Difference between power valves and voltage amplifying valves
This article is mainly about triodes.
It is relatively easy to get large amplification of audio signals by using high μ valves, and thereby obtain large audio frequency voltages. But in these valves the internal resistance of the valve, ri, is very large and therefore also the anode current is small, and off course the power delivered to the load will be small due to a small current through the valve (ia).
I will compare two valves to clarify this, one high μ typical voltage amplifier valve, ECC83 and one typical power amplifier valve EL84 used in triode configuration.
The basic equation for valve operation is:
When Vs (Signal voltage on grid) is at its maximum value, then ia (anode signal current) will be at its maximum too. Let us take a look at the characteristic curves (Ia/Va) for the two valves:
Let’s do some calculations on the ECC83/12AX7 first, this is a typical high μ valve, well known in many preamplifiers and guitar amps.
Anode voltage Va: 250 V, Grid voltage -2 V, Internal resistance (ri) 62.5 K. ohm and amplification factor 100. Load 330 K. ohm
We assume that the peak signal voltage (Vs) is 2 V, just enough to swing the grid to 0.
Let us do some calculations using the previous formula:
Since we calculate power from RMS value, and not from peak value, we have to multiply this with .707 to get the RMS value:
0.51 x .707 = 0.36 mA
And the power out :
Peak voltage over the load:
I x R = 0.00051 x 330 000 = 168.3 volts
Now let us do the same calculations with the EL 84 valve, in triode configuration:
Anode voltage Va: 250 V, Grid voltage -9.0 V, Internal resistance (ri) 2 K. ohm and amplification factor 19.5. Load 4400 ohm. We again assume that the peak Vs drives the grid to 0 volts, in this case 9 volts.
And RMS value:
0.027 x .707 = 0.019 A
Power out is then:
Peak voltage over the load:
I x R = 0.027 x 4400 = 118 volts
From these calculations we can see that the voltage amplification of the high μ valve is considerably larger than for the low μ valve. (168 volts compared to 118 volts) for an input signal that is 4.5 times smaller. The conclusion is that ECC83 is a much better voltage amplifier than EL 84 in triode mode. But with a greater voltage developed over a much higher value of load resistance (330 000 ohms compared to 4400 ohms) the power output is much smaller, actually 38.5 times smaller. The power valve delivers 38.5 times more power with a lower value of output voltage across a load that is much less, actually only 1/75 of the value of the ECC83 load.
The reason for this is mainly the much lower internal resistance (ri) of the power valve, this permits a large AC anode current, for a comparatively small anode voltage.
After this we may draw the conclusion that a valve with a low internal resistance will have the possibility of delivering much more power than a valve with a higher μ which, in most cases, will have a much higher ri, but the valve with the high μ will be a much better voltage amplifier, especially when delivering to a high resistance load such as the input to another amplifier stage or the input of a valve power amplifier. A valve with a high ri will in most cases not be a good choice when delivering signal to a low resistance (impedance) load.
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