fact I will probably derate
to 500W when testing on air, the 650W figure being due to an oversight
on my part in forgetting the 4/pi factor before building the breadboard.
The use of IRF450s or 460s at this rating has a huge overload margin as
they are rated at 15A and only being asked to switch 4A. This is about
the same reliability current margin as used in the Decca TXs.   The
voltage rating at 500V is closer to the 340V rail than the Decca design,
(200V rating at  66V rail) but still very comfortably within limits.
The catch diodes clamp over-voltage spikes extremally well, in fact I
don't see any spikes at all on the breadboard design.   Furthermore, the
TO3-P or TO247 case of the IRF450 family has slightly better thermal
conductivity than the TO3 case - hard to believe, but it is really so !

The 2.5kW PA version proposed reduces the current margin somewhat as
switching current is doubled to the 8A region, but provided an eye is
kept on device temperature, reliability should be good.   The tank
circuit provides excelent isolation against load mismatches.  As far as
I can see, the only potential device destroyer is a low impedance load
or short circuit.  This can of course be detected very quickly with an
RF current monitor, or even a DC current trip.   By the look of the
network, A detuned antenna will reflect back as an increased load
impedance giving an inherent fail safe mechanism.

Just started building the big PSU so far, another project for Xmas to
get this PA going.   
May try an on air test at 500W this weekend, look out for a BPSK beacon
transmission on 137790 sometime during the daytime

Andy  G4JNT 

> -----Original Message-----
> From: Peter Dodd [mailto:g3ldo@zetnet.co.uk]
> Sent: 2000-12-06 16:12
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: Re: Coils and amplifiers
> > While on the subject of power amplifiers the only successful design 
> that I have seen is the one by G3LNP, see 'The LF Experimenter's 
> Source Book', second edition. 
> I have seen solid state amplifiers that use many separate amplifiers 
> with the outputs combined using a ferrite transformer. This might be 
> the way to go in the construction of a less lethal design.
> The Decca transmitters were designed with safety (and 
> reliability) in mind.  
>


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