[sdiy] tube preamp and fx chain

[René Schmitz]

Hi Antti,

Antti Huovilainen wrote:
> 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.

I'm well aware if this effect, infact there is a little circuit at my 
page which just exploits this phenomenon. A tube based expo convertor.

A typical grid current biassing circuit would have its cathode directly 
at GND, and a 10-22Meg resistor at the grid. (Plus an input cap, to 
force the leakage current through the resistor.)
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.

> 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.

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
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 
studied had a cathode/cap combination anyway. (And the ones I quickly 
looked at right now also. Fender Twin 5e8, VOX AC30.)

Well and even if I'd suppose there was sufficient overdrive to the first 
stage, then as soon as grid current would start to flow, a DC bias would 
develop across the cap (which would need to be there to not let the 
leakage current flow through the pickup), which would push the biassing 
back to the linear region. (Audion rectification.)

Cheers,
  René

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