What is preamp tube transconductance?

[Jonas]

I'm looking at a few TDS for the 12A_7 preamp tubes

For a 12AU7 transconductance is listed at 3100 micromhos series / 2200 micromhos parallel

For a 12AV7 transconductance is listed at 6100 micromhos series / 8500 micromhos parallel

what effect does a higher transconductance have on a preamp circuit?

Thanks

[PRR]

At the same current?

Transconductance varies with current. Transconductance goes down as current goes down.

Figure the current in a typical preamp stage, and compare to the conditions where the data sheet quotes transconductance.

For most tubes, the datasheet transconductance is a "show-off" number which is almost meaningless to typical audio preamps. Sometimes it is like a TV VHF Tuner application. The 12AV7's 18mA condition is hot even for a TV tuner.  https://frank.pocnet.net/sheets/127/1/12AV7.pdf

Where do you see series and parallel? In heaters, not triode properties.

[HotBluePlates]

What is preamp tube transconductance?

How much plate-current change you get per unit of grid-voltage change.

I'm looking at a few TDS ...

I don't even remotely like that site for tube data.  Frank's is much better IMO.

For a 12AU7 transconductance is listed at 3100 micromhos ...

Ohm = Volt / Current
Mho = Current / Volt
1000 micromho = 1 milliampere of plate-current-change / 1 volt of grid-voltage-change

Europeans say it simpler as, "12AU7 transconductance is listed at 3.1mA per volt."

For a 12AU7 transconductance is listed at 3100 micromhos ...
For a 12AV7 transconductance is listed at 6100 micromhos ...

At the same current?

Transconductance varies with current. Transconductance goes down as current goes down.

Look at the graph at the bottom of Page 2 of this 12AX7 data sheet.  Transconductance ("Gm") is shown declining as you move left on the X-axis, which is tube plate current.

what effect does a higher transconductance have on a preamp circuit?

Figure the current in a typical preamp stage, and compare to the conditions where the data sheet quotes transconductance.

High-Gm is handy in power output tubes, or maybe in a Reverb Driver Tube that's pulling current through an reverb transformer.  It means, "You get your big alternating current with less applied signal-voltage."

High voltage-gain preamp tubes mostly have low-Gm (though pentodes may be an exception).  Compare 12AX7 Gm to 12AU7 Gm.

We don't need to swing large currents in the preamp; mostly we cannot swing large currents in the preamp.
   - Your preamp tube might have a 300v supply from a filter cap.
   - This 12AX7 tube probably has a 100kΩ plate resistor.
   - It's impossible for this 12AX7 to pull more than 300v / 100kΩ = 3mA; in practice it will only manage something like 2mA.


In theory, Volts = Current x Resistance so "big alternating current = big output voltage."

So we might think a high-Gm tube that can swing lots of current with a small drive signal equates to big voltage-gain.  Except we're limited by the power supply voltage... To get more than 3mA-swing from our example above, we need to reduce the plate-load resistor value (from 100kΩ to maybe 22kΩ).  Except reducing the plate-load resistor value just offset the "increased output voltage" created by the bigger current-swing of the high-Gm tube.  Turns out we're better off just using the low-Gm high-gain preamp tubes instead.

[PRR]

> Compare 12AX7 Gm to 12AU7 Gm.

The 12AU7 Gm is specced at much higher current than any 12AX7 condition.

The 12AX7's Gm at same current is really pretty good at any current the 12AX7 is good for.

As for using Gm to pick-a-tube.... it's complicated. It is generally easier to use the RC-amplifier tables, which also give maximum output (good to know).

[HotBluePlates]

> Compare 12AX7 Gm to 12AU7 Gm.

The 12AU7 Gm is specced at much higher current than any 12AX7 condition.

The 12AX7's Gm at same current is really pretty good at any current the 12AX7 is good for.

Indeed!

I did the work to figure out "what happens when plugging a 12A_7 into a 12AX7 socket."

Funny enough, the 100kΩ plate load and 1.5kΩ cathode resistor choke the 12AU7:  it's Gm in this circuit is only 1350 microhmos, where a 12AX7 should be 1540 micromhos.  The high-resistance circuit lowers plate current to where the 12AU7 doesn't develop its show-off numbers (as you alluded to before).

[Jalmeida]

What is preamp tube transconductance?

How much plate-current change you get per unit of grid-voltage change.

I'm looking at a few TDS ...

I don't even remotely like that site for tube data.  Frank's is much better IMO.

For a 12AU7 transconductance is listed at 3100 micromhos ...

Ohm = Volt / Current
Mho = Current / Volt
1000 micromho = 1 milliampere of plate-current-change / 1 volt of grid-voltage-change

Europeans say it simpler as, "12AU7 transconductance is listed at 3.1mA per volt."

For a 12AU7 transconductance is listed at 3100 micromhos ...
For a 12AV7 transconductance is listed at 6100 micromhos ...

At the same current?

Transconductance varies with current. Transconductance goes down as current goes down.

Look at the graph at the bottom of Page 2 of this 12AX7 data sheet.  Transconductance ("Gm") is shown declining as you move left on the X-axis, which is tube plate current.

what effect does a higher transconductance have on a preamp circuit?

Figure the current in a typical preamp stage, and compare to the conditions where the data sheet quotes transconductance.

High-Gm is handy in power output tubes, or maybe in a Reverb Driver Tube that's pulling current through an reverb transformer.  It means, "You get your big alternating current with less applied signal-voltage."

High voltage-gain preamp tubes mostly have low-Gm (though pentodes may be an exception).  Compare 12AX7 Gm to 12AU7 Gm.

We don't need to swing large currents in the preamp; mostly we cannot swing large currents in the preamp.
   - Your preamp tube might have a 300v supply from a filter cap.
   - This 12AX7 tube probably has a 100kΩ plate resistor.
   - It's impossible for this 12AX7 to pull more than 300v / 100kΩ = 3mA; in practice it will only manage something like 2mA.


In theory, Volts = Current x Resistance so "big alternating current = big output voltage."

So we might think a high-Gm tube that can swing lots of current with a small drive signal equates to big voltage-gain.  Except we're limited by the power supply voltage... To get more than 3mA-swing from our example above, we need to reduce the plate-load resistor value (from 100kΩ to maybe 22kΩ).  Except reducing the plate-load resistor value just offset the "increased output voltage" created by the bigger current-swing of the high-Gm tube.  Turns out we're better off just using the low-Gm high-gain preamp tubes instead.

This is a really great explanation. Thanks!

[Jonas]

thank you for the in-depth help. I don't totally understand everything yet, but still working at it