12ax7 spec sanity check

[craigb]

Hi y'all,
I've been experimenting with a compressor circuit using the GT 12ax7m tube for make-up gain. This is their Mullard clone, and IMO is an excellent sounding tube. Due to space and heat considerations, I am cheating a bit by using just 1 tube for a stereo signal. Let me first say that I know that this is less than optimal, but it allows me to pull off something else that is kind of cool.

Anyway, I have bought 3 pairs of the 12ax7m's over the last year, in July, Oct and Dec. Since I am running stereo through one tube, the internal crosstalk of the tube is an issue. Each of the pairs of tubes I have measures within a db or so, but over the 3 purchases, the crosstalk specs have worsened. The first pair I bought in July measured fine, about 70db of separation. The October pair dropped to around 58db, and the most recent pair to only 48db.

I have talked to GT a bit, and their response is that it's just normal manufacturing variation. They don't select for crosstalk, and in a guitar amp it's not going to matter. Below is a graph showing the measurements. To my eyes it's pretty dramatic, but GT doesn't seem to agree. Again, I realize that I may be just asking too much of the tube, but the first set at 70db shows that a decent spec is possible.

I know that tube manufacturing is very labor intensive and prone to variation. And 6 tubes is not a large sample. But if that were my product, I'd sure be wondering what the heck was going on.

I'd appreciate anyone's comments.
Craig

[grahluk]

IMO tube production these days rates right along with agriculture or wine. You can get stuff from the same brand, same factory, different batch and get very different resultant quality. I'm not so much in the loop anymore but I remember having to be up on who is making what tubes well at any given moment and switching supplies as quality migrates.

[tINY]

That's a pretty huge range. Even if you have the extreme cases, +/- 11dB for crosstalk is pretty big. I wonder how much the other parameters vary.

No wonder every tube amp sounds different!



-tINY

[craigb]

I should add that all 6 tubes measure very closely on noise and distortion. And they should, since that's what GT is selecting for. I have an old Sovtek 12ax7WB laying around that has an internal shield between the tube sections. It measures an excellent 85db of separation. Unfortunately, it sounds like..., well, not nearly as good as the 12ax7m.

[craigb]

update:
I picked up 5 other 12ax7's, all from different manufacturers, and tested them for crosstalk. The chart below doesn't show the Sovtek 12ax7lps, but it measures about the same as the others.

So, I have 5 randomly selected tubes from 5 different manufacturers, and they all come in at around 70db of separation. Versus 48db on the most recent GT 12ax7m that I have. Still not a lot of data points, but it leads me to believe that 70-ish is a reasonable expectation. Interesting stuff.

[tINY]

If you are actually using this on a stereo signal, -70dB of crosstalk is probably better than you need for most recordings...



-tINY