Dynaco and the PAS-2

 Dynaco (aka Dynakit) was an audio manufacturer with a heyday in the late 1950s and throughout the 1960s.  They were best known for their tube (valve) hi-fi gear, implementing the brilliant designs of David Hafler.  Most famous among these was the ST-70 stereo power amplifier.  Somewhat less prominent, but also quite worthy of notice, were the PAS series of tube preamps ("PAS": "Pre Amplifier, Stereo"; there had also been a "PAM").  These used Hafler's clever negative feedback circuits to deliver low distortion and excellent audio quality.

I first encountered high-fidelity tube audio when a friend gifted me his old ST-70 and PAS-2.  I got these units working, and enjoyed the pleasing sound of tube audio playing back my records and cassette tapes.  But of course, being a budding guitarist, I just had to try running my electric guitar (Vox Spitfire) through the Dynaco gear.  The EL34-based ST-70 worked great as a power amp into guitar cabinets, as one might expect.  The line-level PAS-2 preamp didn't have enough gain for anything other than a thin and unrelenting clean tone, on its own.  But I found that I could use it as a front-end to other guitar amps, such as my solid-state Peavey, and it would add all kinds of warmth and character to the tone, either clean or subsequently distorted by the solid-state.  This seems at logical variance with the numerical specs of the PAS-2, which would appear to indicate that it has such low distortion that no detectible "character" should be imparted.  This is the mystery of tubes!

However, being a teenage metal-head, I frequently wanted more than just "character", I wanted acres of raging sustain and distortion.  I knew (or suspected) that it would sound better to get more distortion from the tubes, rather than from the Peavey opamps.  So, I ran my guitar into the "phono" input on one channel of the PAS-2.  Now, I was in a sweet-spot.  Turning up the volume on the PAS-2, there was a progressively crunchier overdrive sound, but with that clarity that tubes can have, allowing the notes to clearly speak despite the crunch.  To get a super-heavy oversaturated tone, I would patch one PAS-2 output into the other channel phono input.  No Peavey (or Marshall, for that matter) can produce a tone like that.

I didn't fully realize the significance at the time, but by going through the phono inputs, I was of course subjecting the signal to the RIAA curve.  This was not generally convenient, since it tended to lead to excessive bass response; however, using Hafler's clever tone controls, this could be largely compensated for by rolling off the bass and boosting the treble.  Interesting effects occurred, however, in the presence of heavy distortion.  E.g., the guitar tone might be mostly under the threshold of extreme clipping, and so the tone would seem to be fairly clean.  With the bass control rolled off, the excess bass response of the RIAA seems "invisible", when playing single-note lines.  Yet, if a big chord is struck, with lots of bass, the RIAA boost will cause this chord to be heavily-distorted; but after passing through the bass-cut, the tonal character of this "explosion" of bassy distortion will still remain relatively thin and trebly.  Hard to describe and perhaps to imagine, but instantly appeals to the ear as a tasty heavy guitar tone, when heard.

So discovering and learning to use tonal effects like this from the PAS-2 became a fascination for me.  I took the guts of my PAS-2 out of its lovely vintage metal case, and built it into a homemade wooden box in a kind of pedalboard format, with a volume pedal; I used this for quite a few years as my main guitar preamp, with several bands and on numerous recordings (with zero commercial success, I must stipulate).

Later on, I started to design my own tube circuits, and I moved on from the PAS-2 to a succession of other gadgets, on my constant wandering quest for new and better guitar tones.  But some of my old recordings with the PAS-2 had set a high bar, tonally: other circuits had good tones, sometimes better than the PAS-2 (which after all was never designed for guitar use), but seldom or never could anything actually replicate those PAS-2 sounds.

I recently dug out of storage, the remains of my old doctored PAS-2 guitar preamp.  As I contemplated restoring it to some kind of functionality, I ended up studying David Hafler's circuit designs, probably more than I ever did back in the day.  And I started to realize, there are plenty of reasons why this preamp sounds unique, and quite unlike most other guitar equipment.  High on the list are the negative feedback loops around each stage (most traditional guitar circuits don't use negative feedback in the preamp), and Hafler's clever tone controls.  These latter are similar to Baxandall controls, in that they produce a true boost and cut, with "flat" inherently in the center position.  (I'm not enough of an expert to tell if, topologically, they actually *are* Baxandall, or conversely if they are Hafler's own design, accomplishing a similar result.)  Traditional tone controls on most guitar amps are derived from the less-flexible Fender/Marshall passive "tone stack", which has an inherent mid scoop that can never be eliminated, regardless of control settings.

Anyway, getting to the motivation for this blog, I have decided to make a somewhat bigger project out of this PAS-2 restoration.  Instead of either trying to revert the pieces that I have back into an original PAS-2 (i.e., suitable for hi-fi use), or trying to re-create the preamp as I had it back in the 1980s (i.e., the pedalboard unit), I will build this unit into a new studio-compatible guitar preamp format, basically a 19" rackmount style box except without the rack "ears", intended to sit on a desk or on top of a guitar amp.  Thus, the project might have some slight chance of being interesting to someone else, who perhaps has a PAS-2 (or PAS-3, same circuit) and would like to try using it with guitar -- although, I hate to see people cannibalize old vintage gear; things from the 1960s weren't quite as old and venerated in 1985, as they now are!  Consider buying new PAS-2 compatible circuit boards in kit form, rather than altering a vintage unit.  In any case, you can always try out your PAS-2 with guitar without making any permanent changes, just by using suitable cables/adaptors to go from 1/4" phone plugs to RCA jacks.

So this blog will mostly cover the design of the front panel layout, switching arrangements, etc., needed to convert the stereo PAS-2 into a form that is more convenient to use as a guitar preamp.