gofar99 Messages: 1926 Registered: May 2010 Location: Southern Arizona
Illuminati (5th Degree)
Hi Good thoughts, but my experience is somewhat different. I find that tubes from one company do not always match characteristics with those from another. Several parameters can differ, Mu, Rp, residual noise and MillerC. Rp is the thing that seems most often different. I have not found Miller capacitance to be much of an issue although in builds with very high input impedance of the second stage it might contribute some. The formulas I use include that impedance in the calculations and compensate for it. One issue I have with some common formulas is that they don't address Rp well. Rp is necessarily included in the resistance feeding most RIAA passive networks. It is unfortunate that many of the formulas don't clearly indicate that it does and I am sure many builders have wondered why their preamps don't sound quite right. Generally I specify the tube type (not just like 12AX7, but if it actually is a 7025, ECC type or something else including the brand) to minimize errors on diy builds. And not to get too far off course some tubes sound different. A classic example of a type I use is the 5751. Most sound similar with one exception the JJ 5751 is way different. In the end I test all my builds on a scope and use my ears to verify that the sound is as I think it should be. In the end the ears are what matters.
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