Vacuum tubes variance in performance is making me crazy!

[shimster]

Hi again folks,
  This is related to my "Duplicate Pedals don't sound the same thread" but I have a very specific question.  Is it normal for vacuum tubes of the same part number from the same manufacturer to perform drastically different?
  Here's what's happening. It looks like I have 3 batches of 12AY7 tubes from EH.  They are stamped with what I assume is a batch number. The numbers are : "07 10" , "08 08" and "08 10".  What's making me crazy is that I used the two "08 08" tubes while I was designing my pedal and they produced an insane amount of overdrive.  After some analysis it would seem that they are naturally more "starved" than the other batches.  The plate voltages I'm getting off of the "0808" tubes is around 9V with a 220K plate resistor and a 12.6 volt power supply.  The other batches of tubes are reading about 11.4 volts and 10.8 volts.  I'm not even comparing the second stage voltages but I'm pretty sure they're also all over the map.
  So, is this pretty normal variation of tube performance?  Man, I love the sound of a real tube overdrive but this is ridiculous.  Should I use trim pots in place of my plate resistors or cathode resistors? Or on the grid? How do you guys deal with this?  This is madness!
  Actually, it's not that bad. This is an amazing learning experience for me.  But it is pretty insane if you're trying to dial in a specific sound...
-Dan

[snap]

what`s wrong? 10.8V +10/-20 %

[R.G.]

...
  So, is this pretty normal variation of tube performance?  Man, I love the sound of a real tube overdrive but this is ridiculous.  Should I use trim pots in place of my plate resistors or cathode resistors? Or on the grid? How do you guys deal with this?  This is madness!
  Actually, it's not that bad. This is an amazing learning experience for me.  But it is pretty insane if you're trying to dial in a specific sound...

Welcome to the real world of engineering.   

Once you fully understand circuits, know how components act and interact, get a familiarity with an encyclopedia of circuits and applications and get the math under control, that then qualifies you to begin understanding what component variances do to you, and start learning how to get what you want consistently in the face of variation.

I once saw engineering described as a disciplined way of making what you need out of whatever you actually have.

[shimster]

Welcome to the real world of engineering.  

Once you fully understand circuits, know how components act and interact, get a familiarity with an encyclopedia of circuits and applications and get the math under control, that then qualifies you to begin understanding what component variances do to you, and start learning how to get what you want consistently in the face of variation.

I once saw engineering described as a disciplined way of making what you need out of whatever you actually have.

Thanks R.G.! In retrospect, I've had 386 opamps perform radically different and transistors too.  It was only when I was reaching for a specific sound that all the little variances started to affect every aspect of my circuit and start to annoy me.  Great stuff!  I love a challenge.  It's not just engineering, it's an adventure!

what`s wrong? 10.8V +10/-20 %

Hehe, I suppose it isn't so bad.  What surprised me initially is that within each batch the performance is quite consistent to my ears.  I came to expect that kind of consistency.  I'm learning...

[brett]

May I suggest in the strongest possible terms that if you see a Germanium transistor, immediately cross to the other side of the street.  Do not, I repeat, DO NOT look directly at it. 

[shimster]

Hey guys,
  One more update. I finally dug up an old pair of tubes that gave me just the right amount of gain in my overdrive circuit.  Can you guess what they are? You probably guessed it, I used 12AX7s (sovteks) to replace my 12AY7s.  The pair of 12AY7 tubes I used in my original circuit behaved an awful lot like 12AX7s.  Confused the heck out of me! I put the old 12AX7 tubes into the circuit and voila! Sweet sweet overdrive! Not sure why I was so stubborn to use only 12AY7s in an overdrive pedal. Anyway, I tested the pin voltages and they look a *lot* like the original pair of 12AY7s.  I'm still scratching my head over how this happened.  All of the other 12AY7s I bought from EH sound almost the same.  Except for this one pair that seems to me to be 12AY7s masquerading as 12AX7s.
Cheers,
Dan