Hi Dick
I use a separate heatsink for each pair of Fets, this method avoids using an
insulated spacer between fets and the heatsink.
It also means you get a better bond to the heatsink
I use two 5 inch fans on the PA one blowing on to the fet heat sinks and the
other one blowing across the PA inductor and LPF. My PA unit runs barely
warm even on QRS.
I use only one 47k resistor gate/ground for each pair of fets, I
get no ringing or noises even using full power.
My PA is modular built so that it is easy to change any individual unit ie
FET/Heatsink, LPF, Driver board.
I use a IC 706 as the rf source on min power and divide down from 13.6/13.7
mhz to get on 136/137 khz.
With a stable antenna system you should have no trouble with reliability but
a wobbly or unstable antenna with a high or varying SWR BEWARE it could get
smoky !! so they say!!
Mal/G3KEV
----- Original Message -----
From: "Dick" <[email protected]>
To: <[email protected]>
Sent: Sunday, April 09, 2006 11:25 AM
Subject: LF: Tech questions G3YXM amp+18 divider
> Hello all,
>
> I'm building a new (and larger) 136kHz transmitter.
>
> The output circuit is the same as the one used by G3YXM with 4 times
> IRFP450.
>
> Each FET will get his own gate input resistor (6R8) and gate-to-ground
> resistor (47k)
> The sources will be grounded as short as possible to ground, while the
> drains of
> 2 FET's will be connected together.
>
> I am now in the stage of putting the components of the output circuit on
its
> correct position,
> and that raised some questions about where to mount what.
>
> I am using a large heatsink 12x15x5cm.
> On this heatsink I mounted a 1.5mm thick copperplate 75x150mm in order to
> transport heat quick away from the FET's.
> Maybe it's some 'overkill' , but I just want to set up a thing that can
run
> for hours
> on full power; just a bit of reserve...........
>
> The 4 FET's are mounted in a row, separation between the FET's is 1 FET
> width (abt 15mm).
> The resistor of the 'zobel-network' is mounted just in the center , with
> wires about 30mm long connected
> to the drains of the FET's.
>
> I do not expect problems, but maybe that 30mm is too long???
> I don't think I can make them shorter without changing the whole component
> layout.
>
> If anyone has experiences with this amplifierstage and the zobel-network
> wire length,
> I would glad to hear.
>
> Another thing coming into my mind is to set up a fixed frequency into the
> transmitter, beside the VXO.
> I do have an 4.9152MHz X-tal, but I have to divide it by 18 to get the
> necessary 273.066kHz.
> Any simple schematic (with easy to get components) for an divide by 18
> circuit?
>
> 73
>
> Dick, PA4VHF
>
>
>
>
>
>
|
