[sdiy] tube preamp and fx chain

[Antti Huovilainen]

On Sun, 25 Jun 2006, René Schmitz wrote:

> But more typically you'll find a cathode resistor which generates the bias 
> and a resistor in the range 470k to 1Meg, and *very often* no input cap at 
> all. Which would controvert grid leakage effects.

At DC yes (IIRC, the grid current was pretty much eliminated at approx 
-0.5V bias). With "high" input signal, the grid current still has a 
dynamic effect.

> Most amps stem from circuits which were in use way before distortion
> became fashionable. Rockabilly 50s. So their designers would have tried
> to keep things linear. If they used a mere -1V bias, and grid leakage

Do you perhaps mean the same designers who used highly scientific methods 
for tuning guitar speaker cabinets, such as Mr. Marshall? ;) (for those 
not in the know, Marshall 4x12" cabinet is pretty much the smallest that 
can physically fit the four speakers).

> biassing, I guess this would be the exception rather than the rule. The 
> effects of grid current were well known, and people were advised to 
> avoid the 0..-2V biassing region. The majority of circuits I have

Assuming ~250V B+ and 100k plate resistor for preamp (seems pretty common) 
and biasing the stage near maximum output swing (150V plate DC voltage or 
so), bias voltage is roughly -1.1V. That's just 0.6 volts from the point 
where grid current starts to flow in the microampere range. You can't 
really bias 12AX7 at -2V with resistive plate load, get max output swing 
and keep it within the allowed voltage limits.

Antti

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