# Balanced Phase Inverters? - Guiary



## Wild Bill (May 3, 2006)

*****
"Anyway, I curious about Phase Inverters. Is it necessary to have a balanced phase inverter? What does this actually do?"
******

Guiary's asked me some tech questions in PM's and I thought I'd answer them here, in case anyone else is interested!

Might keep me from having to answer the same questions too many times!:smile:

Besides, while I don't mind at all helping with questions please understand that I get a LOT of them! My PM inbox tends to jam up. Also, I do have to spend time repairing and building amps so it's more efficient to do it in this forum. GC was kind enough to give it to us to help everyone, after all. 

Besides, there are other guys who post here who know as much or more than I do and who help out, sharing the load.

Anyhow, first we have to understand just what a phase inverter is and what it does.

The PI is the last tube of the preamp string and it feeds the signal to the grid inputs of one or more pairs of output tubes. What it does is to take the preamp signal and make it into two identical outputs, except that they are out of phase. 

Phasing refers to signal waves rising and falling. If they rise and fall in unison they are said to be in phase. If one rises while the other falls and vice versa then they are out of phase.

When the PI makes two out of phase outputs each feeding an output tube(s) then one tube will produce a positive peak of power while the other will be receiving a negative one. The tubes are normally biased so that when the negative pulse gets negative enough the tube will stop conducting and will "rest", or have a chance to cool down. When the PI outputs reverse the first tube will shut off and the other will conduct.

So each tube will only have to work half the time. In practice there's less distortion if we let them work a wee bit longer so that there's some overlap as they rise and fall through zero volts on the input waves. That distortion is called "crossover distortion" and the overlap drastically reduces it.

This is called running the output stage in "Class AB1", which you don't really need to totally understand just yet but if you keep it in the back of your mind it will be useful as you become more of a "wiz".:smile:

Letting a tube only have to work half the time means that you can run it hotter! You can get more than just twice the power out of a pair of output tubes by running them in Class AB1. That's why a Class A amp with only one 6L6 will give only about 10 or so watts of power while a Super Reverb with a pair of 6L6's will give 40-50 watts!

Mind you, the tone of each amp is quite different! Class A amps have less power for a given tube but they sure give a golden bluesy tone!

Some simple and more "HIFI" PI's have what's called unity gain, which means no gain at all. Most guitar amps that are based on Fender/Marshall designs use a type of PI called a "long-tailed phase inverter", which has BAGS of gain!

In hifi applications it's common to design a "balanced" PI. This means that each PI output, while out of phase to each other will be exactly balanced so that when the output waves are combined in the output transformer things will be perfectly even on each side of the combined wave. This helps give the lowest distortion and the most "hifi" sound.

Tube stores will sell you 12AX7's that have been matched and balanced, meaning that since this tube consists of two triode amplifiers contained in one glass tube when you use it as a PI each triode will more perfectly provide that same gain, "balanced" output.

Unless you're a purist jazz player, who the heck wants a hifi guitar sound? 

Most guitar amps also adjust parts values in the PI stage to provide reasonable balancing but being a bit off actually provides a better tone! 

There are a lot of hifi technical ideas that crop up from tube stores wanting to make more money from guitar players that aren't really appropriate. They only make sense in a hifi application. Myself, I would never bother with a balanced PI tube in a guitar amp but in a tube hifi amp it's not a bad idea. Even then, it's easy to get too anal and nitpicky. The human ear is physically incapable of detecting much less than 2-3% distortion. To quibble about .1% vs .2% is frankly ridiculous!

Hope this is useful to some folks!

:food-smiley-004:


----------



## Geek (Jun 5, 2007)

As primarily a HiFi designer, I agree with you Bill.

You don't need a perfect balance. A LTP with different anode resistors is just fine and sonically more pleasing as a sound generator.

For HiFi (or self-balanced PP output guitar amps [no PI, but PP out]) I use an adaption of a common mode NFB PI invented by Fred Nachbuar that gives the ultimate in self balancing performance. BUT, it sounds no different than the tradidtional LTP in the real world guitar amps, so the benefit of excess complexity is moot in a pre-power tube PI.

Cheers!


----------



## Canman (Oct 21, 2007)

*PI's ??*

Interesting Info thanks-

I was wondering?
- what effect an balanced PI has on clipping would it make it more or less symetrical or have no effect at all? 
-If it does indeed effect this, Would using a tube like a 12BH7 which has dissimilar triodes having different gains, in place of the usual. unbalance the the PI and cause more asymetrical clipping?

I was thinking that if this effect was possible it may be used to "tune" a guitar amp by changing the gain of each side of the PI thereby allowing a finetuning of the clipping by ear.


----------



## Geek (Jun 5, 2007)

Hi,

12BH7 is only dissimilar in capacitances (by 0.1pf). As far as audio is concerned, they're identical sections:
http://www.nj7p.org/Tube4.php?tube=12BH7A

Cheers!


----------



## Wild Bill (May 3, 2006)

Canman said:


> Interesting Info thanks-
> 
> I was wondering?
> - what effect an balanced PI has on clipping would it make it more or less symetrical or have no effect at all?
> ...


Listen to the Geek! The triode sections are NOT dissimilar! This is what I mean by how easy it is to take one point from a data sheet and use it as gospel to hype something completely unrelated or inapplicable. You likely got that idea about the 12BH7 from some site where a self-appointed "guru" was actually just trying to sell something. Maybe I'm too suspicious but after seeing so many sites touting the sonic benefits of teflon wire...:smile:

A balanced tube would not necessarily make for perfectly symetrical clipping by itself. Circuit variables could still make things a bit unbalanced. Also, there's no way you could fine tune clipping by ear. Human ears just aren't that good!

No, you need a 'scope, plain and simple. Period. End of story!

This makes things much easier. You can literally see the clipping on either side! Then you can do something much cheaper and easier than screwing around with more expensive tubes. You add a trimmer resistor in series with the plate resistor of one of the PI triode sections. You can then just give the trimmer a bit of a tweak while watching the 'scope and as Kelly Bundy used to say - VIOLA! Perfect balance! With any old tube at all!

You match the circuit, not the tube! Cheaper, easier and far more accurate. Dumble used this trick in a couple of his circuits.

I can perfectly understand the desire to learn and to do things with tube amps but I think the sources pushing tube brand differences as some kind of valid part of the knowledge are not really helping. Anybody with hair in their ears can swap tubes in their sockets and fool himself into thinking he can hear a difference.

It just becomes a cheap copout slowing you down from cracking some real textbooks and learning for real!

Thinking you can make significant differences with an amp by changing tubes is like thinking you're a mechanic who can affect the horsepower and performance of your car engine by changing the tires!

Balanced or otherwise!

:food-smiley-004:


----------



## Wild Bill (May 3, 2006)

Geek said:


> Hi,
> 
> 12BH7 is only dissimilar in capacitances (by 0.1pf). As far as audio is concerned, they're identical sections:
> http://www.nj7p.org/Tube4.php?tube=12BH7A
> ...


Good Lord! .1 pf?

That difference is less than mice nuts! It's gnat nuts!

:food-smiley-004:


----------



## Adicted to Tubes (Mar 5, 2006)

I read your quote and I wonder why it doesn't get bumped off?
Does that mean I'm free to rebut it?
Anyway,there are too many variables to say that the balanced PI tube is better in your Twin.It could be simply a better tube.Those 'dozens' of tubes you say you tried must have at least 3 or 4 that are balanced.how much time do you think the tube store takes to find a 'balanced' PI tube?
You think they sort through 1000 tubes and then charge an extra two bucks for a balanced one? Not likely.


----------



## Wild Bill (May 3, 2006)

Adicted to Tubes said:


> I read your quote and I wonder why it doesn't get bumped off?
> Does that mean I'm free to rebut it?
> Anyway,there are too many variables to say that the balanced PI tube is better in your Twin.It could be simply a better tube.Those 'dozens' of tubes you say you tried must have at least 3 or 4 that are balanced.how much time do you think the tube store takes to find a 'balanced' PI tube?
> You think they sort through 1000 tubes and then charge an extra two bucks for a balanced one? Not likely.


Hi A2T!

thetubestore.com doesn't just use a one at a time tester like we techs. They have what are called "gangbangers"  that test a bunch of tubes at once. This makes it much more practical. I've seen how for bias level matching they run 3 or 4 units at the same time,each holding maybe a dozen tubes. Checking PI tubes involves similar units.

:food-smiley-004:


----------



## nonreverb (Sep 19, 2006)

Paul said:


> I put a matched PI in my 135 W SF twin reverb. _Everything_ sounds better, especially the bottom end. I've since relegate the Twin to Rhode Stage Piano duty, and I'm sure that the matched/balanced PI is part of the "magic". I've tried dozens of AT7's in that spot and the matched one is the best, to my ears.
> 
> But, the 135 W SF Twin Reverb is noted for "Fender Cleans", and I'm not noted as a player of excessive distortion.
> 
> Is the matched or balanced PI tube for everybody? Nope, but it can be a means to an end, if that's the end to which you are heading.


Perhaps, this is one amp that the balanced driver will have an audible effect. The 135 watt twin you have is as close to Hifi as a guitar amp can get. It has an ultralinear output transformer which, in the Hifi audio world is a necessity as the output tubes operate with very low distortion and high power.


----------



## Canman (Oct 21, 2007)

Canman said:


> Interesting Info thanks-
> 
> I was wondering?
> - what effect an balanced PI has on clipping would it make it more or less symetrical or have no effect at all?
> ...





I goofed I meant a 12DW7, I have one kicking around here from an ameg I rebuilt last year.

I dont care, Im happy with the sound of my amps, I thought it interesting is all. I prefer Class A.


----------



## Wild Bill (May 3, 2006)

Canman said:


> I goofed I meant a 12DW7, I have one kicking around here from an ameg I rebuilt last year.


I wouldn't put that tube in a Fender/Marshall type PI. The 12DW7 is made up of one 12AX7 triode with a gain of 70 or more and a 12AU7 triode that has a gain of only 10!

This would seriously unbalance the PI. This won't result in clipping but rather the output tube on the weaker side will be seriously underdriven. You will get lots of distortion but not the clipping kind. Actually, I'm not sure what the hell it would sound like. Might be worth a try just for gits and shiggles!:smile:

The Ampeg used a different kind of PI that only used one triode, or one half of a driver tube. One section for boost and then one section to deliver the two out of phase outputs. This kind of PI has unity gain, or rather no gain at all. It has other advantages which makes it desirable in a V4B or SVT.

:food-smiley-004:


----------



## Canman (Oct 21, 2007)

I figured something like that, I dont even remember where that tube went in the ampeg. I didnt try it. Just some kind of crazy idea I had kicking around after I read the post.


----------

