[AMRadio] Class C final tube swaping

Rick Brashear rickbras at airmail.net
Sun Nov 12 23:27:47 EST 2006

When you had these experiences, were you running two tubes in push pull 
or parallel?  Just curious..

John Coleman ARS WA5BXO wrote:

>	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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