[TenTec] PA current control

[Gary Hoffman]

This seems to be a very clear explanation of how it is done, based on actual
analysis of the circuit.

Thus it seems reasonable to accept that this is how it works.

That being so, I wonder why Ten Tec has gone to the trouble of protecting
the Radio (note I don't say the PA) with such a fast acting device ?

Mine on my Corsair I and II have tripped many times due to "cockpit errors"
such as mis-tuning.

This was obviously not a catastrophic failure....so I wonder what actually
was being protected ?

This is not a challenge of the explanation.  Rather, it is a request for
further explanation of why the protection is there and just what is being
protected.

It sounds like I should be able to load up my ground rod at 100 watts and
never hurt the PA.

Hmmmm.


73 de Gary, AA2IZ


----- Original Message ----- 
From: "Bob McGraw - K4TAX" <RMcGraw at Blomand.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Cc: "Kevin Purcell" <kevinpurcell at pobox.com>; <ken.d.brown at hawaiiantel.net>
Sent: Sunday, September 23, 2007 11:36 PM
Subject: Re: [TenTec] PA current control


> The control of PA current is derived from a R in the A+ line to the PA.
The
> voltage across the R is fed to the 2nd mixer board and driver which
reduces
> drive inverse to the PA current.  The current never gets to max
> destruction value.
>
> Having a Paragon manual in front of me, the current sensing R is R1 {0.015
> ohm 10W} located on the DC power board.  This voltage is fed to U2a which
is
> a level shifter and its output  feeds U2b to which the output if it
exceeds
> the Ic limit of 22 amps at full power, diode D25 is forward biased and ALC
> actions results, therefore reducing drive to the PA stage.  Thus PA
current
> will always be limited to a max of 22 amps for some 300 watts input
> regardless of load Z.  Yes, in theory, it will drive a dead short with
rated
> output.  Remember the Tentec PA is not current limited.  It is drive
> limited.
>
> The fuse, the circuit breaker and power supply limiting serve no purpose
> except
> in the event of catastrophic failure and thus exists simply for the
> protection of
> other components.
>
> Big difference.
>
> 73
> Bob, K4TAX
>
> ----- Original Message ----- 
> From: "Kevin Purcell" <kevinpurcell at pobox.com>
> To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
> Cc: "Kevin Purcell" <kevinpurcell at pobox.com>;
<ken.d.brown at hawaiiantel.net>
> Sent: Sunday, September 23, 2007 7:12 PM
> Subject: Re: [TenTec] (no subject)
>
>
> > The transistor "fry" because they heat up, their current gain
> > increases and the collector current runs away.
> >
> > <http://en.wikipedia.org/wiki/Thermal_runaway#Bipolar_transistors>
> >
> > You aren't going to be able to heat them up in 17nS they are
> > thermally too massive. The time constant in the power supply is not
> > the determining factor. The thermal time constant of the system is
> > the important issue.
> >
> > This is not an over-voltage (punch through) failure which would
> > happen in the RF cycle timescale.
> >
> > I suspect (but don't know for sure) that this time constant is in the
> > hundreds of milliseconds or possibly even longer. So a rapidly
> > responding (but not too rapid) circuit breaker or fuse works as
> > effective SWR protection.
> >
> > On Sep 23, 2007, at 1:32 PM, Ken Brown wrote:
> >
> >> I'm trying to understand this. What part of Ten-Tec's circuit keeps
> >> the
> >> PA transistors from frying in a half cycle at 30 MHz? Looking at my
> >> Omni
> >> VI PA schematic C25 a 33ufd capacitor on the PA board would hold
> >> enough
> >> charge to supply the PA board for well over 17 nanoseconds, by my
> >> calculations.
> >
> > --
> > 73 DE N7WIM / G8UDP
> > Kevin Purcell
> > kevinpurcell at pobox.com
> >
> >
> >
> > _______________________________________________
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> > TenTec at contesting.com
> > http://lists.contesting.com/mailman/listinfo/tentec
> >
>
>
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