Discussion in 'Amplitude Modulation' started by W2NBC, Mar 9, 2019.
Slam it out there! Give 'em the full carrier.
Modulation Essentially negative...
Thank you RCA TT-3
I have noticed data sheets for several tubes from various manufacturers giving specs for class-C plate modulation, stating "Modulation Essentially negative if the positive peak does not exceed 115%". At first I had the impression that certain tubes might not be rated to stand up to full plate modulation, maybe due to maximum voltage limitations of the internal insulation or limits to peak emission, but upon closer examination I noticed that same caveat with tubes known to be designed for normal plate modulation at full positive peaks. The RCA book makes it clear that they are saying is that the tube can safely run at full unmodulated CW ratings if the positive peaks do not exceed 115%. Running at full 100%-plus plate modulation, the maximum ratings must be downgraded to the lower "plate modulated class-C" service to accommodate additional voltage peaks and plate dissipation that comes with full plate modulation.
Some tubes I have seen don't include any plate modulation ratings in their data sheet, but they should still work OK if downrated in proportion to the reduced ratings published for similar tubes in class-C plate modulated service. The ultimate test is the trapezoid pattern on an oscilloscope, to make sure modulation linearity holds all the way to the maximum modulation peak. The sides of the triangle should be essentially straight all the way to maximum. Significant curvature anywhere along the sides of the triangle indicate poor modulation linearity, meaning the tube is not delivering the full required peak emission. Most likely the result of insufficient grid drive, insufficient negative grid bias, a soft or worn-out tube, or that it's a good tube attempting to run way beyond its maximum power capability, or a combination of any of these.