Tube ---- Ei 12ax7

To me the EI 12AX7 is the best production tube of this type (less colored, more defined and extended, cleaner), though I've never tried the new Svetlana's. I have found the newer shiny-plate ones to be as good as the older gray-plate ones. For high-gain stages, like a phono input amplifier, you'll have to go through several of these tubes to find one or two having acceptably low noise and microphonics. But for less critical stages, which is what I'll assume you're dealing with here since you've substituted NOS 5751's, the rejection rate should be within reason. (BTW, the ECC83 is the same tube for these purposes, but the 5751 is not quite - it has less gain, for one thing.)

An approach I was able to take successfully on a few occassions - but which may not be practical or available to everyone - was to get 10-20 of these tubes from a local guitar or repair shop that stocked these tubes, and where I could find a guy working there who was understanding of my requirements and would cut me some slack. What I'd request was to be able to take the tubes home for a few hours, and test them all by simply inserting them into the input stage of the phono preamplifier I was using at the time. I would cull the good tubes (for noise and microphonics only), separating them into those that would work in the phonostage, those that would work in less critical stages (amplifier input stage, guitar amps), and those that were rejects. Then as per my prior agreement with the shop guy, I'd take back the rejects in the afternoon for a full refund. This way I was able to be happy without paying a pretesting premium, which I have unfortunately found to be no guarantee of acceptable performance, and even though pretested tubes that don't stack up can be exchanged, you often wind up paying too much in redundant shipping charges on top of the higher initial cost.

P.S. - Trelja, not only haven't I tried the Svet's, I also haven't tried the LPS Sovteks, which I believe Albert Porter prefers among the current crop (though I think he's OK with the EI's as well). So I should really just say that I prefer the EI's to the Teslas, regular Sovteks, and Chinese 12AX7's (and also latter-day Sylvanias), but not imply that I have gathered all the evidence to say the case is totally closed in my mind.

Just a note, Distortion: As far as I know, all the EI 12AX7's will be the same "model" as one another. The 'Low Noise' designation will simply mean that the particular ones you bought have been pretested and culled by the reseller to represent something like the best 10% or 5% of the tube crop tested (along with being priced higher), while the remainder are sold for less without the 'LN' tag.

Joe, this unfortunately didn't work out from a technical standpoint. I'll be updating the original thread.