Toggle navigation 300guitars.com

Shop

Blog

Articles

Tips

Reviews

Spotlight

E
Book

Gear for Sale

Phase Inverter – What it Does and How it Works

Of all the circuits in a tube amplifier, the Phase Inverter, also known is
the Phase Splitter, is the most difficult to understand by even some experienced
techs. Its function is relatively simple: take a signal input, and create two
outputs, one that is identical (e.g. in-phase) to the original, and another that
is a mirror-image (phase-inverted or flipped phase). Each signal feeds a power
tube (or bank of power tubes) that is connected to each side of the Output
Transformer’s primary winding in the typical Push-Pull configuration.
Single-ended power amp, like those contained in the Fender Champ, which sport
only one power tube, do not require this additional step, and need only a driver
before the power tube to boost the preamp signal to a level usable by the lone
power tube.

So, why is the Push-Pull method of power amplification used when it is
inherently more complex and costly? Several reasons. First of all, it enables us
to use a more efficient amplifier class called Class AB. Whereas single-ended
audio amplifiers ALWAYS run in the Class A mode, which runs the tube constantly
at maximum power (thereby shortening it’s lifespan), Class AB runs each tube
only slightly above the lowest operational point (called “idle”), and each one
is called on as needed to deliver power when necessary. No signal in, no power
use, therefore the tubes remain relatively cool until pushed.

The second reason for using Push-Pull is that unwanted sonic artifacts, such
as hum and odd-harmonic distortion (this is the nasty, raspy kind), are
naturally cancelled in the Output Transformer. Even-order harmonic distortion
(the kind that sounds cool), remains relatively untouched.

Back to the Phase Inverter. There are two designs that dominate guitar amps.
One is the “Cathodyne”, also known as the “Split-Load” (Fig. 1), and the other
is the “Long-Tailed Pair”, derived from a circuit called the Schmitt Inverter
(Fig. 2).

The Split-Load is the simplest arrangement. It splits the signal by virtue of
the fact that the signal appearing at the cathode of the tube is in-phase (this
circuit by itself is called a “Cathode Follower”), while the signal at the plate
is out-of-phase (this is a typical Common-Cathode amplifier). The way this works
is that signal input to the tube at the grid causes a variation in current flow
from the cathode to the plate, producing a voltage swing on the plate,
out-of-phase with the input signal. It is also important to know that the
variation in current ALSO appears at the cathode, as a signal which is IN-PHASE
with the input signal. As long as the plate and cathode resistors are the same
value, the amplitude of the two outputs will be similar, except for the
flipped-phase. This is a very important concept to grasp for later. The main
drawback of this circuit is that it offers no signal voltage gain. What you put
in is what you get out, except one side is flipped-phase. Therefore, an
additional tube stage called a “Driver” is used ahead of it. The Driver provides
the gain, the Cathodyne inverter provides the necessary phase-flip, and they
both live as a happy family.

But wait….what if you could do all that with ONE circuit? Well, you can.
Enter the “Long-Tailed Pair”, presumably called as such due to the dual resistor
“tail” used for critical bias of the dual-triode tube.

I’d like to preface this by listing the three ways we can use a triode tube:
Common-Cathode (where the cathode is grounded and signal is fed to the grid, the
most common type of triode amplifier), Common-Grid (where the grid is grounded
and signal is fed to the cathode), and Common-Anode (where the plate is
“grounded”, not to 0V, but directly to the power supply, which is a “virtual
ground”, also called a “Cathode Follower” and, rarely, a “Buffer”). We already
know that the Cathode Follower does NOT provide signal voltage gain. It DOES
deliver current gain, which is good for circuits like tone stacks that tend to
hog current. However, I digress: the OTHER two amplifer arrangements,
Common-Cathode and Common-Grid DO provide voltage gain, though the Common-Grid
amplifier isn’t quite as effective as the Common-Cathode amplifier in doing
this. The gain for a given signal input is LESS than that of the Common-Cathode
circuit. Save this thought for later.

What the “Long-Tailed Pair” does is use two triodes, one in a Common-Cathode
arrangement, the other as a Common-Grid amp. The signal enters the first stage
(Common-Cathode) in the usual way, through the grid. This produces a voltage
swing on the plate AND the cathode of this stage, as described earlier. The
second-stage, which has a common (grounded) grid, has it’s output at the plate,
like the first stage, so…..just HOW do we feed signal into it? Well, the cathode
is still up for grabs. This is where it gets mighty ingenious. If you tie the
cathode of the first stage to the cathode of the second stage, the variation in
current of the first stage will be superimposed on the cathodes of the second
stage. Here is the circuit again, redrawn and simplified in Fig 3:

Basically, what is happening here is that the plate circuit, which has
voltage gain, sends it’s signal to the power tubes. The cathode circuit, which
is essentially a cathode follower, we already know has NO voltage gain, so the
second stage Common-Grid amp provides the gain. This is the key element missing
in the Cathodyne Phase Inverter (Fig. 1). After voltage gain is applied, the
signal then travels to the power tubes as well.

I stated earlier that the Common-Cathode amp has higher voltage gain than
that of a Common-Grid amp. In an attempt to balance this out, the plate resistor
of the Common-Cathode circuit is slightly reduced, reducing the gain of that
stage. In many tube amps, it will be reduced to 82K vs. 100K for the second
stage. All else remaining equal, lowering the plate resistor value also lowers
the gain of the stage. The secondary effect of this is that the actual signal is
not balanced on both sides, making the output somewhat asymmetrical (i.e. the
positive signal swing is not equal to the negative signal swing). There are a
few amp companies that ignore this and use 100K resistors for both, introducing
a DIFFERENT kind of asymmetry into the signal because of a DC shift. And the
Cathodyne circuit? It isn’t perfectly symmetrical either, because the source
impedances (i.e. the cathode and anode/plate) are different. One (the plate) is
high, delivering lots of voltage, but not much current, while the other
(cathode) is low, delivering lots of current, but no voltage gain. The bottom
line is:

THERE IS NO SUCH THING AS A PERFECTLY BALANCED PHASE
INVERTER!!!

You could match the sections of a dual-triode, match the caps and resistors
in the circuit, and all will still be imperfect. But, my friends, guitar amps
are NOT hi-fi amps, nor do we want them to be. They are not about audio
perfection, they are about tone, and the reality is that the inherent imbalance
produced in the Phase Inverter results in more harmonic complexity, resulting in
a more sonically pleasing end result. Remove the imperfections, and you
sterilize the amp. You know, sometimes even people seem a lot more real with a
few wrinkles on their face!

Now, go in peace. Tonight, you amp junkies can sleep, knowing that, together,
we have solved one of the great mysteries of life!!!

John R. Frondelli is the Technical Services Director at DBM Pro Audio In New York. He has been a
technician for 30 years and has repaired, restored and custom built all
types of musical equipment. Part of his client list includes Bob
Dylan, Lenny Kravitz, U2 and The Who. �

Let Me Repair Your Guitars & Amps

The 300Guitars Shop

Billy Penn’s 300guitars Shop located at 1L Executive Drive in Toms River, NJ
will bring your guitars and amplifiers back to life! Specializing in guitar
& amplifier repairs and restorations in New Jersey.

Guitar
Repairs

Guitar
Restoration

Guitar Amplifier
Repairs

Custom Tube Amp
Building

Guitar Pickup
Rewinding

Guitar Amplifier
Modifications

Click
here to find out more

How to find me

View
Larger Map

Address:

300Guitars Shop
1L Executive Drive

Toms River, NJ
08755
Phone: 848-218-0362

Share this post: on Twitter on Facebook on Google+









Search for: Search

Top Articles

Tube
Amplifier Crackling Noises - What to Look For | 300guitars.com

Hot-Rodding
The New Fender Champion 600! | 300guitars.com

Guitar
Tone Caps | 300guitars.com

Phase
Inverter - What it Does and How it Works | 300guitars.com

300guitars Shop |
300guitars.com

Multi-Effects
Units vs Individual Pedals | 300guitars.com

Pedalboard
Planner Website by Pedaltrain - Build a Virtual Pedalboard! |
300guitars.com

Marshall
Amplifier Date Codes | 300guitars.com

Gear for
Sale - Amplifiers, Guitars, Pedals

Amplifier
Loudness: Watts and Decibels |
300guitars.com

Recent Comments

James
Latimer on Ron
Smith Archtop Guitars

James
Latimer on Ron
Smith Archtop Guitars

Robert
Heyl on Filter
Cap Failure

Jesse
Vukas on New
Jensen P10R-F

Scott
Shannon on Checking
Speaker Impedance With a Multimeter

Archives

April 2015

January 2015

October 2014

September 2014

August 2014

July 2014

June 2014

May 2014

April 2014

March 2014

February 2014

January 2014

March 2013

February 2013

January 2013

December 2012

November 2012

October 2012

July 2012

June 2012

May 2012

April 2012

March 2012

February 2012

January 2012

December 2011

November 2011

October 2011

September 2011

August 2011

July 2011

June 2011

May 2011

April 2011

March 2011

February 2011

December 2010

November 2010

October 2010

September 2010

August 2010

July 2010

June 2010

May 2010

April 2010

March 2010

February 2010

January 2010

December 2009

November 2009

October 2009

September 2009

August 2009

July 2009

June 2009

May 2009

April 2009

March 2009

February 2009

January 2009

December 2008

November 2008

October 2008

September 2008

August 2008

July 2008

June 2008

May 2008

April 2008

March 2008

February
2008

© 2015 300guitars.com

Return
to Top ▲