[sdiy] tube preamp and fx chain

[Antti Huovilainen]

On Fri, 23 Jun 2006, René Schmitz wrote:

>> 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.

Well, as it turns out, I DID measure this, so I'm not just pulling it out 
of my ass :). The grid current is exponential and hence the "turn-on" 
point is somewhat arbitrary. At least guitar amplifiers often use rather 
large grid resistors, so even the few to few tens of microamperes current 
makes a difference.

A typical guitar input stage may sit at 1.0V or so for cathode voltage. 
Especially with high output pickups the input waveform can go over 2.0V 
p-p causing grid current to flow.

> 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

However, there is very often a resistor between the source and the tube to 
prevent RF oscillations. This somewhat isolates the pickup from the miller 
capacitance of the tube.

> never see pentodes in the first stage, and people stick to the 12AX7.

Or more likely because triode distortion just tends to sound better on 
guitar to most people.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova