For Info - The Gizmo PP Pentode Philosophy

[Cressy Snr]

Right, Now Nick has rescued Dr Gizmo's writings from oblivion. (Thanks Nick) I thought it would be good to lift the stuff that inspired me to develop the Barbarik amplifier. It has not been put in one place until now, although much of it is scattered through the "Latest Push Pull Build" thread. It's a long read unfortunately, but is useful as a way to put the pentode design into context.

Looking back, with the benefit of twenty five years hindsight, the design decisions I made, made sense, and are now the foundation of my new push to take push/pull much higher.

FULL CIRCLE PARADOX: You will discover that my experiments with push/pull pentode amplifier was the basis for developing the Futterman OTL-1, and now, once again, this pioneering work is being reapplied to pentodes.

Our goal is not to make a pentode push/pull amplifier sound like an OTL, or directly heated triode push/pull, because that would be like trying to make a banana taste like an apple. There is no way, all things being equal (and they never are) that a pentode amplifier can have the refined sweet aural matrix of a directly heated triode amplifier. And there is no way that a directly heated triode amplifier can have the power and slam of pentode amplifier. Adults have to make choices.

Our challenge is to take what pentode push/pull amplifiers do better than any other type of amplifier and extend their ecstatic aural matrix, and to do that I must narrow my focus, and get very deep into a new tube amplifier aesthetic…one that I pioneered in the 1970s…well to be honest…I didn't really pioneer it…I just took an old idea and used it in a unique way.

In my previous writings I have asserted that pentode amplifiers were, for most musicmaniacs, the key to experiencing, for the first time, the deep musical mystery of rock and roll. My goal is to wean rock and rollers away from their turgid transistor based aural mediocrity. To do that I must create pentode amplifiers that have a new level of slam and sweetness, and affordability…that heretofore has not existed in this modality of amplifiers before SO YOU CAN EXPERIENCE ROCK AND ROLL MUSIC IN A HIGHER ECSTATIC MODE.

STOP: Do you get my drift Mr. Hi-IQ sailor?. Want more refinement? Move over to directly heated triode amps, but you will sacrifice some of that pelvis pumping Britney Spear juice that rock and roll and MTV is all about, at a price you can afford. With directly heated triode amps you need speakers that have higher efficiency too.

FORWARD: What do I mean about taking slam higher? What I am really talking about is the weakness in most "normal" American pentode push/pull amplifier's power supply design. This is a taboo subject in America for some weird reason, but FACT: an amplifier's power supply is in series with music output, and determines ultimate aural matrix quality. It might be helpful to think of your amplifier's power supply as its engine. What good is a beautiful body (the audio circuitry) when hidden under the hood is lawnmower engine?

If you want to experience the outer dimension of rock and roll you need a Dodge Viper V-10 motor with a supercharger and nitrous bottle, and Dr. Gizmo's goal is to develop this level of performance…and I apologize for taking the pomp out of my circumstances, but the way to achieve that is over twenty five years old…and I stumbled on it first..and then exploited this notion in the OTL-1.

Now let's get to the nitty gritty of rock and roll's juice: The foundation of rock and roll is what is happening below 400 Hz and that is where all tube amplifiers are having a problem because their power supply is being drained of energy when heavy bass energy is produced, which causes the bass line of the music to sound very soft and flabby. The reason so many prefer the bass response of solid state amplifiers is because they can deliver the slam at the lower frequencies that make rockers twitch.

Twenty five years ago when sex, drugs and rock and roll reigned supreme I was compelled to discover if I could make my pentode tube amplifier's slam compete with solid state amps, because I couldn't tolerate the compressed, glassy, two-dimensional sound of sand based amps.

A REVOLUTIONARY HYBRID CONCEPT..ALMOST

Dr. Gizmo, when you were probably in a crib, rightfully thought the idea of combing a pentode amplifier with a solid state "amplifier/regulator" made lots of sense, and thought this…. before the AIDS plague, when he was trapped in a vortex of sex, drugs and rock and roll. In effect I was creating an ultra simple "solid state amplifier/power supply" to power a tube audio modulator. Such a solid regulated power supply creates an extremely low output impedance that would dramatically improve bass transient response, eliminate power supply noise, and decouple the power supply from any AC contamination..and not cost much.

PAUSE FOR APPLAUDS: Do you see the direct relationship between these experiments with the OTL-1, which is the same concept taken to ultimate implementation.

In the mid-1970s a new generation of high voltage transistors appeared that could be used in high voltage regulators that would be superb for the high voltage power supply of a tube amplifier. Why not use a transistor regulated power supply for a tube amp?

PAUSE: By the 1970s there were lots of "laboratory/industrial" regulated power supplies that used both tubes and transistors, so this was not a new idea. Many industrial tube amplifiers used active power supply regulation, so this wasn't a new idea. So Dr. Gizmo thought…if the logic applies to industrial amplifiers, why doesn't it apply to consumer amplifiers? No rocket science here.

To further confound the paradox, by the mid-1970s the notion of using transistors as regulators of small signal triodes was common, and was the key to the new breed of high resolution tube preamps. Why didn't this same logic apply to a big output tube…I thought…does a tube know the difference?

If, for example my pentode output stage was running on 500 volts DC and I wanted to have, a constant, actively regulated 500 volts, which means that this voltage would not vary no matter how heavy the load on the output stage, I would have to start with 600 volts. The regulator would then have a "reserve" of 100 voltage, behind the 500 volt constant output, to call upon to meet the demands of amplifier. If the power transformer is properly designed, the regulator would in effect create an infinitely stiff output, that would never vary over the entire frequency range. This meant I needed a higher voltage power transformer, which explains why you see two chassis in the pictures of my "hot rod" pentode amplifiers 1970s amplifiers. The outboard chassis holds the new power transformers and the regulator circuits.

In an unregulated power supply it is "normal" to expect the power supply voltage to fluctuate as much as 10-30 percent under heavy load. With an solid state regulator we are talking about less than 1%.

The term "supercharger' works well here, because exactly the same amplifier topology with these solid state regulated power supplies sounded twice as powerful, especially in the below 400 Hz region…because the power supply voltage never sagged. Can you imagine driving a car up a steep hill and because your motor is so powerful you don't have to give it more gas to maintain your speed?

You name the pentode push/pull amplifier brand from Audio Research, Mc Intosh, et all, I attached solid state regulated power supplies to all of them and got the type of slammy bass response that I thought only sand based amps could produce…with the all of the other benefits of tubes.

And now I have to go further….because it is my job to shred what needs to be shredded.

A PAUSE FOR DEEP PENTODE POWER
SUPPLY MEDITATION

Have you noticed that all modern pentode push/pull amplifier used solid state rectifiers rather than tube rectifiers? Thirty years ago the "tweaker" rule was to replace all of the tube rectifiers with solid state rectifiers because they improved the "slam" of tube amplifiers. The reasons that solid state rectifiers became the rule was, as pentode amplifiers become more powerful it was impossible to use a tube rectifier because they could NOT handle the current needed by gaggles of tubes. Even the smaller amplifier used them because solid state rectifiers cost pennies and tube rectifiers cost dollars.
The benefit of solid state rectifiers is that they aren't current limited (because you can always use a higher current rectifier…like 50 amps…while "normal" tube rectifiers are designed for 1/2 amp or much less ) which means you can use huge power supply capacitors, which draw tremendous current on, turn on, because they look like a dead short to the rectifier. These large capacitors would literally strip and destroy a tube rectifier. You notice that when we use a tube rectifier we use a 100 microfarad capacitor or less, while with solid state rectifiers we can use thousands of microfarads if we want.

So if we need higher current power supplies, that are going to charge huge capacitors, we are going to need solid state rectifiers, and the price we pay is the loss of tonal refinement that is only possible by using tube rectifiers. As you know from my writing: solid state rectifiers produce a nasty spectrum of high frequency hash, and the only way to eliminate that aural contamination is with either a passive "choke filtration" or active power supply regulation..YET..few tube amplifier manufacturers follow this ancient wisdom of making solid state rectification edible.

Why do I mention this? If our design goal is to take the pentode push/pull aural matrix higher, which means taking their power supply design higher, how do we take the slam higher, and not loose the sweetness of tube rectifiers?

ANSWER ONE: Transistor regulators are a brick wall filter to this type of solid state rectifier hash.

ANSWER TWO: Wouldn't it make sense to create two separate, completely independent, power supplies for our pentode push/pull amplifiers?: a solid state regulated power supply for the output stage, so we can achieve maximum slam, and a tube rectified and choke filtered power supply for the front end circuit, so we can maximize the sweet? Why hasn't anyone done this before? Why did God in Her Infinite Wisdom give DR. GIZMO brain damage? Is this some kind of major technical breakthrough. NOT. It is only good thermionic implementation that naturally flows from my passion for creating an optimal tube aural matrix.

ELEMENTS OF PPP BEAUDACITY

My recent article in Listener Magazine on reinventing push/pull delves into this deep. It is not easy thinking in terms of the optimal aural matrix because tradition is strong, so let me give you some concrete examples:

Voltage Amplifiers: The orthodoxy with PPP is to use a 12AX7 because it produces very high gain, and is the least expensive tube. This tube is in the most critical position in your amplifier, yet it is not chosen for its tonal quality, or subtley, or ability to produce complex layers of sound. Have you noticed that the single-ended cult takes their voltage amplifier tubes much more seriously, and for good reason? Why don't designers of pentode amplifiers apply single-ended wisdom to their modality?

Power Supply: This is the ten millionth time I am repeating this..sound quality is proportionate to power supply design quality, and American orthodoxy has not yet caught up with the 1930s..in the 21st century. The soft diffuse sound, the lack of bass transient response, and confused midrange that are common features of PPP is directly related to the absence of stiff power supplies. Rock and Roll demands stupendous power supply stiffness from 400 Hz down because this is the foundation of the music.

The Aroma of Phase Splitters: Every push/pull amplifier has a phase splitter, and there are hundreds of variations. Why is there no discussion of the different aroma of these always less than perfect topologies? Why do members of the single-ended cult care so much about tiny details and the PPP cult doesn't?

The Pentode Power Paradox: Does the same paradox apply to pentodes that applies to directly heated triodes? Are the lower powered tubes more refined? What is the optimal 'screen connection"?

SWEET SLAM

DR. GIZMO'S NEW AURAL MATRIX
A DIALECTICAL THERMIONIC DELICATESSAN

Give Me $45 worth of Parts and I Will
Woodify Your Aural Matrix

The new aural goal of PPP should be SWEET SLAM. Again there is no way for a PPP to have the midrange/tonal refinement of a directly heated triode push/pull amplifier, or, the huge multi-layered sound stage of an OTL..if your speakers are above 95 db efficient, BUT PPP can, if implemented properly, create a deep hot pool of rock and roll magma that will melt you soul.

SWEET SLAM is the optimal aural matrix for PPP because it produces the kind of bass transients that are the one thing that transistor amps do right, and we are able to get the sweetness that tubes produce in the midrange.

What is the point in trying to create a rock and roll aural matrix that is more refined than the aural matrix that is inherent in 99% of CDs? Our mission is to produce the dynamic quality of rock and roll and we want the vocals to speak to us with truth..meaning eliminating the compressed shredded glass sound of transistors. Rock and roll music wants to be played loud…like in… LOUD, I am having sex with my girlfriend on the couch loud.

Does this seem like a new concept? It is for American orthodoxy, but I developed this aesthetic in the late 1970s, and abandoned it for the development of the Futterman OTL, but then implemented it in the MOSCODE hybrid amplifiers…and this is my latest implementation for transformer coupled PPP amplifiers.

Rock and Roll deserve SWEET SLAM.