0
\$\begingroup\$

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?

Circuit picture

\$\endgroup\$
6
  • 1
    \$\begingroup\$ Are you considering replacing the resistor with a short or an open? \$\endgroup\$
    – The Photon
    Feb 25, 2017 at 3:48
  • \$\begingroup\$ 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. :) \$\endgroup\$
    – Raz
    Feb 25, 2017 at 4:17
  • 1
    \$\begingroup\$ 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? \$\endgroup\$
    – The Photon
    Feb 25, 2017 at 4:28
  • \$\begingroup\$ Oh right, short. \$\endgroup\$
    – Raz
    Feb 25, 2017 at 5:39
  • \$\begingroup\$ 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. \$\endgroup\$
    – greg
    Feb 25, 2017 at 12:27

2 Answers 2

2
\$\begingroup\$

Consider if Ra were a dead short (zero ohms). The voltage at the plate would always be 300 volts, regardless of the grid voltage. The current would vary, but the voltage would remain constant.

\$\endgroup\$
2
  • \$\begingroup\$ Right, so, to "separate" the amplified voltage signal from the constant 300 voltage feed so to speak? \$\endgroup\$
    – Raz
    Feb 25, 2017 at 4:19
  • \$\begingroup\$ @raz: yes. the plate load resistor is there for the same reason a collector load resistor is present in a transistor amplifier. \$\endgroup\$ Feb 25, 2017 at 4:27
0
\$\begingroup\$

Viewing the vacuum tube as a transconductor, that is a box producing changes in OUTPUT CURRENT when the INPUT (GRID) VOLTAGE changes, we have this I-V plot:

schematic

How to compute the VoltageGain of this triode_with_Rload? Just as you do for a Bipolar with a Collector Load Resistor: multiply GM * Rload. If this triode has 1 milliMho (milliSiemen) at the operating point (say -2volt on Grid is the Operating point, then Av = 0.001 * 30,000 = 30x. Or 31dB.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.