Wasted half of tube

[LooseChange]

I looked a bunch of the Wreck Schematics. It seems that many schematics utilize 1 1/2 tubes (12AX7) in the preamp with no reference to the other wasted half. Is it really a wasted half of tube?

[RossH]

If you mean not being utilized, then yes. In the Francesca pictures it is the second tube that only uses half.

Ross

[MarkB]

If it really bothers you, the 6AV6 is a seven pin tube with one triode and two diodes. The triodes in the 12AX7 are copies of the older 6AV6. NOS are $3.
Most DIYers don't bother, because you might want to mod the amp and use the 9 pin socket later.

[LooseChange]

It's not a bother it's just the temptation to use it for something... There must be mods that use it effectively.

[Elcabong]

Check the Trainflex file. It shows a couple of way to use this half tube. In the Trainwreck inspired amp I am currently building, I am implementing the single/parallel inputs.

[Robert]

The TR Ruby uses it for a nice effects loop. Check out the schematic.

[SECONDandBOWERY]

Just an idea...

Has anyone tried using it as a cathode follower right after the first triode stage so the tone stack wouldn't lose so much of the signal?

Rock'n'Roll,
--Andy

[UR12]

Just an idea...

Has anyone tried using it as a cathode follower right after the first triode stage so the tone stack wouldn't lose so much of the signal?

Rock'n'Roll,
--Andy

The cathode follower wouldn't give you any more gain. It's gain is near unity. It does give you two things, one is that it is non inverting and the other is that it has a low impedance output. It is interesting that comparing a Marshall to the Express, the Marshall 's guitar signal input comes out in phase with the input to the PI and the Express's guitar input signal is out of phase with the input to the PI. Marshall uses a 33k slope resistor in the Tone stack and the Express is more like a Fender using a 100k slope resister.

I do agree that there must be something cool we can do with the extra triode but I don't think I would use it for more gain as this amp has all the gain I need. If you want more gain then you can play with the 150k resistor value on pin 2 to ground of V2. Maybe a trim pot could be added here to dial the gain in to what ever you want. Kind of a Dumble Wreck (just add goop)

[dobbhill]

My opinion only: by adding another stage in series, you would actually be losing signal. By signal, I don't mean voltage, I mean you would be putting something else in the signal path to corrupt the flow of tone. One of the important things about this circuit is it's simplicity and lack of "stuff" in the way of the pure guitar tone.
I hope this makes sense, and is only an opinion.
D