Emabarrassing question re Extended FPIII

[wera689]

Due to a long story (which I'll not go into), the shunt regulator tube of my Extended FPIII has been broken; and at present, I can't locate the build manual.  As I built this several years ago, I simply can't recall which tube to use in that central position.  As embarrassingly simple as this question is, can someone please remind me what tube I need to get this gem running again? 

Thanks guys.  Don't be too hard on me, ok?

[Grainger49]

It is a third 12AU7.  I looked on the product page for the Extended upgrade since I don't have one.

[wera689]

Thanks, Grainger.  I saw that too, but I wasn't sure I was reading it correctly.  I had it in my head that it was supposed to be a 12AT7 or 12AX7 for some reason.  Any chance one of those would work, too?

[JC]

I would have to recommend against that.  The gain and current-handling are very different.

[wera689]

Thanks JC.  I kinda thought so, but had to ask.  I can't wait to get all my references back at my fingertips!

[Jim R.]

Jim C,

Just curious, but I thought I read somewhere that the regulator tube co9uld be  any number of several twin triodes.  Did I imagine this?

Again, just curious, not questioning your experience.

-- Jim

[Grainger49]

Jim,

Doc has used several different dual triodes as regulators for the FP III Extended but I don't remember if he made changes on the regulator board to accommodate them.

[JC]

You know, guys, I suppose it may be entirely possible that the regulator circuit could be designed in such a way as to not be particular about the tube it uses, and I'll be the first to admit that I don't know the specific circuit well.

But, there are some pretty large differences between a 12AU7 and the AX7 and AT7, as I'm sure you know, and it is difficult for me to conceive of either of the other two being a drop-in replacement for the AU7 in a regulator circuit.  I would have to think that the operating points would need to be too different.

Having said that, I would be pleased to learn that I'm incorrect, because the explanation would undoubtedly be very instructive.

[Jim R.]

Jim,

Yes, I believe you arecorrect.  So, let me re-phrase: you can use other dual triodes for VR tubes with the appropriate changes to the supporting components onthe shunt reg board, etc.

And yes, the 12au7 is the designd tube in stock form, which is really wht was being asked to begin with. :-)

-- Jim