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Thomas Mayer wrote:

I never had a lot of trouble with high transconductance tubes. I use the EC8020 a lot.

Stefano, you might look into this EC8020. I never seen it but just looked into the datasheet and it looks very interesting tube and it might be better candidate to your phonocorrector’s fist stage then the WE 437A. I generally, under other equal conditions, prefer Telefunken/Siemens tubes to any other tubes. My second choose would be the older British tube and only then I would look into all of those RAC/WE/Raytheon. Of cause it is a gross generalization but … I am comfortable with this generalization.

Thomas Mayer wrote:

This is true for RC RIAAs, not for LCR RIAAs. This is one of the big advantages of the LCR technique. The network has a constant 600 Ohms input impedance, if it's terminated by the proper 600 Ohm value. Variation in the driving tubes rp does only affect the loss in the network, but the loss is always the same over the frequency band. So the RIAA curve will not chabge when the tube detoriates or when tubes are changed.

That is not necessarily true. I think any passive RIAA would not be a subject of curve variation with tube aging. Moreover, I would take this argument even further. Even if we have a feedback RIAA, where the gain of the open loop is in the task of direct shaping the RIAA curve, than I feel that aging of the tube and minor change of their gain is not truly a practical issue. There are a lot of people out there who go totally crazy with writing the accuracy of the RIAA curve with .05dB precession. I feel that it is absolutely not necessary. If you want RIAA precession then have your corrector to run multiple curves as 50% of records out there are NOT RIAA pre-equalized. It is not to mention that there was a huge deviation in the RIAA equalizers during the cutting of the records. From what I see the .25dB-.5dB accuracy is plenty for normal record playing. The MC R loading, MM C loading, VTAs, the cartridges themselves give much higher discrepancies then .25dB. so, if you look how the small tubes change gain with time then in most cases it is not significant to affect RIAA too mach. Unless your LP is the only source you use and unless you run your phonostage a few hours each day I would not feel that the tubes aging might be a factor in RIAA running away.

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