Heathkit Apache TX-1

Peter A Markavage pmarkavage at JUNO.COM
Sat Sep 8 13:42:58 EDT 2007

Obviously, if the .1 ohm plate current shunt resistor and the 5.55 ohm
grid current shunt resistor are off due to age, deterioration, or abuse,
your meter readings won't be highly accurate to make these changes in

Pete, wa2cwa

On Sat, 8 Sep 2007 08:13:51 -0700 Glen Zook <gzook at YAHOO.COM> writes:
> 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
> phone.
> 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
> watts.
> 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

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