The Phase Inverter tube. How to choose?

[Diego]

I got a good vault of information last time I asked about new tubes for my Tweaker,
but I still don't understand the FIRST thing about a PI tube.

I read an article by Myles Rose about it, as well as tons of info on other forums, but the information around seems to be more for engineers than for ignorant guitar players like myself.

What does it do to the sound, and for what reason would I want a low or high gain tube in here??
Some guys like the 5751 tubes better, others the 12at7, and others stick to some good ol' ax7.
Some say lower gain tubes like the at7 are better here. Why?

What to do here? I really don't understand the first thing and there's too much COMPLEX information out there.
I'd just like to understand what to choose with tone being the first consideration.

 

[ItsaBass]

Re: The Phase Inverter tube. How to choose?

+1

 

[jeremy]

Re: The Phase Inverter tube. How to choose?

in general get what ever tube your amp is supposed to have in the pi slot. i dont see 5751's used as pi tubes very often. usually either 12ax7 or 12at7

 

[ratherdashing]

Re: The Phase Inverter tube. How to choose?

The phase inverter is more of a functional thing than a tone changer. It's probably the least important tube in the chain when it comes to replacements/upgrades. It's used to create a phase inverted signal for the power amp, and doesn't really change the tone in the way that the preamp tubes do.

 

[trevorus]

Re: The Phase Inverter tube. How to choose?

The phase inverter is more of a functional thing than a tone changer. It's probably the least important tube in the chain when it comes to replacements/upgrades. It's used to create a phase inverted signal for the power amp, and doesn't really change the tone in the way that the preamp tubes do.

Some would say the opposite, as it is one of the deciding factors on how the power amp will behave. I think it depends a lot on how the amp is designed overall that determines the amount of difference a certain tube will make. V1 in any amp pretty much is guaranteed to be the most noticeable change. As far as what tube I like, I generally like a little bit lower gain tube. 5751's are nice.

 

[FuseG4]

Re: The Phase Inverter tube. How to choose?

I dunno, I've seemed to find that the breakup character of the PI DOES matter, in certain situations where the PI breaks up. I tried it with the night train and a boost pedal (to get the signal hot as crap to see if I could effect the PI).

I had two 12ax7s that I first tried in v1. One was a GE, and it was bright as sh*t. The other was a JJ short plate and was smooth and middy.
Tried them in the phase inverter. The GE gave more sizzle to the sound, the JJ was growling more. Both times the v1 was a Japanese 12ax7, a matsush*ta. Same pedal, same guitars, same settings etc. Subtle but noticeable as the tubes were pretty much opposites.


Also in a non-master amp, if you have a weak phase inverter tube, you'll have less preamp distortion at a given spot on the dial. If you then put in a stronger, high gain PI, you'll get more preamp signal to the power tubes. In a non-master amps, PI is one of the things you can tweak slightly to change the ratio of preamp to power amp.

 

[ratherdashing]

Re: The Phase Inverter tube. How to choose?

I'm not saying the PI has no effect on tone, especially in a NMV amp. It has far less of an effect on tone than any of the preamp tubes though (technically the PI is part of the power amp circuit). Every tube imparts something to your tone, so of course swapping one for another will change things.

 

[glassman]

Re: The Phase Inverter tube. How to choose?

This is one of those nearly impossible questions to answer...and all of the previous posters are correct...ALL of them.

You can take three players, all with the same guitar, same effects and same amplifier. You can then swap each of their PI tubes out for a "higher gain" one and you can have three totally different results. One will notice no difference whatsoever, the second will feel his amp is more articulate and touch sensitive while the third feels his is more compressed and lacks punch.

It all depends on how you set up your rig and your personal playing style.

 

[crguti]

Re: The Phase Inverter tube. How to choose?

The role of the PI tube is very important in the amp design. Usually amps are designed class AB. Imaging a pure sine wave where there is a positive signal (peak) and negative signal (deep). Class AB amps have an amplifier or power tube for the positive and the negative part of the signal. One power tube is fed with a signal that is identical (in-phase) to the original signal, and the other power tube is fed with a mirror-image (phase-inverted) of the original signal.
That explain why you usually have 2 power tubes. If you go above in power (>50w) you will need to add another tube in each side, so you end up with 4 power tubes. In the hi-fi world, your goal is the non-distorted pristine sound. Those people talk about matched PI tube, so both sides of the PI tube are matched giving the same signal amplitude for both power tubes (yes, there are 2 preamps in a preamp tube). Guitar amplifiers are usually designed to have a non-matched side because that adds distortion to the sound... what we're looking for.
This is was a brief overview of what the PI does.
Changing the PI with different brand's tubes are tonally insignificant. What you want to experiment is with different gain tubes, like 5751, 12at7, 12ay7 and 12au7. Less gain and feeling but you might like it.

 

[misterwhizzy]

Re: The Phase Inverter tube. How to choose?

Less gain

This I agree with partially...

and feeling

And this I don't...

but you might like it.

And this I do.

I think it depends a whole, whole lot on the amp and how hard the prior stages drive the PI tube. I got a lot more clarity and punch by putting a 12AU7 in my PI. A high-gain 12AX7 really got the power amp sounding too smooth for my tastes, especially in the case of a smooth-sounding tube (i.e. Mullard RI). With a JJ or Amperex 12AU7--which both sound about the same in the PI, by the way--the tone was a lot crunchier, a bit tighter, and definitely more clear. I also got a bit more headroom.

I don't have a whole lot of experience swapping tubes in PI slots, but I'm pretty sure the setup of the prior gain stages makes a big difference in how that tube and the power section both respond. You can affect it with the way the PI amplifies the incoming signal.

 

[STRATDELUXER97]

Re: The Phase Inverter tube. How to choose?

One thing I do know....If the PI tube is weak or just sucks,the entire amp sucks also tonally!

 

[Bludave]

Re: The Phase Inverter tube. How to choose?

I usually put in what my amp tech tells me that the amp needs. I leave it entirely up to him. he's a pro and I trust his judgement completely. He has been repairing my amps for close to 20 years and has never failed me.. besides; like you I am an ignorant guitar player !

 

[Chickenwings]

Re: The Phase Inverter tube. How to choose?

in general get what ever tube your amp is supposed to have in the pi slot. i dont see 5751's used as pi tubes very often. usually either 12ax7 or 12at7

at7. reliability is important for the PI. at7s can take 10x the amount of voltage that an ax7 or 5751 can. ax7s can sound cool and kinda drive a little creating a nice warm spongy response, but they can also crap out pretty easily doing that job too.
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

 

[Edgecrusher]

Re: The Phase Inverter tube. How to choose?

at7. reliability is important for the PI. at7s can take 10x the amount of voltage that an ax7 or 5751 can. ax7s can sound cool and kinda drive a little creating a nice warm spongy response, but they can also crap out pretty easily doing that job too.
http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

Though keep in mind while i incredibly respect Myles there are alot of people who do disagree with him. A few years ago me and Doug Roccaforte had this exact conversation when I was asking about different tubes for my Hi Gain 100 and his opinions were defiantly not the same. There are thousands and thousands of amps out there with 12ax7's in the PI spot with no particular penchant for breaking down.

 

[Chickenwings]

Re: The Phase Inverter tube. How to choose?

you are prolly right. My marshall uses an ax7. I just found i cook them a lot in my fender twin, so i just use at7s now

 

[crusty philtrum]

Re: The Phase Inverter tube. How to choose?

One time i was tracing the signal through my Matchless DC30 clone. What i discovered was that the signal stayed clean all the way up to the input of the PI. The first point where i was able to detect any form of signal distortion was at the PI output once the channel volume controls reached a certain point.

Even with the channel volume controls set to maximum, the signal was clean all the way to the PI input. This is probably true of many or most old-style, non-mv amps.

BTW, matched PI tubes (and power tubes) may well be somewhat of a waste of time unless you have a perfectly balanced output transformer. And if you did have everything balanced, you probably wouldn't like the sound. Most famous guitar amp tones come from the imperfections and imbalances.

 

[crusty philtrum]

Re: The Phase Inverter tube. How to choose?

..... at7s can take 10x the amount of voltage that an ax7 or 5751 can.

Not sure that's correct .... the AT7 can deliver 10 times the current (10 mA) than an AX7 (1 mA). I think you may have your wires crossed, so to speak.

 

[Quencho092]

Re: The Phase Inverter tube. How to choose?

If AT7's deliver 10 times the current, that means that they deliver 1/10 of the voltage if power output is assumed equal?

AT7's are lower gain, so their power value is lower as well, right? The ratio of current to voltage in an AT7 must have a functional impact on the amp.

When I was a tube amp user, I liked a 'balanced triode' AT7 for phase inverter, no V1, and an RCA in V2 for DRRI. Didn't like the duller 5751, absolutely loved the 12AX7M mullard copy in V2 tonally but didn't like the breakup.

12AX7's made the amp sound flabby and mushy. Maybe I would like a 12AX7 in a higher power amp like a Plexi more to make it more breakup responsive, but in <40 watt amps it lowers the distortion ceiling too much.

 

[jeremy]

Re: The Phase Inverter tube. How to choose?

you are prolly right. My marshall uses an ax7. I just found i cook them a lot in my fender twin, so i just use at7s now

your fender was designed to have a 12at7 in the pi, not a 12ax7. marshalls are typically just the opposite

 

[Rick330man]

Re: The Phase Inverter tube. How to choose?

A decent P.I. tube makes a big difference to my ears. Amazingly, those cheap, crappy Sovtek 12AX7As and 12AX7Bs make pretty good ones. As Myles Rose noted, JJ ECC83S tubes do something in the P.I. slot that is not pleasing. I dont' think they do good service there. I think of the GE 5751 as the gold standard P.I. tube. In an amp calling for a 12AX7 P.I., the GE 5751 will bring out the best presence and detail in your amp. That's just my experience.

Lots of folks push the 12AT7 as an ideal P.I. tube. It depends. In some amps, it sounds great in that role. In amps that want a 12AX7 there, the 12AT7 just sounds like it is lacking punch. Let your ears be the judge.