Can a 12ax7 preamp stage be grid biased (instead of the standard cathode biasing scheme)?
The cathode would be grounded, like a "fixed" bias power tube, with a small voltage on the grid to supply bias.
I'm wondering if it would have less gain than the standard cathode resistor w/cap bypass, more current drive (or less) - or if it just plain sounds like cr@p
its called zero bias operation...... a coupling cap and large value resister
are hooked the grid the resister (2.2 to 15 meg.) goes to ground.....
a charge builds up on the grid and provides the bias..... it a form of grid leak
bias...... it works pretty good..... the size of the plate resister determines
the amplification...... its draw back is that when the input is large its output
is asymetrical ...... without a chathode resister the charge has to build up on
the grid which also takes a little time and affects the distortion % of the stage
these sound pretty good until the bias is exceded and then they go into
shut off....
However, I'm interested in the effects of supplying bias voltage to the grid, while grounding the cathode - not letting it build up aka grid leak. This would be similar to biasing a "fixed"/grid biased output tube.
In this case it is a 12ax7 for discussion purposes, but experience/advice with any small signal triode would be awesome
Yes, you can absolutely bias any triode by applying a negative voltage to the grid, grounding the cathode and selecting a suitable plate resistor. Most tube data sheets will include "typical" examples of this type of operation. The gain of the tube will be function of the plate resistor more than anything else; the output swing, mostly a function of supply voltage.
The "grid leak" bias found on early amps was mostly designed to get maximum gain from the tube. With an appropriately large grid resistor, plate resistor and grounded cathode, the gain of the tube will be its mu, so you could theoretically get a gain of 100 from a 12AX7. As Andy Le Blanc pointed out, the problem with this is that a large input signal will produce significant distortion so it can only be used as a first gain stage, and only then with a very modest signal.
all biasing methods work....... the point is to keep the tube operating on the
most linear portion of the plate curve...... for the most gain with the least
distortion....... useing a chathod resistor is the simplest and most economical
way...... but as long a bias is provided for any approach may work.....
12ax7s as a pre for guitar only need a couple volts.....
old texts show a way to battery bias the input of a triode.....
you float the signal generating source with a battery.... that might not be safe
with a guitar amp....... if the bias is not self generated you would still have to
place a blocking cap in the grid circuit to keep the DC volts from draining
out through what ever is connected to the input.......
