Tube preamp low freq loss

[Jeffrey Landgraf]

I recently built a tube overdrive circuit for my guitar rig. It functions
well in almost all respects except one issue that I've not found an answer
for yet. The circuit description is an opamp input stage which drives the
first half of a 12ax7, this is followed by the second half of the 12ax7
which then goes to an opamp output gain stage. If you've seen the Paia tube
mic pre circuit it's essentially the same changed to single ended input and
the gain of most stages have been changed to push the tubes to distortion
levels. If I bypass the tube section the circuit is clean and fat. But
when I start to add the tube section for a mix of overdriven tube, it starts
to sound thin! I've checked the obvious, series caps and values which are
fine (Pspice simulations show -3db frequency response down to less than
10Hz). I've checked the actual circuit caps as well and they are good. I
can rule out the opamp circuits since signal passes through both opamps in
bypass mode. As I increase distortion it gets thinner. I haven't done a
frequency response measurement on the bench yet to identify the actual point
of low freq. rolloff but my guess is it's around the 300-500Hz range.(?) I
did add a 100uf cap to across the cathode resistor of the first tube stage
to increase the gain and there are only 3 other caps which are coupling caps
for input, between stages and output. By the way, the tubes are fed with a
low plate voltage of 48v, plate resisters are 270k, cathode resistors are
2700 (2nd stage) & 1200 bypassed with a 100uf (1st stage). Could this
simply be a result the low bias current and the "starved" plate design?
Any recommendations would be appreciated!

Thanks,
Jeff

[Scott Dorsey]

Jeffrey Landgraf > wrote:
>I recently built a tube overdrive circuit for my guitar rig. It functions
>well in almost all respects except one issue that I've not found an answer
>for yet. The circuit description is an opamp input stage which drives the
>first half of a 12ax7, this is followed by the second half of the 12ax7
>which then goes to an opamp output gain stage. If you've seen the Paia tube
>mic pre circuit it's essentially the same changed to single ended input and
>the gain of most stages have been changed to push the tubes to distortion
>levels. If I bypass the tube section the circuit is clean and fat. But
>when I start to add the tube section for a mix of overdriven tube, it starts
>to sound thin! I've checked the obvious, series caps and values which are
>fine (Pspice simulations show -3db frequency response down to less than
>10Hz). I've checked the actual circuit caps as well and they are good. I
>can rule out the opamp circuits since signal passes through both opamps in
>bypass mode. As I increase distortion it gets thinner. I haven't done a
>frequency response measurement on the bench yet to identify the actual point
>of low freq. rolloff but my guess is it's around the 300-500Hz range.(?) I
>did add a 100uf cap to across the cathode resistor of the first tube stage
>to increase the gain and there are only 3 other caps which are coupling caps
>for input, between stages and output. By the way, the tubes are fed with a
>low plate voltage of 48v, plate resisters are 270k, cathode resistors are
>2700 (2nd stage) & 1200 bypassed with a 100uf (1st stage). Could this
>simply be a result the low bias current and the "starved" plate design?
>Any recommendations would be appreciated!
>

Skip the cathode bypass cap. That's always a bad idea.

THEN measure the actual response. Are you really seeing a low frequency
loss, or are you seeing a huge high frequency buildup due to the distortion
effect? If it's the former, try larger value coupling caps and check to
see what your source and load impedances on the stage really are. If it
is the latter, try adding some RLC equalization networks until it sounds
right.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Jeffrey Landgraf]

"Scott Dorsey" > wrote in message
...
> Jeffrey Landgraf > wrote:
> >I recently built a tube overdrive circuit for my guitar rig. It
functions
> >well in almost all respects except one issue that I've not found an
answer
> >for yet. The circuit description is an opamp input stage which drives
the
> >first half of a 12ax7, this is followed by the second half of the 12ax7
> >which then goes to an opamp output gain stage. If you've seen the Paia
tube
> >mic pre circuit it's essentially the same changed to single ended input
and
> >the gain of most stages have been changed to push the tubes to distortion
> >levels. If I bypass the tube section the circuit is clean and fat. But
> >when I start to add the tube section for a mix of overdriven tube, it
starts
> >to sound thin! I've checked the obvious, series caps and values which
are
> >fine (Pspice simulations show -3db frequency response down to less than
> >10Hz). I've checked the actual circuit caps as well and they are good.
I
> >can rule out the opamp circuits since signal passes through both opamps
in
> >bypass mode. As I increase distortion it gets thinner. I haven't done a
> >frequency response measurement on the bench yet to identify the actual
point
> >of low freq. rolloff but my guess is it's around the 300-500Hz range.(?)
I
> >did add a 100uf cap to across the cathode resistor of the first tube
stage
> >to increase the gain and there are only 3 other caps which are coupling
caps
> >for input, between stages and output. By the way, the tubes are fed with
a
> >low plate voltage of 48v, plate resisters are 270k, cathode resistors are
> >2700 (2nd stage) & 1200 bypassed with a 100uf (1st stage). Could this
> >simply be a result the low bias current and the "starved" plate design?
> >Any recommendations would be appreciated!
> >
>
> Skip the cathode bypass cap. That's always a bad idea.
>
> THEN measure the actual response. Are you really seeing a low frequency
> loss, or are you seeing a huge high frequency buildup due to the
distortion
> effect? If it's the former, try larger value coupling caps and check to
> see what your source and load impedances on the stage really are. If it
> is the latter, try adding some RLC equalization networks until it sounds
> right.
> --scott
> --
> "C'est un Nagra. C'est suisse, et tres, tres precis."


> Skip the cathode bypass cap. That's always a bad idea.

Scott,

Yes, I agree I need to measure the actual response which is my next step in
debug. This should point to the RC pole where I'm getting rolloff if
that's the problem. Why is it a bad idea to add a cathode bypass cap? This
is straight out of many tube handbooks including the Audio Cyclopedia as
well as other reference books. I haven't seen any mention as to any "bad"
effects of doing this. Also, you mention high frequency buildup as the
waveform moves toward a square wave. I've looked at the output waveform on
the bench using a sine wave input and don't see a sloped, scooped or
otherwise strange square wave. I would assume this would lean toward
coupling cap (I have not tried the complex signal from a guitar signal).
Would this buildup (and loss of low freq) have anything to do with the loss
of even order harmonics?

Thanks for the advice,
Jeff

[Scott Dorsey]

Jeffrey Landgraf > wrote:
>
>Yes, I agree I need to measure the actual response which is my next step in
>debug. This should point to the RC pole where I'm getting rolloff if
>that's the problem. Why is it a bad idea to add a cathode bypass cap? This
>is straight out of many tube handbooks including the Audio Cyclopedia as
>well as other reference books. I haven't seen any mention as to any "bad"
>effects of doing this.

Cathode bypass will give you an additional low frequency corner, it means
an additional capacitor in circuit (and therefore additional nonlinearity
due the cap), and it gives slightly poorer linearity of the tube itself.
It's common on a lot of circuits out there to increase gain at the
expense of linearity. But you want it out because you don't need much gain
anyway and it makes your circuit harder to model.

Also, you mention high frequency buildup as the
>waveform moves toward a square wave. I've looked at the output waveform on
>the bench using a sine wave input and don't see a sloped, scooped or
>otherwise strange square wave. I would assume this would lean toward
>coupling cap (I have not tried the complex signal from a guitar signal).
>Would this buildup (and loss of low freq) have anything to do with the loss
>of even order harmonics?

No, it has something to do with the additional of odd order harmonics. When
you distort something, you're making a square wave with a bunch of high frequency
trash. When you do it with a cheesy starvation-mode circuit, you'll tend to get
more higher order stuff than you would if you had the tube working at normal
levels, and also the distortion is asymmetric so you're getting some even order
stuff too.

But the whole notion of distorting a signal IS to add harmonics... and the more
you add, the more topheavy the signal will sound. EQ can help you with this.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."