5879 tube data sheet question

[tubenit]

I have always loved what I hear as a fat chimey tone with 5879 tubes.  From what I read this is a low-medium gain tube?

5879.pdf


It looks like the "amplification factor" for the triode is 21?  I don't see any indication for the pentode amplification factor?


I am curious how the pentode amplification factor compares to a 12AX7? 


12AX7 datasheet(1/4 Pages) GEC | TWIN TRIODE


With respect, Tubenit

[Merlin]

The datasheet quotes:
Plate resistances approx 2 megohms
Transconductance 1000 umhos

mu = gm * rp
Therefore:
mu = 0.001 * 2000000 = 2000

As you can see, pentodes have colossal mu! It's also very difficult to determine accurately, so it's not a very useful metric which is why datasheets don't quote it.

[Jonas]

i also love this tube. Why is 12.6 v heater operation not listed? I looked at a few other pentodes which only show 6.3v option in the TDS? 

[kagliostro]

Why is 12.6 v heater operation not listed?

Because of construction, It was thinked that a single heater winding will be better in a tube devoted to low noise apparatus like HiFi and a double winding will add construction difficulties and require more space


Franco

[HotBluePlates]

I don't see any indication for the pentode amplification factor?

I am curious how the pentode amplification ... compares to a 12AX7? 

mu = gm * rp
Therefore:
mu = 0.001 * 2000000 = 2000

As you can see, pentodes have colossal mu! It's also very difficult to determine accurately, so it's not a very useful metric which is why datasheets don't quote it.

Further, the formula we use to determine in-circuit gain recognizes that the external plate load resistor forms a voltage-divider with the internal plate resistance of the tube.  Amplification = Mu x [Rl / (Ra + Rl)]

For a Triode "Rl" (the external plate load resistor) is usually bigger than the internal plate resistance of the tube.  But the pentode's internal resistance is almost always higher than the plate load resistor, so we don't use that formula to estimate in-circuit gain of a pentode.  That's because if you put 100kΩ in parallel with a 2MΩ internal plate resistance, the total resistance is "nearly 100kΩ."

So as Merlin noted here, we calculate Pentode Gain as "Gm x Plate-Load."  Within this framework, there are approaches that seek to maximize Gm and different-approaches that use lower-Gm and a larger plate load resistor.  Gm for the tube is controlled by choice of screen & bias voltages, with higher idle current leading to higher Gm for the tube.