Dave the bass wrote:OK. Swotting again.
Now then, this Transconductance measured in 'mhos', I've seen that detailed in some Data sheets. If a unit of resistance is 'negative ohms' it becomes an indication of 'gain' or amplification yes?
Just checking I've got this bit right before going on.
Not quite as bright as Sunday what with it being a Monday .....
No, its not a measure of gain, but a measure of the transconductance of the valve (you could consider it the current gain of the valve). It can be used to calculate the voltage gain of the valve, but only if you know the anode resistance (ohms law again).
gm = Âµ / ra
Âµ = gm * ra
Remember that we measure the gain of the valve as the ratio of signal voltage in to signal voltage out. The fact the valve is a transconductance device means it converts a change in voltage to a change in current. To convert current to voltage we need to pass it through a resistance (ohms law again), so the above equation shows how the gain of the valve is a result of the gm multiplied by the anode resistance.
For example, the 5687 has a reported gain of 18.5, and a transconductance of 11500 Âµmhos (micro mhos) So we can find its anode resistance by
ra = Âµ / gm
ra = 18.5 / 11500e-6
ra = 1608 ohm.
So to bring it on topic, if we have a 50H choke, how will that play with a 5687?
The -3dB point of a LR filter is f-3dB = R / L * 2 * pi, so its 5Hz. So 50H is ok, but more might be better. 220H gives 1.2Hz
Though remember that we hen we talk about the gain of the valve, its the internal gain, the actual gain in a amplifier will be less than Âµ as we will be seeing the effect of the anode resistance in parallel with the load resistance, so as the load resistance gets larger the gain of the circuit approaches the Âµ of the valve.
Also remember that the gm of a valve will vary with the op point, so the figures given are only valid at one set of current and voltage conditions. The less a valves gm varies with current the more linear it is.
Little known fact, coherent thought can destructively interfere with itself leaving no thought at all, that’s why I prefer incoherent thought.