[Amps] Class A for AM

[Bill L. Fuqua]

"We were chatting about a circa 1970 screen-modulated AM rig.  Those things are also "efficiency-modulated" and so operate at about the same efficiency as a pure linear amplifier."

   Control grid modulation is efficiency modulation because it changes the class of operation of the tube. But is screen grid modulation? The tube is still operating class C and but at lower power when not being modulated. However, the tube must be able to provide 4 times that power on modulation peaks.  If it is operating 75% efficiency at no modulation and 75% at 100% modulation you are not efficiency modulating the tube. 

     How efficient is a plate modulated amplifier when you add in the extra power to operate the modulator?

    A 4-1000A running all out class C can easily produce 3.5kw all day long in low band FM service. It is not hard to to push that to 4kW on modulation peaks if it is spending most of its time producing 1kW at reduced screen voltage. 
    But if you plate modulate the same tube you can get about 2.6kW output carrier.  But at what cost?  You need a plate modulator producing 1650 watts ( the 3300 w plate input power divided by 2). This requires two other tubes running around 60% efficiency, maybe a pair of 4-400As ) adding to the total power used, lots of iron in modulation transformer, modulation choke etc. 
    Yes there is some efficiency improvement but it is a lot more complex. 

    Taking into account the complexity of plate modulation vs screen modulation I suspect that it would been a lot easier to make a 1 kW broadcast transmitter using one large tube (say a 4-1000A) and screen grid modulation than the standard ones using pair of 4-400As modulated by pair of 4-400As.  But, I think you could not sell AM broadcast transmitters that were not plate modulated due to the bad rap that screen grid modulation has gotten.  “Real transmitters are plate modulated”.  Another myth along the same line that direct FM is better than indirect FM even though the resulting signals are identical. Some amateur repeater companies still make that claim.
     I still like my Desk KW even if it is plate modulated. HI HI HI. 
And my Viking I, II, my Valiant, B&W 5100, home brew 813 and others. But, I think if I was starting from scratch I may use screen grid modulation and a big tube. 
73
Bill wa4lavmprovement but it is a lot more complex. 

    Taking into account the complexity of plate modulation vs screen modulation I suspect that it would been a lot easier to make a 1 kW broadcast transmitter using one large tube (say a 4-1000A) and screen grid modulation than the standard ones using pair of 4-400As modulated by pair of 4-400As.  But, I think you could not sell AM broadcast transmitters that were not plate modulated due to the bad rap that screen grid modulation has gotten.  “Real transmitters are plate modulated”.  Another myth along the same line tha