Phase Inverters, Even-Odd Harmonics?

Randy Beavers · Post by **Randy Beavers** » 28 Feb 2005 6:53 am

Tube amplifiers have a phase inverter tube, or something similar. It's my understanding that this has to do with reproducing even and odd harmonics. I've noticed on tube power amps part of the signal, although slight, seems to be missing. I've never noticed this on solid state power amps. If someone can explain this in layman terms to me, I'd appreciate it.

John Daugherty · Post by **John Daugherty** » 28 Feb 2005 7:29 am

Randy, a phase inverter is used in amps that have a "push-pull" output circuit (2 power tubes operated push-pull}. The signal from the preamp is split into 2 signals. The postive portion of the audio waveform is fed to one tube, the negative portion to the other tube. There are also push-pull transistor amps (as well as complimentary-symetry).
If the biasing on the output tubes is correct, it will reproduce the waveform applied to the output circuit. If not... it will cause distortion.
I can't tell you why you hear different sounds from a tube amp as compared to a transistor amp. I am sure some one can help us on that.
I quit working with tubes many years ago and prefer transistors because of their dependability (and lack of noisy, dirty, tube sockets).

Michael McGee · Post by **Michael McGee** » 28 Feb 2005 8:24 am

Push-pull, you say?

Well, so that's why tubes sound so good????!!!!! :>)

(let the flames begin...)

mm

John Daugherty · Post by **John Daugherty** » 28 Feb 2005 9:54 am

Mike, I knew that (push-pull) would hit someone between the eyes.
I only had one pushpull guitar. Couldn't tune that rascal.
I should have clarified on my previous reply that the phase inverter circuit has nothing to do with amplifying (harmonics or otherwise).

Rick Johnson · Post by **Rick Johnson** » 28 Feb 2005 10:00 am

John
You gave an excellent description
of the phase inverter tube, I can
only add that the 180 degree signals
sent from the preamp section to the
power tubes can go to a pair of tubes
also. Like a Twin's 4-6l6GC's, each
split signal will go to each pair as if it
were a single tube smaller wattage amp,
then as you said it goes to the output
tranny to be greatly amplified.
As to the dirty sounds from different
amps, I would guess that the values of the resistors and tone caps in the preamp section will drift over time and the amps will sound different. Thats why I would
only replace the electrolytic power caps
They loose power handling capabilities
and can short out.

------------------
Rick Johnson

Ray Minich · Post by **Ray Minich** » 28 Feb 2005 12:20 pm

What? And no mention of crossover distortion? I'm shocked

Keith Hilton · Post by **Keith Hilton** » 28 Feb 2005 12:34 pm

Actually a push pull amplifier is only one class of amplifier and really explains the operation. Class AB and so on. I built one in college Linear Electronics class a year or so ago. John is correct about the positive and negative. Think of it in terms of a sign wave, with half the sign wave above a line and half below the line. Above is the positive half of the sign wave and below, the negative half of the sign wave. The problem with push pull amplifiers is the point where the sign wave crosses the line from positive to negative. If there is a mis-match the positive part of the sign wave will not join up--exactly--with the negative part of the sign wave when it crosses the line. When this happens you get distortion, and the amount of distortion relates directly to the mis-match where the sign wave crosses the line from positive to negative. When I built my amp, I ordered matching or what they call complementary transistor units. By the way, I built my push pull amp from transistors, but the construction and concept is the same for a tube push pull amp. All the guys posting here are my friends, so I hope I didn't stick my nose in where it did not belong. I won't get into the discussion of which sounds best, tubes or transistors. I suppose that is a personal preference anyway. Your friend Keith Hilton

Ray Minich · Post by **Ray Minich** » 28 Feb 2005 1:42 pm

Keith, I have a Peavey Express 65 amp that is all tubes except they put several Vactrols in it. I still haven't figured out why

Chick Donner · Post by **Chick Donner** » 28 Feb 2005 2:12 pm

A push pull amp will cancel even order harmonics. Not all of them, because of the "imperfectness" of the amplifier.

Bill Hatcher · Post by **Bill Hatcher** » 28 Feb 2005 2:20 pm

There are amps that use a phase "splitter" circuitry and amps that use a phase "inverter" to drive the outputs. The inverters do not cancel out any frequencies as such, they balance the even and odd order harmonics.

The splitters cancel some even order harmonics.

I would have to question what you seem to think is missing in a tube amp signal as opposed to a solid state signal, since in every solid state amp I have ever had to "tolerate" the sound of, I immediately missed the upper harmonic structure that was simply clipped off resulting in a very sterile sounding flat signal. No bright sparkle on the top at all. I have never experienced this in a tube amp.

I hate SS amps that use transistors for the pre and the power section. For electric bass, the combination of a tube pre with a SS power section is pretty good. For electric guitar, all tube is the classic ground zero sound.

I know this is not a thread on SS Vs.Tube amps, but when is the last time you saw an ad that touts a tube amp built to sound like a transistor amp???

Randy Beavers · Post by **Randy Beavers** » 28 Feb 2005 2:48 pm

Some good explinations, most all of which I don't understand but...

At present I'm using an Alembic F-2B tube preamp, then to the power amp section of a Peavey. It really doesn't matter if it's a 500, 400, 112, 1000, etc. The tone I'm hearing is the best I've ever had with this combination. I have had, and still do have some great tube amps. However I feel that what I'm hearing now with the tube pre and the SS power is a fuller signal. Richer in harmonics than an all tube combo. That's why I was wondering about the even and odd harmonics, if the tube power amp reproduced them as well as solid state. Maybe it's all in the Alembic?

Bill Hatcher · Post by **Bill Hatcher** » 28 Feb 2005 3:43 pm

Randy.
What you need to hear is the Alembic F2B into a 100wt. tube power amp. You would then not only hear all the harmonics you don't hear with a SS amp, but you would then "feel" the natural compression of a tube amp. Have you tried this yet? If not, see if you can borrow a nice tube power amp at least 100watts and let me know what you hear.

Randy Beavers · Post by **Randy Beavers** » 1 Mar 2005 6:41 am

Bill, one thing's for sure, you were "right on the money" with your recomendation for the Alembic preamp. I'll have to give this a try also.