Inside the mind of boutique ampbuilder Eric Pritchard

(Eric Pritchard | Posted 2012-01-05)

Inside the mind of boutique ampbuilder Eric Pritchard

The world of boutique guitar amplifiers probably was born in the early 1980s with the premier of Mesa Boogies heads, combos and cabinets.Thirty years later, guitarists more than ever are forsaking Marshall stacks for smaller amps tailored to give a big sound, made by small manufactures. One such manufacturer is Eric Pritchard, headquartered in Berkely Springs, West Virginia.

MusicGearReview will be giving you a hands-on look at Pritchard's Black Dagger (pictured) in the near future. To get you in the groove, Eric agreed to write a short history of his amp-building career, one which provides a unique glimpse inside the mind of a pioneer.

My college degrees are in Mathematics and Electrical Engineering, and when I was in school, transistors had just entered into the curriculum and tubes were taken out. This was the first stimulus for Pritchard Amps, although I did not know it then. Later, I worked for the US Navy mostly with circuitry that had to function in spite of being irradiated with nuclear radiation. This required extremely detailed analysis of circuitry, because neutron radiation reduced transistor gains and gamma radiation caused every diode and transistor to conduct. Further, the analysis had to be proven by real circuits being exposed to radiation in various test facilities. This experience also would be quite important later with amp design.

While working for the Navy, I started my machine shop and built custom camera equipment, including computer controlled cameras. It was at that time I met Paul Reed Smith through the brother of a client who needed custom photographic equipment. Paul had a problem with machinists in Annapolis, MD, and so I helped him with his problems when he had a third-floor walkup guitar repair shop at 33 West Street. To get the beginnings of guitar manufacturing shop, I did the following:

• I installed the electric service in a garage that he was renting;
• I designed and built the copying router, a ‘Duplicarver’ on steroids, for him to carve guitar bodies and necks from hand carved masters;
• I designed and built guitar body lamination glue presses; and
• I designed and built many router and drilling fixtures.
• I designed and built a fret slotting adapter for a milling machine that was equipped with a digital table measuring system to precisely position the frets.

When Paul finally decided that he would build guitars professionally instead of playing them professionally, I helped further:

• I designed and built neck profiling and body profiling jigs for a shaper;
• I designed and built a fingerboard profiling jig for a router;
• I designed and built a fingerboard fret slotting machine that cut all the fret slots in seconds;
• I designed and built a neck carving machine to carve the back side of a neck complete with blends into the headstock and heel; and
• I designed and built numerous other fixtures and manufacturing aids.

When computer controlled milling machines got up to being able to beat the manual copying router, I helped further still as I:

• Consulted on the revamping of the guitar making process to make it compatible with the new machines;
• Designed the fixtures to adapt the machines to guitar making; and
• Built some of the fixtures once I got my computer controlled milling machine.

White wine, red wine, glass and flesh
During this time vacuum tubes became scarce in the Western countries, so in 1988 we started researching what became the PRS Harmonic Generator guitar amp, which still has a cult following. I took the very first version to the Virginia Avenue Shop when Carlos Santana was there. He tried the amp and tried to explain its shortcomings, saying that "it sounded like white wine and that it should sound like red wine." Since I am not a wine drinker, I had no clue. He tried to clear up the matter by stating that it was "like glass, and it should be like flesh" and pinched my arm to drive the point home.

Paul and I then took a third-generation amp to see Al Di Meola, whereas he said it was "dead and it should be alive." No clue there, either.

I designed the PRS Harmonic Generator with an approach common to engineering, doing it in sections in lieu of attempting to emulate the tubes themselves. The Harmonic Generators used resistor-diode arrays to create non-linear characteristics. There were ones to produce even harmonics of preamps and ones to produce the odd harmonic of output stages. The direction did not work very well. Moreover, the fate of this amp was sealed by the first President Bush when he began bombing Iraq during the winter NAMM show in 1991. Dealers feared that this fourth-rate military power would ruin our economy, so they dropped instrument/gear lines and did not pick up new ones. Consequently, Paul dropped the project at a substantial loss.

Deja Vu
I kept the project going as Deja Vu Audio, to continue the idea of making amps that sounded like the classics. The first problem was to analyze what went wrong with the PRS Harmonic Generators. Primarily, what they did was not follow the basic structure of the classic amps. The harmonic generation and the modification of frequency response filters had to correspond to the classic amps; thus, I designed various tube emulators.

After numerous tries I came up with a tube emulator that could replicate the waveforms, albeit proportionally, of a vaunted flat-plate 12AX7. To prove this circuit out, I ran it in parallel with that tube and showed oscilloscope waveforms that laid one on top of the other. To prove it out harmonically (harmonics are not easy to measure on oscilloscopes), I built a generic two-stage microphone preamp so that I could compare its harmonics with the data in Russell Hamm’s "Tubes Versus Transistors - Is There An Audible Difference?" After adjusting the bias on my tube emulators, I got virtually the same harmonic versus overdrive that Russell Hamm did. The upper harmonics were just a bit more subdued and that is a good thing.

From there I built some amps using these emulators only to find that the tube sound still was not there. With all of the tricks in the bag gone, I decided to check engineering for a close connection to physics. It wasn’t and still isn’t.

Engineering, quite unlike Mathematics, makes substantial assumptions to get things done. One such assumption presumed that harmonics were not produced by human hearing. Consequently, amplifiers should not produce harmonics either. That is the basis of total harmonic distortion (THD) and inter-modulation (IM) tests. But the engineered amplifiers are cold, stiff, and thin because engineers do not value harmonics. Musicians, on the other hand, prefer warm, fat, full-bodied, and resilient sounds. Hmmm, well Carlos was right! The body of red wine and the resilience of flesh were needed, and that required that I examine the design of output stages, and it turned out that there is a lot going on in vacuum tube output stages:
1. They interact with the power supply to produce sag and help produce compression.
2. The pentode screen grid voltage drops produce compression.
3. The push-pull structure minimizes cancels even harmonics and produces a dominant third harmonic.
4. The pentode gain characteristics, particularly when there is no feedback, produces expansive harmonics that reduce the blanketing effect on the tone when the tubes clip.
5. My output stage also produces expansive, open harmonics, which tend to offset the blanketing clipping harmonics.
6. The output stage produces ‘clean’ distortion, which is great for players who can play since they can hear their notes.

With the advent of the National Semiconductor output stage chip, I redesigned the output stage with it because it was not only cheaper but had over-voltage, over-current (short circuit), and over-temperature protections for greater reliability.

The design of the Pritchard Amps’ output stage then required a lot of extra circuitry to give this high-fi output stage the warmth, fat, body, and resilience of tubes – and then required still more circuitry to control its wattage while maintaining these characteristics. Hence the Watts Knob found on my amps.

When someone wanted to build a cheap solid state amp and use my emulators, we rapidly found that these emulators were just too expensive. And so, I started on an inexpensive version, and that began a parallel development of high-end and low-end amps. The high end amps were designed quite closely to existing tube amps and the low-end ones took some liberties. Well, the low end ones sounded better. Hmmmm? When my effort at getting funding failed, I decided to go forth with a compromise version that took the best of the high-line and low-cost amps.

Getting it right
Since I do not play, I had to ask for evaluations from many players; but since the language of sound is not standardized, the words can have many meanings. Consequently, players’ impressions often are translated to other senses, such as red versus white wine. One of the more baffling descriptions was the lack of dimension, i.e., ‘not 3D,’ or ‘too fast.’ And usually I never got a player to check out amps twice. I was dealing with an electronic enigma behind a linguistic mystery. Phil Zuckerman solved that problem by sticking with the program for a decade and more. Phil and I had to work to get past the language barrier to actually be able to discuss the character of the sound and what we might change.

And so, in working with Phil, I would translate a tube effect to solid state with an exaggeration so that it would be heard – then I cut it back until it sounded good. The result is an amp that has been optimized without regard to the limitations of tubes, which is possible because solid state has to have extra circuitry to create the tube sound – circuitry that is separately tweak-able from gain, etc. That really created the change from Deja Vu Audio to Pritchard Amps, largely at the suggestion of Paul Bechtold, a writer for Vintage Guitar.

(Stay tuned for an upcoming review on the Pritchard Black Dagger Amp, which will address features found in Pritchard Amps, as well as its quality compared to tube amps!).

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