using the Dynaco PAS-2 as a guitar preamp

   In this project, I had the chance to test out an idea I've had for a while, which I plan to use in my own original guitar amp design and elsewhere -- now that I know it works.  It's a more-informative pilot light for tube circuits.  The idea is to use the red and blue LEDs from an RGB LED triplet (must be common-cathode), red indicating the heater voltage, blue indicating the plate voltage.  At power-up, the light starts out red.  As the tube rectifier begins to conduct, the blue fades in and the final colour is a nice shade of purple, for "ready to play".  When the power is switched off, the red goes dark almost immediately, but the blue persists, fading out slowly: which gives a clear visual indication of when the high voltage capacitors are safely discharged. Of course, it could simply be done with two separate LEDs on the panel; using the RGB LED is just a cute gimmick. https://youtu.be/qPnR_FwKMtM I powered the red LED from the +11VDC heater li...

  Here's a representation of my current front panel design.  This is actually a PostScript file which I write and update by hand; if you want a copy (or perhaps, the PDF version), please contact me -- I don't know how to attach such files, here (and/or, one can't). I will make this PDF file into a nice-looking panel, using my usual panel technique; actually, it should be called "Phil and Ben's Panel Technique", because my friend and mentor Phil Williams (umn.edu) invented the technique, and I have since extended it slightly.  To wit: (1) Print the PDF file on regular white printer paper.  Since my final panel is about 16" wide, I divide the portions of the panel across multiple pages, in order to print on regular paper.  After printing, I carefully use scissors and/or a paper-cutter to trim the pieces down to just outside the black rounded borders. (2) Spraypaint the blank front panel, white.  Let first coat dry.  (This is the main limitation: must be ...

The Dynaco PAS2 / PAS3. As mentioned, this preamp will be housed in a 19" rack compatible box, without the actual rack-mounting "ears".  I.e., the width of the box will be under 17", probably 16" or 16.5". When I design tube circuits in rack format, I often intentionally don't make the box height an exact multiple of the standard 19" rack "unit height".  This is to discourage tight vertical spacing of tube gear in racks.  Of course, when carefully set up by a professional, the rack should have enough directed airflow that units get plenty of cooling even when tightly packed in.  But failing that, even when slapped into a closed-box rack in a haphazard fashion, my devices will obtain some measure of cooling for themselves by enforcing small airspaces between units.  If compactness is an issue, don't use 19" racks in the first place!  So anyway, the height of this preamp will probably be about "2.5 U", so to speak. To a ...

 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 o...