[sdiy] tube question 2: characteristics , DC versus 20kHz

[René Schmitz]

Hi Martin,

(Btw, I saw the posts at Frihu's site, I'm lurking there.)

> is it true that e.g. a triode stage large signal characteristic 
> measured at DC differs from those measured at -say- 10kHz or 20kHz?

The characteristic at higher frequencies depends on the tubes dynamic
capacitance (IIRC Cdyn = Cag * mu + Cgk) and the drive impedance of the 
previous stage. Usually that is quite high, in first order rP || Ra (rp 
is Ri in german literature). As such it isn't a tube property but a 
circuit property. If you have a different characteristic at 20kHz 
already you have a problem I would say. (Typically a 12AX7 stage with a 
220k plate resistor trying to drive the next triode stage....)

> I read something like this every now and then. What is the reason? 
> Parasitic capacitance or time of flight of the electrons?

You can exclude the latter by a simple reasoning: How fast are the 
electrons when accelerated by 100V+. How long does it take to travel say 
5 mm? It comes into play at frequencies well in the megahertzes.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159