Heathkit Apache TX-1

Glen Zook gzook at YAHOO.COM
Sat Sep 8 11:13:51 EDT 2007

Heath was very "generous" with their specifications. 
Frankly, if you check the RCA specifications for the
6146 and 6146A/8298 tubes you will see that they are
rated for a MAXIMUM of 67.5 watts INPUT on AM phone. 
This equates to 135 watts input for a pair of 6146
tubes.  Yes, you can run more input power but the tube
life is going to be reduced substantially.

Johnson rated the Viking II (which uses a pair of 6146
tubes in the final) at 135 watts maximum input on

If you cut the input power to no more than 140 watts
on CW and 120 watts on AM you will find that your
tubes will last MUCH longer and the station on the
other end will not be able to tell the difference in
your signal.  The usual increase in life over the
rated power of the tubes is often between 5 and 10
times the life at rated power.  Also, running the
tubes at the Heath rating can reduce the tube life by
at least half and often considerably more than when
running at the rated input power.

As such, running the final amplifier at 120 watts on
AM can increase the tube life by at least 10 times and
often by over 20 times than if you run them at 150

Also, the 6146 has a VERY weak grid.  If you reduce
the grid drive to between 4 and 5 mA for the pair you
will get considerably longer tube life and the output
power should not vary from the higher grid current. 
The 6146 will work fine with as little as 1.5 mA per
grid although I like at least 2 mA per tube.

RCA originally specified 3.75 mA maximum grid drive
per tube then reduced that to first 3.5 mA and finally
3.25 mA.  However, at even 3.25 mA the grid is
operating at the "edge" and it really needs to be
reduced substantially for long tube life.

Glen, K9STH

--- Michael Waldrop <w5rkl at YAHOO.COM> wrote:

The original specification state 150 watts input on AM
fone. With 6ma of grid drive, 750VDC on the finals,
and a resonant plate current of 210ma, that's 157.5
watts input. The output is 120 watts on 75 meters into
a dummy load or my G5RV. That's about 70% efficiency,
not bad for a 48 year old transmitter. I have replaced
all the power supply electrolytics before any testing
(no NOS caps, only new caps).

Glen, K9STH

Website:  http://k9sth.com

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