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Tube preamp

J

Jeff Johnson

Jan 1, 1970
0
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).
 
P

Pimpom

Jan 1, 1970
0
Jeff said:
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
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).

Except at very low plate currents (relative to normal operating
levels), triodes have quite a linear transfer with a positive
grid. With zero bias, a high source resistance could result in a
distorted waveform at the grid input due to grid current on the
positive half cycle. Otherwise, the input-output characteristics
should not show serious clipping just because the grid goes
positive.
 
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.

Are you sure that the R1 potentiometer is actually working properly
when in the extreme position and is not erratically open ?

Indirectly heated small signal tubes have such strong electron
emission from the cathode that it will drive a floating grid slightly
negative.

Look at various small signal RF/IF/audio (especially pentode) stages
from previous decades, they often have the cathode directly grounded
and a few megohms resistor from grid to ground, with the signal AC
coupled to the grid.

The electron charge cloud will bias the grid to several volts negative
respect to ground if completely floating. The resistor just leaks
excessive charge in order to make the bias more predictable.,
resulting typically to -1 .. -2 V, which is enough for small signal
operation.

This grid to ground potential can be measured with a voltage meter
with sufficiently high input impedance ( > 10Mohm).
 
R

Rich Grise

Jan 1, 1970
0
Jeff said:
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?

Grid-Leak Bias.
(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).

Hope This Helps!
Rich
 
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