What is a phase inverter?

[Riffmeister]

And what does it do? I have always heard about the preamp tube that is in the phase inverter spot, but as usual have no clue what that means

[Hellbat]

In it's purest form a phase inverter takes a sine wave and 'flips' it. So if you have a 2 volt peak to peak signal that is going...

0, .25, .5, .75, 1, .75, .5, .25, 0, -.25, -.5, -.75, -1, -.75, -.5, -.25, 0

The phase inverter will output...

0, -.25, -.5, -.75, -1, -.75, -.5, -.25, 0, .25, .5, .75, 1, .75, .5, .25, 0

[Riffmeister]

Good lord! That is a mess to me

Let me rephrase the ?, what difference does it make if I put a lower gain tube in the phase inverter slot, or why would I want to do it?

[DonP]

In a tube amp running Class AB, you need to feed one side of the power amp an in phase signal, and the other side of the power amp an out of phase signal. The phase inverter tube has the job of splittling and amplifying the signal(s) before feeding them into a power amp.

[DonP]

Good lord! That is a mess to me

Let me rephrase the ?, what difference does it make if I put a lower gain tube in the phase inverter slot, or why would I want to do it?

Try it and find out. (edit: read my following post first)

Every amp is different, and each reacts differently.

The only way to tell if it's good or bad is to use your ears.

[DonP]

And what does it do? I have always heard about the preamp tube that is in the phase inverter spot, but as usual have no clue what that means

Most modern amps use the same peramp tube in all sections of the preamp to keep it simple.

Back in the old days, they's use different tube in different slots to get better performance. Fender for example uses a 12AT7 in the reverb driver and phase inverter in the old days. This tube has less gain than a 12AX7, but can handle more power, possible driving a small speaker for about 1 watt.


I'm not sure what you are trying to do, but if you don't know what you're doing it's probably not a good idea to be doing it.

Stick with using the correct tubes.

[Twitch]

The important thing to consider with the phase inverter tube is to have a matched tube in that slot, meaning both triodes in the 12ax7 are "matched" like power tubes would be. However, finding a 12ax7 thats matched would be luck of the draw as its a crap shoot at manufacture how close the 2 triodes will be as theyre not manufactured with that in mind. They can be tested to pick out the ones that are closely matched, Eurotubes supposedly can do this, however, honestly you probly wouldnt notice any great loss or gain in installing a matched phase inverter.

[Newc]

Good lord! That is a mess to me

Let me rephrase the ?, what difference does it make if I put a lower gain tube in the phase inverter slot, or why would I want to do it?

Look at it like this:



Here, a stereo signal is almost perfectly matched on both sides.


But, by inverting the phase of one side,



You have frequency cancellation and focusing.

However, if by default the amp puts those two signals opposing one another, then the phase inverter will align both sides, which prevents the frequency cancellation and keeps the signal from being as tightly focused, eliminating any unwanted "scooped-mids" tone that might occur naturally.

A lower wattage inverter will still invert properly, but at a reduced amount, so the opposing signal is not as similar, providing a different tonal characteristic.


At least, that's how I understand it in terms of audio (wav files). Should be the same for electrical circuits, though.

[DonP]

The important thing to consider with the phase inverter tube is to have a matched tube in that slot, meaning both triodes in the 12ax7 are "matched" like power tubes would be.

I sometimes questions that. If the phase inverter is matched, and the circuit is blueprinted to have and exact opposite signal, and the power amp tubes are perfectly matched, you stand to have a very cold sounding tube amp, because the class AB style power amp tends to cancel even order harmonics the better it's matched.

I'd rather have some sloppyness in there to have a better sounding amp. Look at the classic Fender and Marshall amps from the 60s and 70s. Tolerances were 20% on most component back then.

So I say, go by your ears as to what sounds best.

[Spivonious]

The only reason I can think of to get a lower gain tube for the phase inverter would be to minimize any distortion before the gain stages.

And am I correct in believing that the tube itself is not doing the inversion, but rather amplifying it after it is inverted to make up any lost signal?

[rstites]

I'd rather have some sloppyness in there to have a better sounding amp. Look at the classic Fender and Marshall amps from the 60s and 70s. Tolerances were 20% on most component back then.

I generally agree with your points, but don't buy that some sloppiness produces a better sounding amp. Everyone used 20% tolerance parts for years, and amplifier stages were designed for them not to interfere with the function of the stages. Higher tolerance parts are nice because you can stay closer to a classroom type design, but in the end you really aren't getting better overall performance out of the system. It's just easier getting there. (Note: this is going way tangent form the OP's question I know! )

The only reason I can think of to get a lower gain tube for the phase inverter would be to minimize any distortion before the gain stages.

And am I correct in believing that the tube itself is not doing the inversion, but rather amplifying it after it is inverted to make up any lost signal?

The entire stage, including the tube, are doing the inversion. Without the tube (or other active device), there is no inversion. The most accurate way of saying it is that the tube is amplifying the signal. It happens to also be producing a mirror image, inverted signal. We are able to take advantage of that and use the stage to drive a class B (technically A/B for audio) amplifier and make the most efficient use of the power section. It's really more of a useful byproduct than anything else.

[Riffmeister]

Guys, these answers are way more technical than what I thought I was asking about. I just wan'ted to know what happens when changing different kinds of tubes in the phaseinverter spot

[Newc]

Well, for starters, your wallet gets one tube lighter, and your amp gets a tube that hasn't been cooked as much as the others.

[Spivonious]

Oooh, another question.

Is the phase inverter tube only used in the inversion stage, or do both signals pass through it, since 12A_7 is a dual triode.


In other words...

Normal------>12AX71------>normal----->gain stage
Normal------>12AX72------>inverted--->gain stage

[DonP]

Oooh, another question.

Is the phase inverter tube only used in the inversion stage, or do both signals pass through it, since 12A_7 is a dual triode.


In other words...

Normal------>12AX71------>normal----->gain stage
Normal------>12AX72------>inverted--->gain stage

There are many phase splitter/inverter circuits. Some can get by with a single 1/2 tube, but have no gain.

The cicuit used by Fender that Marshall copied is the long-tailed pair. Needless to say, the "pair" means the entire tube (both halves).