Which Tube Should I Use ?

[tubedude]

Your experiences do not match mine. I have seen plenty of crappy tubes from the US, Europe and Japan. I have also seen outstanding tubes from Russia, chech republic and China. The premium US and European tubes were better than what we see today, but the average tubes were quite comparable.

Our purchasing and screening processes are complex, because we are also screening for precursors that might indirectly influence reliability. It's all proprietary information.

But without revealing any secrets, i'm guessing you have a rejection rate of about 10-12%
You know i did have ten Tesla silverplate 12ax7's that were nice.

 

[agedhorse]

It depends very much on the production lot and how tightly spec'd the tubes are.

I went through a good sized batch of NOS GE/RCA 12AX7 tubes a while back and was surprised at how high the screen-out rate was, they were worse than our average new production tubes. I don't know how much of this is due to the time passed and how much is due to the inherent quality of the tubes themselves.

 

[tubedude]

It depends very much on the production lot and how tightly spec'd the tubes are.

I went through a good sized batch of NOS GE/RCA 12AX7 tubes a while back and was surprised at how high the screen-out rate was, they were worse than our average new production tubes. I don't know how much of this is due to the time passed and how much is due to the inherent quality of the tubes themselves.

Unless they are gassy, i wouldn't think the materials would degrade much, unless mechanical shock was strong enough to shift the grids. Might truly have been marginal tubes to begin with.

 

[rickdog]

I seem to remember hearing that towards the end of the mass market tube era 40 years ago the U.S. manufacturers were not investing in their tube production lines, equipment wore out, and tolerances got sloppy. So NOS GE and RCA tubes from that time really might not live up to expectations.

 

[agedhorse]

I seem to remember hearing that towards the end of the mass market tube era 40 years ago the U.S. manufacturers were not investing in their tube production lines, equipment wore out, and tolerances got sloppy. So NOS GE and RCA tubes from that time really might not live up to expectations.

They always had issues, the scrap rate of the tube industry was quite astonishing actually.

 

[tubedude]

On one of these threads someone posted a pic of a mountain of tubes that didn't meet QC. There must have been millions of tubes in that scrap heap.

 

[agedhorse]

On one of these threads someone posted a pic of a mountain of tubes that didn't meet QC. There must have been millions of tubes in that scrap heap.

The failure rate of tubes of all origins is high, especially the initial failure rate, but there were lines of British tubes that had such abysmal microphonic defects that it's a wonder they could ship acceptable tubes at all.

It's ironic, that in today's precision CNC environment, there isn't a company taking advantage of the precision in order to improve first pass yields (low hanging fruit). On the other hand, I can imagine tube snobs rolling their eyes at better quality more consistent new production tubes, now complaining that they don't make them like they used to...

 

[bucephylus]

Here is my preferred reference on preamp tubes. It does NOT cover current production tubes. And, one other thing to be aware of is that some preamp circuits are more sensitive to tube changes than others. Which is an interesting topic in itself. In some senses, designers may prefer a preamp that is insensitive to tube choice. But, I have always preferred preamps which allow the player to easily hear differences between tubes. One of those two sides to the discussion things.

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