Last Few Touches on the PAS Preamp

With the PAS input selector switch wired, and the new boards installed, it's time to put some tubes in it and fire it up.  Would be nice to hear some music coming out of it after all this time!

I'm fortunate to have a handful of Dynaco-labelled Telefunken 12AX7 (or ECC83 in European nomenclature) tubes.  These are the tubes that Dynaco originally shipped with the PASs.  They were manufactured by Telefunken in, as it says "West-Germany."  You can see on this one that the lettering comes off very easily - handle any tube like this with care so you don't wipe off the lettering, or you wipe off a little as possible.

If you have a small signal tube like this with no markings and wonder if it's a Telefunken, turn it over.  Telefunkens have a small 'diamond' shape molded into the glass between the pins.

I say all of this because the Telefunkens are reputed to have the 'best' sound of all.  I've never done a comparison, but I will say that the Teles go for big bucks, so use 'em or sell 'em!

Here we have our PAS 3 with all the new stuff installed and tubes (aka 'toobs') ready to go.

The 1 mfd caps actually did fit.  Here's to hoping they don't have so much capacitance that instead of mucho bass, we wind up with mucho motorboating.

Looks pretty neato with all the new boards and new wiring in it, I have to say.  You can see how I routed the input wiring and the output wiring (from near the front between the two PC boards) up off the chassis.  I also kept the wiring for the two channels apart where I could to reduce crosstalk.

Now ve get ze CD and put it into the ancient Sony CD player.

I bought this CD a couple months ago and still haven't heard all of it, so this is a good opportunity.  I understand that Debussy cat could write some good music.

Of course we have to plug the PAS into an amplifier so we can hear it.  I don't have an amp on the workbench (yet) so I grabbed my little Vox DA-5.

A digression here on the DA-5.  This thing is amazing.  I don't know how they did it!  It's a 5-watt guitar amp that has an array of effects (chorus, reverb, delay...the usual) and different gain settings.  It also has a power switch to let you run .5 watts, 1.5 watts, or 5 watts output.  And one of the best features is you can run it on batteries if you want!  It comes in different colors - I chose the "Classic" Vox styling.

Just the thing for practice or small (I used it busking) gigs.  Most highly recommended.

Now back to our regular blogging.

After hooking it all up...it works!   It actually sounds pretty good through a geetar amp too.

I took it up to my main system (Dynaco ST-70) and found I had a noticeable hum.  It took me a while, but it turned out that the ground that runs off the Blend Switch (remember that?) needed to be moved just a hair.  An inch in the wrong direction made a massive change.  With that done, it's very quiet...much quieter than my other, still stock PAS-3.

Now on to a couple last touches.

The PAS power transformer is pretty...um....marginally designed.  I'm sure Dynaco went as cheap as possible to save money, but the stock transformer runs notoriously close to its limits.  So, to reduce the load a bit, I decided to run an LED instead of the stock incandescent pilot light.

You can see the original bulb - it kinda sorta points in the direction of the pilot 'lens.'

I ran a couple of tests right off the 12 volt wiring going to the pilot light.  I wound up using a blue (5 volt I think) LED in series with a 1N4007 diode and a 1K dropping resistor.  The LED now is running at about 2.5 volts, and is still plenty bright.  It draws less than 10 milliamps, compared to the whopping 150 ma that the original bulb drew. 

I need to get a shot of the blue LED in action...stay tuned.

And...I have a parts bin full of Dyna stuff left over.