Ian Bell wrote:
I am having a strange problem with what appears to be
distortion due to grid current in a 12AX7. It's a CC stage
with a 3K Rk bypassed about 120K in the plate and a 300V
supply. Anode sits at just over 200V and cathode just below
2V.
I would not expect to see any distortion due to grid
current with an input signal of 250mV rms at 2KHz but I
do. There's a 1Meg from grid to ground and the test
oscillator is fed to the grid via a 100K resistor.
Analyzer is connected across grid to gnd. Tube off, there
is no distortion with the 100K in cct or shorted. Tube
powered up I get about -33dB 2H measured at the
grid, -39dB 3H, -59dB 4H.
Short the 100K and the 2H drops to to below -70dB.
I tried changing the tube but the result is the same. What
is going on?
I could spend a while calculating the drop in grid
resistance and the rise in grid voltage due to signal, and
compare the two, but maybe you've already done that.
What's the grid resistance or current *supposed* to be, and
how should it change with voltage?
Do all the grids of all the valves in the world behave in
broadly the same way, regardless of their purpose?
At low anode voltage does grid current rise steeply? Are you
using a typical operating point and load, or have you made
it up yourself? 120k seems very low for an ECC83.
Do you need the 100k? If so, could you use a lower fixed
resistance multiplied by feedback, and could that
arrangement alleviate the grid current problem?
The only mention I found of grid current in datasheets is on
page two in this:
http://www.tubezone.net/pdf/12ax7ecc83.pdf
Where it appears to note (*) that the distortion shown in
the table is due to grid current.
Another thing, in passing, is that your test circuit had me
musing on the difference between a grid stopper and a source
resistance. Calculations would be easier if you swapped that
100k to the other side of the 1meg leak, maybe.
Ian