# Vacuum tubes

#### Pavlo138

Joined Feb 26, 2009
28
Hello all,

I am currently studying for the CETa exam through ETA-i. In the amplifier section Vacuum tubes are discused and so are the subsequent calculations for biasing such antiquity

I understand so far that the input signal is introduced via the grid, if I am interpreting the diagram for a VT correctly, and that the plates, I assume, are much like the collector and emitter of the BJT that I am far more familiar with.( Very easy to calculate the voltage-divider bias)

What is not clear though, is how he goes about deriving the values for the different biasing components to produce the gain he desires. For example, he is using values from a dynamic transfer graph, with curves for various loads, corresponding to a grid voltage and a plate voltage. However, his values for plate current do not seem to match the intersection of the load resistance with the plate voltage. It's quite confusing on how it's presented, and of course it's only a study guide, so the explanation is brief. Basically I can follow what he is doing, but I dont know why he is doing it that way. I looked in the different online books we have here, (very impressive by the way) but could find nothing on VT's.

If anybody would happen to have some guidance to add, or a link to a good online resource for understanding Vacuum tubes and biasing more completely I would greatly appreciate it, thank you.

#### hgmjr

Joined Jan 28, 2005
9,029
Notice that for vacuum tubes the grid is biased negative relative to the cathode.

This is usually accomplished by placing a resistor from the cathode to ground. That way the quescient current flowing between the cathode and the plate will result in a positive voltage on the cathode.

hgmjr

#### Pavlo138

Joined Feb 26, 2009
28
Now, to my understanding there are two plates, the one attached to ground through that resistor is then the cathode? and the other one, the output side, is the anode? It is quite different then how transistors operate.

#### hgmjr

Joined Jan 28, 2005
9,029
Yes, a vacuum tube is more similar to a FET than it is to a BJT.

hgmjr