Assembling a Dynaco PAS4 klon

Questions suggestions: tubular.well <( -at- )> gmail.com

As i never got my hands on a Dynaco PAS4 i will attemt to recreate one. A circuitboard from Roy Mottram is the starting point.

Roy's manual
Dynaco/Panors manual

Change history : Initial: 201025
Improved power off behaviour: 210221

Pictures will follow Empty board Input selector mains transformer enclosure

Board fixes

Double fault, removal of copper and addition of missing connection

E88CC internal screen connects to ground

I have no -3V bias, ground this ( Use a LED in cathode instead)


The use of odd component values forces me to build composit resistors

One of the ECC83 sockets

Higher powered resistors mounted higher for better cooling


As i have no -3V on board to use as bias to V2 and V5 i use a white LED. It is mounted in a free grounded hole and a wirewrap wire connects to pin 3 of the V2 and V5


LED soldered in place.


Wirewrap wire inserted before mounting tube socket


A long forgotten technology : wirewrapping. It's perfect for making remavable connections or strapping. It also exists connectors that fits on wirewrap pins to make connections outside the board.


Tubes in place verifying filament function.

Now i have applied B+ from an external powersupply. No fire not even smoke !
Starting with riaa stage. Some bugs sorted out :
- 240k instead of 240 ohm in R12 and R112
- one white led was only 2.44 Volt, tested several variants but 2 RED LED in series seemed best, then gave 4.04 Volt
- R12 and E112 changed to 300 ohm to reduce current in E88CC Dist now 0.06% at 1Volt out ( app 10mV in @1khz ). This THD includes HUM from the workbench.


Work in progress, as the object is flat on a wooden table hum is present everywhere.


First fitting of modules in box


I could only find board-mounted pots, thus soldering was tricky

Case Closed ! eller Locket På som vi säger i sverige


Dull front view


Backside.


whats under the lid ?


An unedited picturer of my 3580A analyzer. This is 380mV into line and 1V out

***** News *******
As some hum remained in the filament feed i decided to increase the transformer output with 3Volt to 15 Volt. This is done by adding some turns outside existing transformer. See transformer modification The end result is that hum from shorted riaa input is 4.4mV (left) and 3.5mV right channel measured at the line output.

final (?) version with transformer mounted different and a silent fan installed.

Improvements regarding power off surge

I have been somewhat concerned about the turnoff surge. There is a relay that shorts that output at power on, the same relay will short the output as soon as the +15Volt used for filament drops. But at power off B+ will drop fast, in fact a "bump" is heard at power off.

But an simple idea will fix this. An increased B+ cap that keeps the unregulated B+ ( 325Volt) until the relay deenergized and shorted outputs. All tubes are fed with a regulated ( +270V) power and as long as the regulation works there is no drop to the tubes. Test with a experimental 470uF confirmed that this works, the amp is dead silent at power off !


Honor the board source at tubes4hifi.com

Feel trapped ?