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This page discusses the effects of tube aging on the RIAA filters and other amplifier stages ... there is a lot of discussion on tube aging today, both because the influence on tube aging on circuits like phono amps (or other stages with filters) is larger than normal, but also since all of us seem to e searching for those NOS - never been used before - tubes to use in our latest projects. And since everybody on eBay seems to have tube testers, and tube matching seems to be more important than making good designs in the first place, I decided to look into the effects of tube aging in my DIY projects.
I've doubted whether to put this page in the RIAA cookbook or in the tube background section. I've decided to put it here since it deals primarily with the physical effects of tube aging. When desiging your RIAA circuit it is good to keep this page in mind. Remember that it is nearly impossible to make a tube phono design that is always low noise and always correct in (passive) RIAA filter without spending massive amounts of money to cater for things you'll probably not hear. Sometimes your amp just needs some maintenance, and small tubes have a very long lifespan with good parameters.
Should you nevertheless be worried about anyway, than you can always build a solid state amp (which for phono is not a bad idea really), use lots of feedback or or or ...
And remember, I'm not the ultimate tube expert, I'm just getting the info I think is missing from lots of other places.
When tubes age, the cathode emission descreases. 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 (e.g. Hickok) 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.
Now 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.
Let's first look at the most common stage used for amplification of signals, the Common Cathode amplifier. The important formulas are copied from the first page.
Based on the same pricinples it is possible to calculate the effects on the output impedance of the CCS amplifier.
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 isalmost minimal. With cathode bypassing
however the effect on the output impedance is strong (for CCS).
But of-course there is no free lunch: If tubes are not bypassed the gain is
also a lot lower.
xxx