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Re: [TenTec] Amplifier Question

To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Subject: Re: [TenTec] Amplifier Question
From: "Paul Christensen" <w9ac@arrl.net>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Wed, 29 Aug 2007 16:32:17 -0400
List-post: <mailto:tentec@contesting.com>
> The relay in the Omni VI+, when used to key the amp, does not provide 
> delay
> for RF output thus the amp is likely to switch hot.  The TX EN is the
> correct way to delay the RF output from the Omni VI+ when using the 
> internal
> keyer.

TX OUT/EN is fine for amps with such a handshake mechanism.  But for QSK 
amps without this feature, TX OUT is a poor choice for a control line.  In 
many amps, using TX OUT as the QSK control signal results in severe 
hot-switching.  More than ten years ago, I used TX OUT to key several Alpha 
amps, and it was a total disaster on the air.   Unless you sample the amp's 
transmitted waveform on a scope, you may never know the amp is hot-switching 
and causing key-clicks.  For these amps, it's best to sample Ten Tec's "T" 
voltage line.  This provides ample delay to avoid hot-switching, but not so 
much delay as to kill QSK.

One method of using the "T" line on the Omni Six involves physically 
disabling the loud external amp relay.*  A transistor's open collector is 
normally used to key this relay.  The open collector trace can be cut from 
the relay coil and brought out to the RCA PTT key line, thereby bypassing 
the amp relay.  Control for the switching transistor is then moved to the 
"T" voltage line through the transistor's biasing resistor.

The transistor used by TT has a relatively low Vceo rating, owing to the low 
voltage (12VDC) on the relay coil.   While I was making these changes, I 
replaced mine with a 2N5551 which has a Vceo of +140 VDC.  An even better 
choice is to use a HV MOSFET type -- or even the Jackson Press "Key-All" 
board.  Some amps have very high DC (and AC) key lines so one really needs 
to know the voltage and current demands placed on the amp key line before 
embarking on the modification.

Paul, W9AC

* Although the Omni's menu allows you to disable the amp relay, doing so 
will deactivate the key line transistor altogether but it's needed if the 
"T" sample is chosen. 

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