The AM Forum

The thing that I would like some help on is understanding what would be better for the rig, and tubes,running more plate volts and less current or more current and less plate volts. The input is working out to the same and the power output is about the same either way.

2000 volts and lots of current?
3000 volts and much lower current? 

C

hi Clark .... I say 3kV and lotsa current ....

just fooling around ..... assuming not violating any tube spec (especially oxide cathode tubes in 4cx1000 class, etc)
a tube tends to last longer properly operated at lower cathode currents .... case in point : commercial vhf repeaters
I used to work for GE Lynchburg Va and learned from the old hands about this .... a tube is supposed to run at full ratings for a minimum of 1000 to 2000 hours in transmitting service .... running it properly at less plate current tends to give it longer life .... I think GE had a goal of 10000 hours in 50/50 duty service ... my experience with the old Prog line repeaters is that they could do this .... a bit less plate voltage than max a good deal less current than max

That makes sense  In the eimac data sheet they are clearly using the voltage for more power output. The reason is that they show the same plate current on all three voltages, 2000, 2500 and 3000.  I run the thing at legal limit which is way way down for this kind of rig, I can load it at 3000 volts which will be low plate current or 2000 volts with lots of current. The same power output is had. I cant tell a difference on the scope.

Is the mod iron ratio a factor here?

C

If you run 3000 volts the peak modulation will be 6000 volts at 100%. Tank parts need to handle a lot higher peak voltages. Operating Z will be higher so you want to run a voltage (operating Z) that works with your mod iron.

After going back to the book and reading again, This is what I was missing.  While the input will be the same at higher volts and lower current vs. lower volts and higher current, The match to the mod iron will not be the same.  I am now calculating this out so I can further understand how much room I have each voltage setting.   Sound right??

C