Dynaco Pas 3X

The attempt to build a ”NEW” Dynaco Pas 3X in 2025!

Pas 3X scraps from a forgotten time!

Found a very old cardboard box from the seventies that contained the shown parts of a Dynaco Pas 3 and a thin aluminum box,  with what looks like an oversized power supply....the power supply consisted of an extra transformer with 2 x 1.3A 12V windings and some old Siemens "huge" 330uF/300V capacitors...as well as 7812 /7912 regulators.

...wondered why so much effort was made to make a huge external power supply when the Dynaco draws a few mA on +B and 1.2A + 300mA : 1.5A for glow 4 x ECC83 + 1 x 12X4.?

When the cardboard box appeared, the mood was high, but it quickly dropped when it turned out, that the 4 bass and treble pots had never been soldered...nor the 4 switches for tape, loudness, filter and power...the two original printed circuit boards had been soldered, but not used according to Dynaco's instructions. The old original Dynaco capacitor with 10 - 20 - 20 - 30uF/450 was measured according to the specifications, but looked bad, Dynaco's transformer has never been installed in the original box.

An electronics person who knew about tubes bought this kit (probably sold by Audioscan) to experiment, but never got around to a finished preamplifier....

Hope there is an assembly guide with photos for this Dynaco Pas 3 (maybe X?) ...Many cable connections are missing? ...the manual found:

www.thetubestore.com/lib/thetubestore/schematics/Dynaco/Dynaco-PAS-3X-Schematic-Owners-Manual.pdf 

...and this is a 3X as can be seen by looking at the four tone controls....

I got many EZ 80 and 81's i stock but they all need 6.3V for heaters .....and also a lot of different rectifier tubes that can deliver more than 12X4 , but I don't think Dynaco's transformer can withstand to deliver the higher current! I stay with the 12X4....

As a DIY’er of tube amplifiers, it would be nice to have a Dynaco Pas 3X in standard trim as a kind of reference amplifier. So here we go.....

Pas 3X PSU

As a replacement for 4 capacitors in the Dynaco Pas 3X that were assembled in one and the same aluminum capsule, "www.tubes4hifi.com" has developed a new power supply PCB. .. 

But on the PCB the new 230VAC fuse holder has been placed at the "wrong" end, which faces the PC-6 phono PCB. 

This is why I made my own power supply PCB for the Pas 3X !

Always keep the AC wires (heaters) and DC high voltage towards the transformer and away from the sensitive phono PC-6 PCB. 

It is the undersigned who is responsible for the PSU drawing shown, which means that the 230VAC fuse is moved out as chassis fuse according to Danish Demko standard! 

...the original version of the Dynaco Pas 3X's kit is not available with a serial 230VAC fuse and certainly not as an external chassis fuse! ..but "f" away from the PCB!.... 

Dynaco's original transformer has a 12VAC output for 12X4 as a rectifier? On the other hand, Dynaco chose to make a voltage doubler in the power supply for 2 x 12AX7 in series. This means that 24VDC is supplied to the 12AX7's heater wires. On my DIY power supply PCB, 2 pieces of 33uf/450V capacitors are shown.... the correct voltage is 450V, since the first main capacitor +B startup reaches over 405VDC.... the 12X4 tube can withstand according to specs. 40uf as max. load! 

Here is a jpg of the Dynaco Pas 3X where the 12X4 has been replaced with 1N4007 rectifiers, and Dynaco's four-pole capacitor with 10uf – 20uf – 20uf – 30uf has been changed to four discrete capacitors of 2 x 33uF/450V and 2 x 100uF/450V as well as 2 x 3300uF/50V for the DC filament power supply for the four 12AX7/7025 

Wrong place for the fuse, and the fuse is not used in this position!

Inside a Pas 3X bought in Sweden a few caps changed in the PC-6, but PC-5 linestage is original 1969

In an electrical device with an incoming 230VAC mains lead there should be a fuse...... also applies to Dynaco Pas 3X. 

As previously mentioned, there was no fuse holder included in this Pas 3X kit... nor was a serial fuse drawn on the diagram immediately after the AC cable? 

...upon disassembly, the slide switch for Pas 3X showed signs of wear after several years of switching on and off .......

the idea: relieve the mains switch with a relay.... that said slide switch should only "carry" the few milliamperes that the relay coil draws.... ..suggestions with relieving the mains switch and serial 230VAC fuse....or only with 230VAC fuse? 

....it had to be  1.0 A "fuse" for the four AC outlets.... ..together with Weidmueller DRI 314730LT 230V VAC relay that draws less than 10mA...this means that the Dynaco Pas 3x's somewhat fragile 220VAC mains switch does not have to "carry" any more power than turning on the Weidmueller relay....new diagram of the switch function. 

...with external 230VAC socket and ditto chassis fuse, the Dynaco 3X begins to look like an approved standard hifi device for home use. A self-adhesive thin fiberglass print must be made as insulation between the relay and chassis, but the test showed that the start-up function works electrically.....

... a relay that has a 230VAC coil, and where space is limited, you have to think about it... 

Since in my opinion no more holes should be drilled in this Dynaco Pas 3X chassis, the solution was a self-adhesive print with a thickness of 0.8 mm, which should act as insulation. The 3M adhesive is heat-resistant to well over 100 degrees Celsius and therefore suitable for keeping this Weidmueller relay firmly anchored on Dynaco's metal mounting plate. 

After placing the relay on the print adhesive surface, it is impossible to remove the relay with your fingernails. The opposite side of the print to the metal chassis is also coated with heat-resistant double-sided tape... a super strong tape used for mineral glass grinding. 

Should the worst happen that the relay comes loose at very high heat, the contact points will still not touch the metal plate, as all relay outlets are insulated with shrink flex. 

....a noise capacitor remembered as the single-pole Dynaco mains switch probably doesn't quite meet D-mark standards ....

...final assembly according to Dynaco's recommendations....a couple of long signal cables for the "tape" slide switch....must be improved with a couple of plastic cable holders.

Also note the two 1N4007 rectifiers  that are there to protect 12X4 against Peak Inverse Voltage, which is a consequence of the tube's Reverse Recovery Time (RRT) . If the diode's RRT is lower than the tube's, the tube is protected from the current peaks that the tube is exposed to, when it has to change current direction. There may also be some noise that can be reduced. 

Dyna Pas 3X component look PC-5A & PC-6A ...update! 

Copper tracks ...and a solder pad that has overheated due to excessive heat during soldering...that was also the case with both amplifier boards in this Pas 3X. 

....for the same reason I was looking for some copper rivets ¨...I think it was from RS...but I had to buy 1,000 pcs, and they were not quite optimal.... ...I sat and looked through old storage boxes....found 50 pieces of hollow rivets copper..tinned from Holland collar: 2.3mm length: 2.8mm tube thickness: 1.6mm tube hole: 0.95mm 

The 50 tube rivets can easily be 30-40 years old and I don't remember where from... I managed to repair the 3 - 5 damaged holes...a small vice, conical rivals and a mandrel...there were now some fixed-mandrel solder islands to work with.....it was necessary to grind another mandrel so that the outer tip reached down below the tube rivet's hole thickness of 0.95mm...the tip of the mandrel measures 0.7mm 

Should I complain, the collar diameter of the rivets too small..thinking 2.6 - 2.8 mm maybe up to 3.0 mm would be better for tube fittings.. 

Silver oxidation...impossible to solder on silver with black oxidation

Method:

Baking soda (sodium bicarbonate):

Is a salt of carbonic acid that can be used as a leavening agent in baking. 

Baking powder:

Is a mixture of baking soda and an acid (such as tartaric acid or citric acid). 

To clean silver surfaces, or silver-plated electrical contacts, I have successfully mixed baking soda and baking powder in water and stirred the combined powder together to form a thick paste. By either dipping the oxidized silver items into a warm “soup” of baking soda and baking powder, or applying the thick paste with cotton swabs to the contact points, I was able to remove oxidation on silver surfaces.

After application, leave the “paste” for about 10-15 minutes, rinse with warm water, dry with a hot air blower, after which it is recommended to apply Ford contact grease to all silver contact points, as Ford contact grease keeps the silver oxygen-free!

OR...the easy way shown below....

             Clean silver after 10 min.                                                                    Baking soda in hot water

                     Clean solder tabs                                                                   Oxidated solder tabs

Pilot lamp 1967 VS 2025

Taught by a former Pas 3X owner, who has long experience, the Pas 3X mains transformer is designed "right to the edge". 

He recommended reducing "unnecessary power consumption"! 

Since the intention is to preserve the 12X4 rectifier tube, the focus was directed to the pilot lamp. The remains of the Pas 3 x have not been used much...and the inserted 12 Volt bulb which looks original draws 122.8 mA AC. 

It is possible to save some milliamperes , as a 1N4148 in series with 1K to a 3 mm high bright, white LED draws about 9.4 mA DC....but with far too much brightness. If you look a little away from the strong light, the LED with one rectifier diode is only on for half the period, and it is possible to see the diode blinking...a blue 3 mm high bright diode draws about 9.05 mA ...never mind, it will be 4 x 1N4148 instead...with a potentiometer in series with the diode, the brightness from the white 3mm high bright matches quite well and the resistance ended up at 2740 R...

The good thing is that the consumption drops to about 3.80 mA AC..! ..measured across the 2740 R resistor there is about 9.45 VDC corresponding to 3.45 mA DC ..there is about 2.62 VDC across the white diode....the "savings" are about 119 mA from the glow winding. 

You can buy a "modernized" Pas 3X pilot lamp as sparepart...or make one yourself 

  The Dynaco Pas 3X construction has gathered some good info 

Experience:

According to experience from a former Braun employee with extensive electronics experience, there is no electromagnetic field inside a PAS 3/3X?...and that supports one's (my) surprise about the absence of shielded coaxial cables...David Hafler's engineers must have done their homework, as the electronics guy from Braun states that coaxial cables in this tube preamplifier do not add extra hum protection?...more about improved PC-5 /PC-6 print.... attached jpg.

My thoughts due to information regarding electromagnetic field that is not found in a Pas 3?...

....for me is it not completely obvious?

Important! :

Cable pulls with single tinned cobberwire of 0.6 - 0.7 - 0.8 mm are better to install than soft wire because rigid wire holds its shape well in Pas 3X.

Regarding cable pulls, wires between printed circuit boards and power / inputs / outputs are important to pull close to the chassis.

The manual's picture is optimal... except if you install a "power board". With a new power supply board, experience shows that power and ground can be pulled on the TOP side, by pulling wires around the board and close to the chassis.

Reducing hum:

A tip: Pulling an extra jumper between eyelet 1 and eyelet 8 on the PC-5a...... even though there is a connection between these points, an extra thicker wire helps reduce hum in the right channel! 

...info as I see it: Caused by induced hum in the right channel, a too "open" PCB layout with too thin copper tracks from ground to ground, where there is a possibility of radiation comes in.

Dynaco's very important three wafer switch:

The channel selector switch is important. It grounds (almost) all inputs that are not in use..e. you don't hear the radio when listening to vinyl. Those who choose to replace the original switch contact lose this feature.

Choose original values:  (exept for 4 x 7,5 nano caps)

In PAS 3X it is recommended to change the "black capacitors" "Black Cat" as most of them on both riaa - and line cards are from about 1970 ...so 55 years old...replace with the same values!

There may be noise from the anode resistors... which can be changed to metal film 1/2w .....no problem at the last listening test...as always: both 1 W / 2 W and 3 Watt have been ordered for both PC-5 & PC-6 boards...otherwise completely unnecessary, since only one anode resistor approaches a power output of 1/3 watt...

Dynaco patent:

An unnecessary change in the PAS 3X is to remove the tone controls, as they are completely disconnected in the middle position......although not the same for Pas 3...see change in appearance of 4 potentiometers in Pas 3X with "2 offset" notches. 

Dynaco's patent was that the four tone controls in the middle position are completely disconnected from the circuit. There will be an explanation later in the process for too much bass boost by activating the bass potentiometers.

Dynaco Pas 3X approx. 1 K output impedance:

The output impedance of about 1K is a minor problem for both the line output and the tape recorder output.

This can be solved with an external cathode follower..a cathode follower diagram as used in the Marantz 7

Things that can be improved with Pas 3X:

1. By using PC-5a and PC-6a as double-sided PCBs, you can reduce induced hum.

2. If you want to drive the output stage with a lower impedance, a buffer should be used. There is not enough power available internally in Pas 3X...so it must be an external buffer....e.g. a cathode follower with 2 x ECC82s in an external box.

3. The mains transformer in Pas 3 is designed "right to the limit".... all measures that can reduce the load are good suggestions.

E.g. eliminate 12X4 .... replace the pilot light (a bulb) with an LED. Big problem with Pas 3 mains transformer, but compatible more powerful mains transformers can be manufactured..... The volume pot can fail or become inaccurate .... a 250k ALPS can replace the original pot....... a manufactured adapter board can be purchased for easy installation. Transformer, Alps and adapter can be purchased in Sweden....

George and Dave Gillespie (posting on audiokarma.org and writing on tronola.com) work with Dynaco's and are largely in agreement on most things including modifications to Dynaco Pas 3X.

Text should be checked when searching for Dynaco Pas 3X .... the text from George Rennenkamp has not been read closely, but the text is well documented....Be careful reading about "other improvements" to Dynaco Pas 3X...... the reality was different after G. Rennenkamp's review.

As a beginner and first-time owner of a Dynaco Pas 3X preamplifier, it was good that this information appeared before purchasing various components and a lot of soldering work.

Added the missing 1,0 uF capacitors at the output and voltage points at the PSU

Now that the power supply and amplifier board for Pas 3X are finished, there are still: .. many wires to be soldered and connected!

Dynaco's PC-5 and PC-6 boards were made for retro axial American "Black Cat" capacitors. 

..My choice fell on radial MKP 10 transfer capacitors from German Wima, which is why the assembly in this case was more difficult, because all "legs" had to be extended. 

The question was... new pcb prints or reuse the original ones?....the start will be with original Dynaco boards....it's possible to buy axial 630V / 1000V capacitors that fits in PC-5 /PC-6. ....these axial capacitors can be purchased, but I know the MKP 10 Wima sound quality! 

 If the very "open fields" on the old boards cause increased noise, new board layouts will have to be added. 

Found four colors of the 0.8 mm tinned cobberwire similar to the original wires used by Dynaco. 

After cleaning the tubes, screws and chassis ... the time finally came for test assembly and assembly of finished parts for Pas 3X Printed circuits and solder islands have been repaired..... 

Narrow copper circuits for the heaters have been reinforced with a tinned 0.4 mm copper wire, as an almost invisible break had occurred at one point... less resistance to the heaters is always preferable. In the attempt to get as close to Dynaco's original values ​​as possible, it was necessary in two cases to put together a couple of capacitors..... it may well be pointless and impossible to hear the difference... the reason for the attempt was to hear how a Dynaco Pas 3X sounds with 1960's values. 

A 1.5 square (15 AWG) wire as a "ground-runner" is soldered to the chassis via a 3.2mm solder lug 

Today d. 11. april 2025....230VAC the Dynaco Pas 3X was powered up. 

At first start-up, the AC supply was delivered through a vario transformer, but the amplifier was unaffected all the way up to 230VAC, where the power button was constantly monitored. 

The amplifier is assembled from bits that were either found around, manufactured or assembled because several components were not available in DK. 

In this Pas 3X I had changed the power supply, added a pair of 1N4007 rectifiers, and changed the actual +B and grund wiring compared to the original wiring and the four-pole capacitor from Dynaco.... 

Therefore it was a start-up that was expected to have some voltage differences, but all AC and DC voltages were within approx. 5-6% ..even with the four old RCA 12AX7, which were not measured in any way...just plugged in. 

So I thought...this can't be half wrong...forget the oscilloscope...on with Mitronic's 100 Watt mosfet amplifier and the Marantz CD50 .....it didn't take many minutes before I forgot all about the audio chain and the Philips dac TDA1541A with single crown....

This old Pas 3X preamplifier sounded good, especially in the midrange, but also the bass...the bass was "tight" and without "rumble"...,,good thing I kept the 12X4 together with a new up-to-date psu....

 The nervousness about having created a 50-60Hz "hum amplifier" because the distance from the rectifier tube to the new ground point had increased a bit.....(and with all ground wires moved up on top of the metal chassis plate) was fortunately unfounded, as there was no audible hum at all!

... and that the amplifier's low "hiss" seems to come from the 4 x RCA 12AX7.... Good thing I had some recommendations from a hifi guy Sweden, who has helped to find the best setup for this old tube preamp. 

Dynaco had a patent on the bass and treble controls which are neutral in the vertical zero position. The Dynaco sounds just fine with all tone controls in neutral position, but then comes the part that does not work optimally. The tone controls are far too sensitive as the "bass boost" at just 10 degrees of rotation on the bass knob is excessively large. I had otherwise used the four 7.5 nano in front of the 750K potmeters for the bass control, but that was not enough. The same applies to the 400K pot for the treble where I took the closest capacitor value of 20 nano...it became 22 nano, but it should probably have been smaller? The same story applies to the loudness knob...there is too much boost high and low. But if the amplifier runs in flat, neutral position, the Dynaco Pas 3X sounds very musical to my ears and has many fine details. We'll have to see what the oscilloscope says........? 

I have to reduce the bass boost from the 750K pots.... 

But I'm very satisfied that the amplifier just worked perfect the first time I turned the power on.... 

A new PSU on the top chassis for Dynaco pas 3X meant that the total length from 12X4's  pin 7, to the high voltage input on the power supply board was measured to 14 cm.

In comparison, there was 2-3 cm from pin 7 to Dynaco's four-pole decoupling capacitor..... the cable here becomes 11 centimeters longer, with the risk of collecting noise and hum.

.... a B+ high voltage wire must be pulled from the bottom of the amplifier to the PSU board on the top side of the chassis...

We need to use shielded cable and I found RG174/U, a 50 Ohm shielded cable .... but only the shield must be used!!

The idea was to connect the 12X4's ground point with the RG174/U's shield, replace the central wire in the RG174/U with a silver-plated AWG 24 (approx. 0.5mm Ø) solid mounting wire......At the opposite end towards the PSU, the shield is terminated 1 cm before the PSU's +HT input...the shield and center wire are enclosed in black shrink-wrap.

Along this composite coaxial cable from 12X4 pin 7 runs the secondary AC center wire of the mains transformer in parallel( the zero volt AC center)...the high-voltage wire is shielded completely from the output of the 12X4 to the input of the PSU supply. 

The Pas 3X is completely "hum-silent"....I assume that this wiring contributed to a total hum-free preamplifier.......?

It is recommended to solder  the middle of the RCA output frame and lead the center wire to tag #1 on the PC-5 !!!

Regarding too much "bass/treble boost" with the tone controls and correspondingly too much raising of bass and treble with the loudness button, have to be the next and final question concerning Pas 3X.

Note that the distance between #1 and #8 has been strengthen with a 0,6mm wire in order to reduce hum in the right channel....we must have succeeded, because the Pas 3X is completely silent!

Note the 3.3 nano 630V MKP across the on/off switch....Note the original groundpoint connected to the screen of RG174/U....Note the +B wire inside the screen , in parallel with the secondary zero ACVolt mid-wire....both going to the new PSU....Note the two 1N4007 rectifiers to secure the 12X4 tube....Note the white LED and 4 x 1N4148 as bridge!

Dynaco Pas 3X 

DIY finished in april 2025