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I was told once that a phase inverter tube should "just work" and that it has no effect whatsoever on the tone of the amplifier. Is this true?

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Everything "should just work", but in the real world how many things do? We get to repair/replace things that have worn out all the time. –  Anonymous Feb 6 '11 at 23:50

3 Answers 3

It's much too long to quote, so I'll link to Myles Rose's paper about the Phase Inverter and quote this:

Before you go to a pricy output set of tubes and a possible need to rebias the amp think about a simple phase inverter change. There are no amp adjustments necessary when you change the phase inverter.

It makes sense that the phase inverter is very important. It's feeding the signal from the pre-amp into the power-tubes and has to make that signal change very cleanly, without adding any distortion. At least that's the idea for an amp running very cleanly.

With a lot of amps we rely on the pre-amp stage to provide the distortion, so the power-amp tubes really should only amplify. But, there's a really nice distortion you get when the power-tubes are getting pushed, and, according to that article it's the phase-inverter that is responsible for that sound. So, making sure that tube is in good shape is important otherwise the output-stage distortion will be weak or flabby sounding instead of fat, like it should sound.

All the tubes degenerate as they are used so it's a good idea to swap them out periodically. A good amp tech. will be your best asset when trying to figure out when that should be.

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The phase inverter is the final amplifier of the chain. If you overdive it, it will distort. The distortion depends on the tube(some tubes will distort sooner than others or differently).

A phase inverter is simply an amplifier configured in such a way to give the original amplified signal and another signal 180 degree's out of phase to drive the push pull amp.

There are two things that can make the amp sound different due to the PI. If it is overdriven and/or if it is asymmetric.

It depends a lot on the tube itself and on the type of phase inverter circuit used.

In any case the tube will have about the same effect as any other tube in the chain. Any tube will have more of an effect the harder it is driven. All tubes will behave approximately the same if they are kept in in their linear range. Even bad tubes can sound good if not driven hard enough(All physical devices behave linearly within some range).

There are more important things in a tube amp that effect the sound than the tubes(if the tubes are working). EQ, pre-amp bias type/level, Full load current, tone stack type, feedback amount, etc...

What makes tube amps sound different is generally not the tubes since all tubes pretty much behave very similarly even when distorting.

Whatever minute differences you'll get from changing good tubes don't compare to changing the EQ(EQ has the most drastic effect on an amp's distortion).

Of course we are assuming working tubes.

The reason why some tubes do work better typically have to do with the type of circuit the amp is built around rather than the tube itself. I know this sounds contradictory but the point is that some circuits happen to be out of spec and some tubes can handle it better(or worse depending on your perspective). What this means is that it's more of a problem with the circuit itself than something special about the tube and instead of changing the tubes you can fix the circuit. Of course it's easier for people to change tubes rather which is why you get so many people doing it and claiming some tubes are better than others.

Given that there are only like 4 vacuum tube manufacturers left in the world and 99% of the tubes you see are just rebranded versions goes to show that tubes don't make all that much of a difference.

The more distortion you are after though the more the tubes come into effect up to a point then all tubes will sound almost identical.

The idea is that the pre-amp is where you get your distortion by overdriving the tubes there. Unfortunately this can happen in the PI too trying to get more gain which causes them to distort. Because of the circuit difference it will distort differently which will have a different effect.

If you are comparing PI tubes make sure you are driving them as hard as the amp allows to get the real difference else they will all sound almost identical(i.e., the whole point of saying they are operating in the linear region).

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To follow up a little on what AD has said... This used to be considered good advice: "A known microphonic tube, with no other flaws, can often be used as for the phase inverter stage." After reading the answers to this question, I now question that rule of thumb as I do most things Aspen Pittman says... –  Anonymous Feb 13 '11 at 4:10
    
The problem with a PI tube is that it is suppose to do 2 things: Generate an EQUAL and inverted signal to the original and drive the power stage. If the signal is not equal you'll have unbalanced transformer currents that could burn up your OT since they are designed to assume no DC current(an equivalent class A transformer would be much much larger for the same power). You also get a more distorted signal when this happens. If the PI cannot drive the power amps enough you will have lower power as the output will sag under the load. –  Anonymous Feb 13 '11 at 19:46
    
I've played around a lot with all kinds of combinations and IMO it doesn't really matter. Some people will like it one way and some will like it the other. In most cases you can play around enough with the component values of the circuit to make any tube sound like any other tube if they do not have any major issues. –  Anonymous Feb 13 '11 at 19:49
    
The reason he says a microphonic tube will work is because any noise should be inverted and cancel out in the power amp. This isn't always true because a paraphase inverter that uses two tubes and they won't be perfectly balanced and microphonics in one tube will not necessarily be exactly reproduced on the output of the other. –  Anonymous Feb 13 '11 at 19:53
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The same goes with each aspect of a tube amp. Maybe some people like underbiased tube sound while some don't. I personally prefer to use unbypassed cathode biased tubes because it cleans up the sound much better(more of a hi-fi approach) but you lose gain that way. Some people might not prefer that for their guitar sound and that is fine. To each his own. Best thing to do is learn what you like and how to get it then you'll not have any issues with tone. Learn to mod your amps and you'll be able to dial in what you want. Who cares what other people say. They don't have your ears. –  Anonymous Feb 13 '11 at 20:01

Apologies for answering my own question, but after performing some empirical experiments the answer in short is: yes. A phase inverter has an effect on the tone of your amplifier. I performed some experiments with the following tubes:

  • NOS 1981 Tesla 12AX7
  • NOS 1968 RCA 12AX7
  • NOS 1980 Mullard 12AT7
  • Standard new Chinese JJ 12AX7

I matched all the following combinations for the PA and the PI in my Orange AD-30R and recorded a clip of me playing (almost) the same riff with my Les Paul R8. The settings and volume were identical on each clip, and they were recorded through a SM-57 into an Apogee Duet with Logic Pro on my Macbook.

After I have A/B'd these clips, I can tell that there are obvious differences between all of them. Take the two clips with the JJ's for the Preamp tube for example. There is a very noticable harshness to the clip using another JJ in the PI, while the Mullard is less so--fatter and more evenly spaced. There is, of course, also the fact that the Mullard is a 12AT7 and the JJ is a 12AX7. That could attribute to the fatter sound due to higher headroom and a much lower gain rating.

As you can tell in each of these clips, there is a noticeable difference between the JJ phase inverter and the Mullard phase inverter. I have ran additional tests with the RCA and Tesla tubes as phase inverters, and in each case the amplifier sounds different—so the difference in tube types isn’t completely the reason for the tone difference.

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