Preamp tubes vs phase inverter tubes

[E-rex]

Does the 2nd 12ax7 (in a lite IIb) contribute much to the tone? What I'm getting at is, should I put an expensive tube in that position or just in V1

Or in a JA-TMB, does the 3rd 12ax7 matter all that mcuh for tone?

Just trying to figure out best use of my better 12ax7's

Thanks much!

[NitroWoman]

I usually save my best tubes for the earlier preamp stages. I don't hear much difference in different PI tubes.

[andre_orwell]

There's an interesting piece called mosfet follies on geofex that talks about different stages in tube amps and what can be replaced with solid state without impacting on the tone. Unfortunately the "mosfet phase inverter" has never appears. Got me interested though. Has anyone here ever tried such a thing?

[rjgtr]

Any change has an effect on tone. Usually the first stage has the most effect with the PI having the least. Having a balanced or close to balanced phase inverter can also make a noticable change. Some like a balanced PI and others don't.

I've heard a few amps using MOSFET for certain stages of an amp and while they sound good, you can still hear a difference between the circuit without them and with them. I think the only safe place for SS in the signal path is the effects loop, especially if it switches out when not in use. The devices you are running it into are likely SS so it is a moot point to use tubes for it.

[zaphod_phil]

You certainly could build a MOSFET PI. However, FETs, MOSFETs and silicon transistors don't compress when overdriven, so the end result wouldn't be beneficial.

[keith]

Some like a balanced PI and others don't

How does that affect tone/sound? Is it cleaner vs more distortion?

Keith

[rjgtr]

Some like a balanced PI and others don't

How does that affect tone/sound? Is it cleaner vs more distortion?

Keith

A balanced PI will be generally cleaner sounding and more balanced. It is especially important for Hi-fi applications. A balanced tube will also distort more evenly, which some like and others don't. I'm of the school that feels as long as the two sides aren't wildly different in gain, it's fine. I don't think a balanced tube is necessary in a guitar amp and a slight imbalance results in a richer tone. It's all a try it and see if you like it thing, like voicing an amp with different brands of preamp tubes.

[keith]

What determines the PI balance. The 470k (or whatever)resistors from grid to tail? the coupling cap values? the PI plate resistor values?

[zaphod_phil]

A balanced PI will be generally cleaner sounding and more balanced. It is especially important for Hi-fi applications. A balanced tube will also distort more evenly, which some like and others don't. I'm of the school that feels as long as the two sides aren't wildly different in gain, it's fine. I don't think a balanced tube is necessary in a guitar amp and a slight imbalance results in a richer tone.

I would go further and say that a well balanced PI is both highly desirable in hi-fi and undesirable in guitar amps. Why? A well balanced PI and power amp will produce an output largely devoid of even order harmonics (apart from any coming from the preamp). Even harmonics make the tone sound warmer, sweeter, richer, even when to our ears the amp appears to be running completely clean.

What determines the PI balance. The 470k (or whatever)resistors from grid to tail? the coupling cap values? the PI plate resistor values?

That depends on the type of PI. With a long-tailed pair PI, as found in the 18W and most Marshall and Vox amps, as well as some Fenders, the two load resistors and the PI tail resistor determine the balance. Here's an article you may find useful - http://www.bonavolta.ch/hobby/en/audio/split.htm Just note, that what they describe as the Schmidt and the Long-Tail Pair PIs, are in fact two vartiations of the same kind of PI.

[keith]

Thankyou for the link.
On a long-tailed PI, what would be an example of balanced vs unbalanced.(components) Or, looking at a schem, what would cause someone to say, thats balanced (or unbalanced).

[zaphod_phil]

Higher tail resistor values give more balance, but also reduce the available output swing. Also slightly dissimilar load resistor values (eg 100k & 82k) help correct imbalance within the PI. So the 18W PI has a relatively high tail resistor (47k or 56k), but equal sized (100k) load resistors. A Marshall Plexi or Fender Bassman will have dissimilar load resistors (82k & 100k), but with a total of about 15k worth of tail resistor (10k + 5k pot).

[rjgtr]

So the kicker with most guitar amps, given the 10-20% range in component values, is that even when you buy a balanced 12ax7 (or AT) you might not be balanced after all!