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Re: Preamp bypass circuit



Hello Jon,

I use an external sequencer, like the ARR unit.

I grab the mic PTT before it hits the FT-847 and go into the ARR
sequencer. ON PTT ground to TX, the sequencer take +13 VDC off of the
mast mounted preamplifier and the 2 ea SPDT coaxial relays (legal
limit rated), sends PTT to the PA, then PTT to the IPA then PTT to the
FT-847... After transmitting I release the mic PTT for RX mode and the
sequence is reversed.

A sequencer will also handle the DC power and relay control, of your
preamp relays..

ARR is ADVANCED RECEIVER RESEARCH in CT, see QST for their website.

Stan, WA1ECF




"Duffin, Jon" wrote:
> 
> I have recently built up a GasFET preamp for use on the 2 meter band on
> AO-10.  To switch the preamp out during transmit, I have built up a RF
> sensed circuit which bypasses the preamp through a relay.  The problem is
> that I can observe a breif RF transient that would go into the preamp (I
> actually have not hooked up the preamp to the switch circuitry yet) before
> the relay switches into the bypass mode.  I am fearful that this RF
> transient would be enough to blow out the GasFET.  Does anybody have any
> hints or ideas how to supress this transient so I won't be replacing the
> FET?
> 
> My rig is a FT847, and so I though another idea would be to switch the
> preamp out of the transmit path by using the 847's PTT accessory jack.  But,
> I don't think that the results would be any different unless there is a way
> to delay the power output stage from coming up at the same time as the
> switching of the PTT accessory jack signal.
> 
> How does everyone else get this to work on their homebrew preamps?
> 
> Thanks
> Jon
> KK7RZ
> 
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