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This page discusses the effects of tube aging on the RIAA filters and other
amplifier stages. When tubes age, the cathode emission decreases. This is what
tube testers try to test: either they test the emission (cheaper testers) by
measuring current through the tube when it's connected and behaving as a diode,
or the expensive testers test the transconductance of a tube.
There is a company also claiming to test the tubes for gain.
How does the decreased cathode emission affect the sound then? In general there are two tube parameters involved: The S / GM / Transconductance of the tube decreases which will result in less gain for the same input voltage. Do we care? Well we do for the sake of total gain of the amp, but in most cases we will just turn up the volume a little more. For some phase splitter circuits etc. it can be an issue if both pats of such a phase splitter are not the same tube and age diferently (for example we do not use double triodes).
But for phono amps another parameter is more important. If tube aging affects the plate resistane (r_a) of the tube. In the case that effective plate resistane will increase significantly, the output impedance of the tube will incrase as well. Remember that for CCS the output impedance is the effective impedance of Rp and r_a in parallel. Therefore the value of R1 in series with Z_out of the tube will increase and our filter is not calibrated anymore.
If we know what parameters will change over time, it's interesting to see what effect these changes have on the gain of an amplifier and its output impedance.
It is possible with the formula above to calculate the influence of tube aging for a particular setup on the output impedance. I did some calculations in excel and some things are clear: For example if the cathode is not bypassed with a capacitor, the influence of tube aging almost minimal. With cathode bypassing however the effect on the output impedance is strong (for CCS).
The circuit below is from Loekie, and there are several phono circuits alike in design. Some have ECC83/12AX7 as the first tube, others ECC88/6DJ8. But one thing in common: often the first stage of the amp is a CCS amplifier.
Especially for passive RIAA filters, the output impedance of the tube driving the filter network is part of the RIAA filter itself. Look for example to the circuit of Loekie above. As you can see, the output impedance Ru of a ECC83/12AX7 tube with 150k plate resistor and bypassed cathode resistor is 43k Ohms. Most components in the figure do have more or less constant values, except the values of Ru and Cmiller.
The last two are dependent on the tube and on the tube constans of these tubes. However tube constants are not really constant anyway. As tubes age, the cathode emission decreases over time.
There are several ways to help achieve correct RIAA filtering:
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