Tube preamp stage

[powerampfreak]

Hi all,
Me and a friend at work argue about the behaviour of a simple tube
preamp stage found in most guitar amp front ends. I know I'm right but
he says he's right. Well, here we go:
Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
cathode resistor. The tube is a standard ECC83 (12AX7). Across the
1.5k resistor a 1u cap is in parallell.
In my eyes this will introduce a slope at around 100hz (1/2piRC)
simply limiting low frequencies.
My friend (former...) says that changing the 1U cap to a 50u cap will
increase gain. This is true, but this also affects the low frequency
gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
But my opponent says this is not true.
I'm not used to tube circuits, just transistors stages, but I know the
above applies to transistor stages.
So, would anyone just tell me I'm right in my point?
Thanks,
Regards,
PAF

 

[Jim Thompson]

Hi all,
Me and a friend at work argue about the behaviour of a simple tube
preamp stage found in most guitar amp front ends. I know I'm right but
he says he's right. Well, here we go:
Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
cathode resistor. The tube is a standard ECC83 (12AX7). Across the
1.5k resistor a 1u cap is in parallell.
In my eyes this will introduce a slope at around 100hz (1/2piRC)
simply limiting low frequencies.
My friend (former...) says that changing the 1U cap to a 50u cap will
increase gain. This is true, but this also affects the low frequency
gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
But my opponent says this is not true.
I'm not used to tube circuits, just transistors stages, but I know the
above applies to transistor stages.
So, would anyone just tell me I'm right in my point?
Thanks,
Regards,
PAF

You be right.

...Jim Thompson

 

[martin griffith]

Hi all,
Me and a friend at work argue about the behaviour of a simple tube
preamp stage found in most guitar amp front ends. I know I'm right but
he says he's right. Well, here we go:
Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
cathode resistor. The tube is a standard ECC83 (12AX7). Across the
1.5k resistor a 1u cap is in parallell.
In my eyes this will introduce a slope at around 100hz (1/2piRC)
simply limiting low frequencies.
My friend (former...) says that changing the 1U cap to a 50u cap will
increase gain. This is true, but this also affects the low frequency
gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
But my opponent says this is not true.
I'm not used to tube circuits, just transistors stages, but I know the
above applies to transistor stages.
So, would anyone just tell me I'm right in my point?
Thanks,
Regards,
PAF

Why don't you build it and measure it?


martin

 

[Eeyore]

Hi all,
Me and a friend at work argue about the behaviour of a simple tube
preamp stage found in most guitar amp front ends. I know I'm right but
he says he's right. Well, here we go:
Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
cathode resistor. The tube is a standard ECC83 (12AX7). Across the
1.5k resistor a 1u cap is in parallell.
In my eyes this will introduce a slope at around 100hz (1/2piRC)
simply limiting low frequencies.
My friend (former...) says that changing the 1U cap to a 50u cap will
increase gain.

At low frequencies only. Midband gain will be the same.

This is true,

Not exactly, see above.

but this also affects the low frequency
gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
But my opponent says this is not true.
I'm not used to tube circuits, just transistors stages, but I know the
above applies to transistor stages.
So, would anyone just tell me I'm right in my point?

Basically yes.

Graham

 

[Bob Eld]

Hi all,
Me and a friend at work argue about the behaviour of a simple tube
preamp stage found in most guitar amp front ends. I know I'm right but
he says he's right. Well, here we go:
Imagine a standard circuit with 100kohms anode resistor and 1.5kohms
cathode resistor. The tube is a standard ECC83 (12AX7). Across the
1.5k resistor a 1u cap is in parallell.
In my eyes this will introduce a slope at around 100hz (1/2piRC)
simply limiting low frequencies.
My friend (former...) says that changing the 1U cap to a 50u cap will
increase gain. This is true, but this also affects the low frequency
gain of the stage. Instead of 100Hz, it will go down to around 2Hz.
But my opponent says this is not true.
I'm not used to tube circuits, just transistors stages, but I know the
above applies to transistor stages.
So, would anyone just tell me I'm right in my point?
Thanks,
Regards,
PAF

You are right, sort of. But, in the bass region below 106Hz, the break point
of the 1u, 1.5k resistor, the gain is falling. Using a larger cap will
increase the gain in this region by moving the break point lower. It won't
increase the gain at high frequencies. However, the kicker is what happens
to the gain in the critical region from 100 to 300 Hz where the fundamental
frequencies of notes around middle C are. Here the gain will increase one to
three dB depending on the exact frequency. This increase coupled with the
increase in the bass will give the sound added umph or the perception of
more "body" and power. Depending on the speakers and other factors, you'll
probably like the sound of the added cap better than without it.
If there is feedback around this tube stage, all bets are off.

 

[powerampfreak]

At low frequencies only. Midband gain will be the same.


Not exactly, see above.


Basically yes.

Graham

Thanks for all replies. I kind of love the situation when telling my
opponent he's out bicycling (that's how we say in Sweden
when somebody is not right..
BUT - I thought that bigger cap would increase the gain over the
entire spectrum. Not only pushing the breakover point lower.

Have I missed something?

 

[Eeyore]

Thanks for all replies. I kind of love the situation when telling my
opponent he's out bicycling (that's how we say in Sweden
when somebody is not right..
BUT - I thought that bigger cap would increase the gain over the
entire spectrum. Not only pushing the breakover point lower.

Have I missed something?

The larger cap will indeed increase the gain over the entire spectrum
*minutely*. The effect will simply be too small to notice though and only
measurable with the finest equipment with digital readouts that can measure
differences of 0.01 dB.

Graham

 

[powerampfreak]

The larger cap will indeed increase the gain over the entire spectrum
*minutely*. The effect will simply be too small to notice though and only
measurable with the finest equipment with digital readouts that can measure
differences of 0.01 dB.

Graham- Dölj citerad text -

- Visa citerad text -

Sorry, BUT - the capacitors impedance decreases with increasing
frequency.
Why doesn't the gain increase more than in the 0.01dB range?

 

[Eeyore]

Sorry, BUT - the capacitors impedance decreases with increasing
frequency.
Why doesn't the gain increase more than in the 0.01dB range?

Because, once the cap becomes a 'virtual' short to AC, the characteristics of the
tube itself determine the gain.

Graham

 

[powerampfreak]

Because, once the cap becomes a 'virtual' short to AC, the characteristics of the
tube itself determine the gain.

Graham- Dölj citerad text -

- Visa citerad text -

Oh, so you mean when caps Z is low enough the tube will provide
maximum gain?
I see....hmmm

Thanks for your always informative replies!

Regards

 

[Eeyore]

Oh, so you mean when caps Z is low enough the tube will provide
maximum gain?
I see....hmmm

Yes, I found that an ECC83's mutual conductance at 1mA for example is ~ 1.2mA/V. This
is a bit like having a 'perfect' tube with a cathode resistance of 830 ohms. Once the
caps' impedance becomes small compared to that 830 ohms, the gain won't increase
significantly. I do think 1uF is rather on the small side though.

Thanks for your always informative replies!

You're welcome.

Graham

 

[powerampfreak]

Yes, I found that an ECC83's mutual conductance at 1mA for example is ~ 1..2mA/V. This
is a bit like having a 'perfect' tube with a cathode resistance of 830 ohms. Once the
caps' impedance becomes small compared to that 830 ohms, the gain won't increase
significantly. I do think 1uF is rather on the small side though.


You're welcome.

Graham- Dölj citerad text -

- Visa citerad text -

Some guitar amp manufacturers seem to be using limited gain in each
stage and instead use multiple stages to achieve the necessary gain.
Some designs completely omit the cap. I think it boils down to
personal taste of each designer, and what makes two amps sound
different in the end.

Regards