Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Rebuilding the Dynaco PAS

One project that I've had rattling around in my brain for a long, long, long time is to rebuild my Dynaco PAS-3 preamp.  When I say a long time, I mean like years!  Finally I got a round tuit and gathered up the parts.  I've actually been doing research for this for a while and had some ideas of what I wanted to do to improve it. So it's actually a rebuild, restore, refurbish, resurrect project!

If you're not familiar with the Dynaco company, the short version is this:  Dynaco was an audio components company based in Philadelphia.  It was started in the late 50s and the original company went out of business by the late 1970s.

Dynaco produced a range of stereo preamplifiers, amplifiers and FM tuners.  They are still quite well respected today by audio nuts.  One of the most unique things about Dynaco is that all of the components they built were available as kits or factory built.  It's believed that the vast majority of the pieces that were sold were kits.  In fact, some of the labels actually say "Dynakit" rather than just "Dynaco."

There is a lot of information on the internet about Dynaco and if you have any interest, I encourage you to search out more.  You'll also discover the fact that there are many modifications that have been cooked up over the years to improve the sonic performance of the gear.  Not that they sound bad at all stock.

I own  two Dyna preamps, two complete ST-70 (stereo amplifier, 4 EL-34 output tubes), and a couple of FM-3 stereo tuners.

I have a PAS-3X as the preamp in my main stereo system - I've had it for about 12 years.  Here it is on the bench.  When I first got it, I undid some crazy modification involving an extra tube that someone had done to it.  I also did the usual upgrades such as put in Orange Drop capacitors.

But my PAS, which dates to 1966 I believe, could use a good rebuild.  The old Mallory can filter cap is still in it, and the selector switch is starting to become unreliable - channels are dropping out, for one thing.

So I culled together all my research and decided I would replace the can cap with a new modern-style filter capacitor board, replace the selector switch with a modern one, upgrade the phono and line circuit boards, change some components on the phono board to make it correctly follow the RIAA equalization curve, and finally, experiment with some new coupling caps.

First order of business is to build up the new capacitor board.  I went with the SDS Labs board - I bought the kit from Triode Electronics.  They offer it as a bare board with no parts, or as a kit with parts - I opted for the latter.

The assembly is very straightforward.  If you take this on, be forewarned that the "instructions" are little more than a diagram.  It helps to have the original schematic in hand to ensure you're putting everything where it should go.  The only thing that made me put my ol' noodle into operation was the 1N4007 rectifier diodes.  They're for the conversion of the filament voltage to DC.  The original PAS used a selenium rectifier, and the diodes just replace that.

But there is space on the board to use a couple of diodes to replace the 12X4 rectifier tube, which I didn't want to do.  The diodes to replace the tube are labelled "D1" and "D2" on the board.  The diodes for the filament circuit are "D3" and "D4."   I originally figured that D1 and D2 were for the filaments, but I just checked the schematic and figgured it out.

Once you go through the usual installing-of-the-components on the board and soldering 'em up, choo wind up with a whole new filter supply.

It's really nice that the main filters and the filters for the filaments are all in one place.  Another nice touch is a fuse for the primary of the AC power line, something Dynaco omitted in the original preamp.

I mentioned new PC boards for the preamp and here they are.  I actually went back and forth deciding if I really needed new boards - after all, the originals could probably be reused.

But I saw these on the you-know-where auction site and went for it.  I'm very glad I did.

They're made by a bloke in New England who runs a company called Vintage Electron.  They specialize in a lot of Dyna stuff.  These are replacements for the stock PC-5 and PC-6 boards.  You can get them as bare boards or stuffed.  I went for stuffed this time, but I will be sprinkling a few of my own components on them.  They are beautifully made, and even have ceramic sockets for the tubes.  When we get to the removal of the old boards, you'll see the night-and-day difference between the old ones and these.


Post a Comment 0 comments:

Post a Comment