[TenTec] QSK w/amplifier

[Paul Christensen]

> I tapped into the T-line and keyed a transistor which in turn was used to
> key the 405 Amp's relay.

Same here...

When I owned an Omni Six, I decided to modify the external amp relay circuit 
to support a super-fast, high-voltage key line for use with an Alpha Seventy 
amp.  The Alpha uses a +55VDC key line and is beyond the maximum specified 
Vceo voltage rating of most small-signal switching transistors.

First, I replaced the Omni's relay driver with a 2N5551 NPN transistor and 
brought out the collector to the RCA connector.  The 2N5551 is Vceo rated 
for +140 VDC.   This allowed me to directly key the amp but it created 
another problem:  the timing of the open-collector was causing the Alpha to 
hot-switch.  I had modified the Alpha for full QSK by using a 
voltage-doubling circuit that initially doubled the keying voltage to the 
vacuum relay and thus increased the initial snap time of the relay.  But 
when using the Omni Six, the problem was not the leading edge, but rather 
the trailing edge of the amp's keyed waveform.  For many years, I have been 
using a scope to monitor my transmitted output.  Had I not had the scope, I 
probably would have never caught the hot-switching issue before replacing 
several expensive vacuum relays.

The next fix involved finding a better sample point for the Omni's keying 
transistor.  That turned out to be Ten Tec's "T" line.  Sampling the T 
voltage line resulted in fast amp switching on both the leading and trailing 
edges of a the keyed waveform.  I further went on to create a quasi 
Darlington pair for better isolation from that +55 VDC line.  Two 2N5551 
transistors were cascaded to create the Darlington.  A diode on the base 
added further isolation.

If I had to go back and re-visit this exercise today, I would use a 
high-voltage MOSFET.  When I owned a Ten Tec Pegasus, I looked at the amp 
keying transistor, and they designed this part of the circuit correctly: a 
TO-220-style transistor is used with a high-voltage Vceo point -- even 
though Ten Tec specified the keying line for much less than it was capable 
of switching.

At least one mod exists on the Internet to create an amp key line by 
utilizing an RCA "Y" adapter connected to the Omni's TX OUT/TX EN line with 
the third RCA connector sent to the amp key line through a small steering 
diode.  That solution works fine for amps that protect against 
hot-switching, but it will not work correctly for amps that are full-QSK but 
have less sophisticated sensing circuitry.

It would be interesting to see what type of amp keying transistor is used on 
the O2 and Omni Seven.

Paul, W9AC