[sdiy] STUPID tube questions (there are no stupid questions, only stupid answers...)

[René Schmitz]

Hi Eric, Martin et al,

>>(Typically a 12AX7 stage with a 
>>220k plate resistor trying to drive the next triode stage....)
> 
> 
> Caused by Miller effect. Use a pentode.

Either that, or use a triode with lower rp and lower the plate resistor.

>>I saw an old app. note where a zener is used as a "grid voltage regulator".
>>It replaces the cathode resistor and bypass cap. 
>>It allows you to adjust the tube operating point, using only a single supply.
>>It seems necessary if a tube circuit is to handle +/- 5-10 volt signals at
> 
> the grids.
> 
> <groan> I give up. 

People have been using batterys to generate the grid potential in the 
old days. In fact using zehners, batterys, resistors, diodes and all 
that does work. The resistor/cap arrangement however does have the 
advantage that the DC has higher negative feedback, so the stabilisation 
for the quiescent current is better. This is very important for some 
tubes (the ones with high transconductance tend to "run away").

Cheers,
  René

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