# What is the purpose of resistor Ra in this simple vaccum tube voltage amplifier circuit?

Came across this in a book, there's a brief explanation of why there's said resistor in the circuit:

" ... it should be obvious that if we apply an audio signal voltage to the grid (i.e., vary the grid voltage) then current in the valve is bound to vary too. We can put this current variation to good use by putting a resistance in series with the valve so that a corresponding audio voltage is generated across the resistor, which is also called the load. "

Yet I can't quite understand the need for the resistor to be there in order to create the second amplified voltage signal, won't the amplified voltage signal be there anyway whether there's a resistor or not?

• Are you considering replacing the resistor with a short or an open? Feb 25, 2017 at 3:48
• None, I'm not yet actually building this but trying to get the concept/theory of why each element in the circuit is placed there and what is it doing. :)
– Raz
Feb 25, 2017 at 4:17
• Okay, but you asked "what if the resistor weren't there"? Which situation are you asking about? Replacing it with a short, or with an open? Feb 25, 2017 at 4:28
• Oh right, short.
– Raz
Feb 25, 2017 at 5:39
• Tubes as often designed as "valves" because they act like a current valves. Since they are voltage driven, it is necessary to turn their output current variations into voltage variations. U = R × I a simple resistor makes this job.
– greg
Feb 25, 2017 at 12:27