J
Jeff Johnson
I'm playing around with my amp's tube preamp(12AX7). I have set to pots for
the bias for both degeneration and cathode, i.e.,
R1
|
+-C-Gnd
|
R2
|
Gnd
R1 is the degeneration resistance and R2 + C is the cathode bias.
What I don't understand is that when R1 and R2 are almost 0(< 100 ohms) I
still get a rather decent signal even with the grid has a signal of at least
1V p2p. I would expect a significant amount of clipping on the positive half
cycle. I have checked the voltage on the grid and it is 0 and the cathode at
around 200mV or lower.
I get the input signal and output signal on and they are rather very close.
Changing the bias from around 0 ohms to 2.5k ohms essentially just starts
clipping the peaks. This is fine except I would expect at around 0 that the
positive part of the signal would be completely clipped.
What am I missing? (I've checked everything and all the measurments are
correct). All I can think of is that the grid is being driven positive but
the tube's plate resistance isn't dropping a significant amount to create
clipping(the tube could be bad).
the bias for both degeneration and cathode, i.e.,
R1
|
+-C-Gnd
|
R2
|
Gnd
R1 is the degeneration resistance and R2 + C is the cathode bias.
What I don't understand is that when R1 and R2 are almost 0(< 100 ohms) I
still get a rather decent signal even with the grid has a signal of at least
1V p2p. I would expect a significant amount of clipping on the positive half
cycle. I have checked the voltage on the grid and it is 0 and the cathode at
around 200mV or lower.
I get the input signal and output signal on and they are rather very close.
Changing the bias from around 0 ohms to 2.5k ohms essentially just starts
clipping the peaks. This is fine except I would expect at around 0 that the
positive part of the signal would be completely clipped.
What am I missing? (I've checked everything and all the measurments are
correct). All I can think of is that the grid is being driven positive but
the tube's plate resistance isn't dropping a significant amount to create
clipping(the tube could be bad).