What is the input output impedance of tubes?

[Alan0354]

I want to know the formulas of small signal (grid always more negative than cathode) input and output impedance of the 12AX7 in Fig.1 to Fig.4







This is my guessing:

1) the small signal input grid current is not signal dependent to input signal. So I assume it is like a set Rin shown in Fig.4 depends on the bias condition and plate voltage.

2) The input is mostly capacitance which is the combination of Cgk from grid to cathode and Cm Miller capacitance due to gain from the plate.

[Tooboob]

I think you guessed right.

[Alan0354]

I did some search, seems like it's just that simple. Too simple for my comfort. The fact that it has +ve and -ve grid current has to have some sort of resistance!!! But I can't find any circuit model analysis. Anyone has more info?

[teemuk]

Vacuum Tube Preamplifier Analysis and SPICE Simulation

[jazbo8]

I'm pretty sure the subject of the common cathode amplifier is covered in detail in RDH4. While you are at it, why not look up the input/output impedance as well for other valve voltage amplifier stages - common plate aka cathode follower, and common grid.

[Alan0354]

I am hoping for easy and quick answers. from the model Teemuk provided, seems like my first guess was right, the impedance does not change much between common cathode with or without cathode bypass. The Miller cap is the only one that play the major role.

[jazbo8]

I thought you did not like the simple answer... you have gotten just the negative bias case, the input impedance is not Rg when the grid is positively biased, in SPICE models, that is usually modeled as a forward biased diode, which isn't always right (but perhaps ok for a real rough estimate).

[Merlinb]

I did some search, seems like it's just that simple. Too simple for my comfort. The fact that it has +ve and -ve grid current has to have some sort of resistance!!! But I can't find any circuit model analysis. Anyone has more info?

The input impedance of a valve is mainly capacitive, especially when the Miller effect is present.
There is also lead inductance, which I would estimate around 10nH.

Then there is the resistive part, which is highly non-linear due to the nature of the grid current:

• At large bias voltages it is positive and very large, say 1 to 10 gigohms. Varying with anode voltage and gas presence.
• Between about -10V and -1V it briefly becomes smaller and negative, say -0.5 gigohms.
• Then at bias voltages hotter than about -1V it becomes positive again and rapidly drops to a couple of k-ohms as the grid becomes positive.

[Alan0354]

The input impedance of a valve is mainly capacitive, especially when the Miller effect is present.
There is also lead inductance, which I would estimate around 10nH.

Then there is the resistive part, which is highly non-linear due to the nature of the grid current:

• At large bias voltages it is positive and very large, say 1 to 10 gigohms. Varying with anode voltage and gas presence.
• Between about -10V and -1V it briefly becomes smaller and negative, say -0.5 gigohms.
• Then at bias voltages hotter than about -1V it becomes positive again and rapidly drops to a couple of k-ohms as the grid becomes positive.

Thanks for the reply, this really helps.

So in all intended purpose in guitar amps, the grid RESISTANCE is like an opened circuit ( as they are in over 100Mohm range) when the grid is not 0V or +ve. Then drop to Kohm when going +ve. When in small signal where the grid is always negative, we only have to deal with Miller cap.

Thanks

[Alf]

The input impedance of a valve is mainly capacitive, especially when the Miller effect is present.
There is also lead inductance, which I would estimate around 10nH.

Then there is the resistive part, which is highly non-linear due to the nature of the grid current:

At large bias voltages it is positive and very large, say 1 to 10 gigohms. Varying with anode voltage and gas presence.
Between about -10V and -1V it briefly becomes smaller and negative, say -0.5 gigohms.
Then at bias voltages hotter than about -1V it becomes positive again and rapidly drops to a couple of k-ohms as the grid becomes positive.

This is clear to me !
So I am experimenting with a different tube , an ecc85 tube and the bias voltage of the first stage is 1.85 V
what would the impedance be and how do you calculate this exactly ?

When I plug in my guitar it still sounds a bit mid emphasized , when I switch my onboard Anderton clarifier on it sounds much better ( this is low impedance out), more highs. But I want it to sound good too without the clarifier.
So I think I need to make the input impedance higher .

Can anyone explain to me how to do this ?

[jazbo8]

The Anderton "clarifier" is an EQ unit, are you boosting the highs when it is engaged?

[Alf]

I have the Clarifier in my Les Paul custom as an active tonecontrol . I usually have the controls on five , so in the middle I think but the mids are cut by the device. So that is part of why the highs get accentuated more I think.

[jazbo8]

The Clarifier has a flat response when the bass and treble controls are set to 5, but it does buffer the pickup from the load, so I think that's why you are getting less HF roll off when it is engaged. But unless you are running really long cables and/or have a very large grid resistor in you amp, there should not be that much of a difference.

[Alf]

Yes , I think you're absolutely right.
But I went through the component values again and noticed that I didn't have a 470 Pf cap in the tonestack but an 80 pf and that's a big difference so I went up to 330 pf now and have more highs now. And I noticed that I liked hte small amount of crunch I had with the 80 pf value so I think I'm going to make a switch to halve the 330 to 160 or lower.

Alf

[jazbo8]

That's seems like a good and simple mod to shape the HF response.