Complete Restoration of a Dynaco PAS Pre Amp

This is a story of rags to riches; turning an abused Dynaco PAS 2 into a very modern PAS 3.

The Dynaco PAS 2, for those who are unaware, is a pre amp kit introduced by Dynaco in 1960 to compliment the Stereo 70 (ST70) that came the year before (1959) and other various other power amplifiers in the company's lineup. The Dynaco PAS has developed a cult following, and there are an immense amount of boards being produced for those interested in restoring such archaic technology :) CurcioAudio, Tubes4HiFi, AudioRegenesis, ClassicValve and others have created drop-in boards to restore the PAS, whether you want to maintain the same circuit or drop in something completely different.

I chose to use reproductions of the original PAS 3 boards and rather than recreate the rudimentary selenium rectifier with standard capacitors, I chose to make use of the high-tech ClassicValve PAS Regulated Power Supply.

ClassicValve Regulated Power Supply

The PAS is a design that was built to fit a budget, so a number of other things are recommended to be replaced. The 2 pole, 6 position switch is rather archaic compared to what is being produced today. A $10 switch would likely be an improvement. I opted to go with an Elma 2 pole, 6 position switch 04-1264 ($60 SRP) so that I wouldn't have to replace it down the line.

A Stock PAS Switch

The rear input and output board also leaves something to be desired. Many consider this to be one of the most drastic areas that Dynaco cut costs on the kit. Cheap Nickel plated RCAs on an old PCB board don't really inspire confidence.

Stock PAS IO Board

So... let's have a look at the splendid PAS 2 example that I was able to source:

Original PAS Back

Original PAS Back Detail

Original PAS Insides

Original PAS Outside and Bottom

The previous user left me with a lot more work that I would have had if I had a pristine PAS 2 / 3. There were a number of unusual mods, including plexiglass mounted RCAs, a 9 pin tube mounted in the middle, a non-descript transformer that was halfway mounted, some additional holes cut out of the back, and a powdercoated front panel that was rubbed with armor-all. No worries, one has to rise to the challenge.

The PAS was completely taken apart, a few parts were set aside, most were disposed of as they had been pillaged by mods. The chassis was sanded down and prepped for powder-coating.

Chassis Prep

The new boards were sourced and populated with my favorite parts, Kiwame and Takman resistors and Sonicraft caps. 

Old boards versus new boards

Curcio Audio has a number of nice tutorials on the PAS, including one to make the Phono board a bit more accurate. I performed those mods, along with bypassing the tone controls. I sourced a new input output board from Tubes4Hifi to connect to the Elma switch. I also picked up an Alps volume control (100K) and balance control, along with PCBs to make soldering a little easier.

Mostly done

The wiring was completed using Neotech PCOCC in teflon wiring, Kimber wiring and SPC in teflon. Russian teflon bypass caps were added to the main board (mounted to the front), and Russian PIO were used on the phono board. A power LED board was sourced from ClassicValve, and a new transformer was sourced from TriodeElectronics. After all was said and done, I sent off the PAS to Sal at Dynaco-ST70.com to look over all my wiring and fire it up. I wanted to have an expert give it the final eye and do the final wiring.

Here's the final build:

PAS guts from the right

PAS guts from the left

Inside Detail

Inside Detail

Bottom wiring

Outside with new gold front panel from ebay

Outside Photo 2

Rear, notice rear vent and Cardas output RCAs

Some final details were the Cardas RCAs added to use the holes made by the initial owner, a power supply vent to eliminate the ugly holes in the chassis, the bottom slots were covered using slightly larger than stock power receptacles from Radio Daze, and the front panel was replaced with a gold anodized version from Michael Nipomo on eBay (he typically only sells black and silver, but had this gold one as a one-off).

This little baby was an amazing amount of work, but it came out well and I'm really happy with it. Good luck to everyone out there restoring their PASs :)

Wondering where I got all my parts for this build? Check out a list of my favorite online vendors here.

The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind. 

A Custom Dynaco ST-70 Kit

I'd imagine that the venerable Dynaco ST-70 is the most well recognized tube amp in the world. It was introduced in 1959 and was sold until mid-1990. Over 300,000 of the ST-70s were produced, and more continue to be built as kits from companies like Tubes4HiFi and Triode Electronics. The Tubes4Hifi recreates the chassis exactly as it was (aside from heavier gauge stainless steel) with holes for screw-in posts, pre-amp power draw tube sockets up front, the stereo/mono switch, and other nostalgic things that aren't used today but will make your build look authentic. The Triode Electronics kit eliminates the superfluous holes in the chassis and makes use of a cap board inside rather than the tall quad cap, along with a different driver board.

Also popular are rebuild kits with newly engineered driver boards that one can use to improve an existing ST-70. There are new boards from Tubes4HiFi, Mapletree, DIYTube, WelborneLabs , Classic Valve Design (the high tech DynaMutt) and likely a dozen others out there, all with their own flavor and philosophy. After reading so much about the amp on various forums, I had the burning desire to put one together ;)

I personally chose the kit available from Bob Latino from Tubes4Hifi as the cornerstone of my build. The kit is well documented and it seems that there has been a lot of thought in the parts selection. The kit includes the VTA70 driver PCB which was introduced in 1987, appeared in Glass Audio magazine in 1989 and continues to be offered today. Of course, building a kit stock is not my style, so the chassis and transformer bells were sanded and powder coated. I snuck in a few little upgrades too, including:

• Teflon Tube Sockets and PCBs
• Kimber TCSS wiring
• Cardas RCAs / Binding Posts / Input wire
• Kiwame / Takman Resistors
• Mundorf Coupling Caps
• Nichicon 100uF Electrolytics
• Pilot light
• Teflon solder strip for pentode / triode switch wiring
• Heavy Duty On/Off Switch
• Custom brass input and name labels
• IEC Plug

Holes had to be drilled/cut for the new RCAs, larger switch, pilot light, solder strip and IEC. Since the chassis is 16 gauge stainless, it takes a bit of effort. The binding posts required a little bit of modification but fit in the existing slots. I used black lacquer coated phenolic boards and drilled them out to hold the posts in place.  The tube compliment is Sylvania JAN input tubes, Winged C output tubes and Sovtek rectifier.

Here's a few pics to enjoy:

ST-70 Running - Click to Enlarge

ST-70 Back - Click to Enlarge

ST-70 Front - Click to Enlarge

ST-70 Guts - Click to Enlarge

It was a fairly easy build and a recommendable kit. The sound is lush and engaging.

Looking for Mundorf Capacitors for your build (among other great DIY parts)? Check out AuDIYo.com - Quality High End Audio Components and Parts.

The Fine Print:

Please remember that building circuits and performing circuit modifications can be dangerous to you and/or your surroundings and should only be performed by a certified technician. The owner of this blog and all associated parties can not / will not be held responsible if you attempt a build or modification posted above and cause physical harm to yourself or your surroundings. Many electronics contain high voltages that can kill, and mods, if performed improperly, can be a fire hazard. Please keep this in mind.