[sdiy] tube preamp and fx chain

[René Schmitz]

Hi all,

Antti Huovilainen wrote:
> On Thu, 22 Jun 2006, Tom Arnold wrote:
> 
>> I don't remember what guitar primadonna made the original statement 
>> but to
>> paraphrase "There is something magic about the interaction of the pickups
>> and that first 12AX7 tube".  I've tried to follow that rule.  Guitar ->
> 
> 
> IF any such interaction exists, it may well be due to the grid current 
> of the triode (strange as it may seem, the current flows from grid to 
> cathode even when grid is below cathode). This would present a nonlinear 
> impedance to the pickups.

Nope. Thats why they give the input tube a negative grid bias. The 
amplitudes are so low that you can't forward bias the grid, and the bias 
is so far negative that the grid leakage current can be neglected.

The pickup has inductance, and the 12AX7 has high gain, and thus 
multiplies its Cak and Cgk to a high Cdyn. Now if you parallel a 
capacitance and an inductance, what do you get? A resonance. Thats also 
why the cable sometimes alters the sound. All since this is a high Z 
thing. Thats also why you never see pentodes in the first stage, and 
people stick to the 12AX7.

Cheers,
  René

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