[AMRadio] Class C final tube swaping

John Coleman ARS WA5BXO wa5bxo2006 at pctechref.com
Sun Nov 12 23:14:21 EST 2006

	I do not have a ranger or have I had a ranger, but I just can't see
how replacing a class C tube will change the power.  As far as I know the
ranger is a class C output and not a linear amplifier. To me a class C
circuit is a switch which gains its efficiency from the fact that it doesn't
spend much time in between saturation and cutoff.  The perfect device would
be a switch that is either on or off and is switched on and off at the
carrier frequency.  Tubes are not perfect switches hence they dissipate some
heat in transit from on to off and back.  Changing the loading or the supply
voltage will change the power.  In other words putting in more lights or
bigger lights is increasing the load making more light in the room.
Changing the supply voltage up or down will also change the brightness of
the room.  Putting a larger or smaller switch in the wall will have very
little effect until the switch is made too small then it will heat and burn

Here is an example that I am familiar with.
	I have run pairs of 813s, 812s, 304TLs, and now 250THs.  I have yet
to see any difference the output or tuning when I turn off the filament in
one of the pairs.  I first had this happen on the rig with the pair of
304TLs.  I ran 1700 Volts at 550ma total plate current from the power
supply.  The power supply was separate and had its on meters on it.  I
addition the 304TLs had separate plate chokes and were shunt coupled to the
tank circuit.  This allowed for each of the 304 to have its on plate current
meter.  When all was balance right each tube pulled 225ma from the power
supply.  One day the heater connection on one of the 304TLs went bad as they
or known to do sometimes and nothing changed on the scope.  I didn't even
know it until I happened to look up at the rig and noticed a lot of extra
color from one tube and the other was dark.  The power supply current did
not change and the one tube that was left was doing all 550 ma of plate
current by itself.

So I went from a VIRTUAL 608TL to a 304TL and no change it operation except
more color on the one tube. 

I did the same thing on the 250th rig except I broke a tube. So I just
pulled it and only had to re-dip the plate because of the slight change in
capacitance from the missing tube.  No change in output no load change.

So will some one please tell me the theory of how putting a smaller tube in
the ranger will by its self make the ranger half power????

And please don't misunderstand me here, I'm not saying it can't, I just want
to know how it does it.  It just seems to me that putting in a smaller
switch will just ensure that you will change the switch more often.  

I have theorized that the 2E26 must pull more screen current than the 6146
and there by have less voltage on the screen and thereby have less
acceleration to the plate current.  In other words similar to reducing the
screen voltage on the 6146.  But this would cause a change the parameters
and conduction angle requiring a reduced load in order to maintain the
proper Q in the tank.  Improper Q, conduction angle outside the normal 70
degree window, these things can cause a lot of improper modulation
characterizes.  Maybe I'm just too picky!

John, WA5BXO


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