[AMRadio] Class C finals


John Coleman ARS WA5BXO wa5bxo2006 at pctechref.com
Mon Nov 13 20:22:59 EST 2006


	Jim, you and I are on the same track here.  Many folks make the
assumption the final tubes in a class C circuit are what determines the
power. I look at it from the other end, in that it is the voltage and
loading which determines the power and you have to find a tube that can
handle the switching and plate dissipation.  

	So it would seem that if you replace the 6146 with a 2E26 then in
order to get the proper operating parameter for the 2E26 one would need to
reduce the loading.  So why not just reduce the loading on the 6146?

	To me, it is like replacing my 250th with a 100th and then being
forced to reduce the loading to keep the tube in the proper operating
parameters for efficiency.  Where as if I had a rig designed for a 100th and
then plugged in a 250th instead all plate parameters could remain the same.

	It would be interesting, if some one wanted to try it, that a DX100
using a pair of 6146s could be tuned to 100 watts output and then one of the
6146 removed and the user would have to reduce the loading in order to keep
the remaining 6146 from meltdown.  And if the reduction in load was made to
the point that the 6146 was drawing 1/2 of the plate current and output was
50 watts or less then plugging in the next 6146 would not increase the power
unless the load was increased again.  At least that is my theory!

	I wonder if any one has tried the ultra efficient class C circuit
that can achieve 90% efficiency.  It involves, 3rd harmonic shaping circuits
in the grid tank, and 3rd harmonic traps in the plate.  RCA built a
broadcast transmitter using this design. The theory is that you square up
the input wave form so as to switch the tube on and off.  The third harmonic
squaring of the input wave causes this switching time to shorten up a lot.
The 3rd harmonic trap in the plate tank represents a high Z to the third
harmonic and passes the fundamental to the main tank and load circuit.
Using this circuit a single 6146 could put out 90 watts with 100 watts input
and run cooler than the pair in the DX100.  It might not be quite this good
since it is a tetrode with an indirectly heated cathode. 

John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim Candela
Sent: Monday, November 13, 2006 5:14 PM
To: Discussion of AM Radio in the Amateur Service
Subject: Re: [AMRadio] Class C final tube swaping

John,

   You made several good points with your push pull amps. One big difference
here is that the 2E26 is indirectly heated, and possesses an indirect
cathode with limited emission capability. Those directly heated thoriated
tungsten filament tubes you mentioned are all high pervience whereas the
6146, and 2e26 are not. So replacing a 6146 running 90 watts DC input Class
C with a 2E26 will not give you the same power input or output since the
2E26 won't be able to swing the plate to a low potential at high current
like the big brother  6146 did. I would worry about the 2E26 turning red,
and going into sweep tube heaven since the efficiency will be poor unless
the plate load impedance is lightened up.

   For those who have tried this, how well did the 2E26 modulate? Ever look
at the pattern in trapezoid mode? ;-)

Jim





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