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

[Czech Martin]

Yes, this is basically what I thought.

So it seems that the first stages of a tube amp
are not very frequency dependend (the "EQ"
network excluded). This fits to my experience
that the preamp sound has a shaver-buzzing quality,
lots of high frequency stuff. Not much unlike
a transistor fuzz, but there is certainly
a difference to the transistor circuit when
it comes to intermodulation.
Somehow chords never get that muddy on a tube
amp than it sounds on the transistor solutions
I have.

Certainly things get different in the power amp
stage, where large inductances, large currents
and very large signals come into play.

Can't wait till I get all the parts to start soldering...


m.c.

> -----Original Message-----
> From: René Schmitz [mailto:uzs159 at uni-bonn.de]
> Sent: Donnerstag, 28. August 2003 13:01
> To: Czech Martin
> Cc: Sdiy (E-mail)
> Subject: Re: [sdiy] tube question 2: characteristics , DC versus 20kHz
> 
> 
> 
> 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
> 
> 
> 
>