Extended Foreplay 12AU7 regulator tube

[triode]

Hi,

Just wondering how critical the shunt regulator tube (middle one) to the sound in Extended Foreplay III?
Is it worth to use some exotic NOS tube there?

Also, how we know if this regulator tube need to be replace? I guess, the tubes in signal path are easier to spot if they're going weak (loss of gain, loss freq extension, etc); but what about the regulator tube?
If this regulator tube fails without noticed, would it damage the rest of the pre-amp?

Thanks 

[Paul Birkeland]

You can pull the regulator tube out while the preamp is running and nothing is going to happen more than maybe a little pop as the contacts release. 

The signal tubes will get fuzzy sounding when they wear out, but I bet it would be pretty tough to hear a regulator tube that's really worn out, but at some point the cathode voltages will drop below the threshold of operation of the TL431, and at that point you might hear a little noise that otherwise wouldn't be there.

[triode]

Thanks Paul

[Natural Sound]

So, if one suspects the regulator tube is failing a voltage check for regulated 150V would be prudent? I'm guessing the same would be true for the 0D3 in a non-extended version.

[Paul Birkeland]

You would see Kreg voltages on the middle PC board drop significantly when the 12AU7 ages.  I'm not sure I've ever seen a worn out 0D3.