Input(!) needed on Dynakit PAS. A little urgent...

[Eustacius]

Guys,
I spoke to a retired gentleman today about his PAS-2. He wants to sell it to me, and I'm tempted. I know how to meddle with valve/tube circuits only in theory (and a little by ear) and I'm eager to start (humbly).

Question 1: Is it worth it? I know some mods that have to be done and some other that is good. If fairly carefully refurbished, could it be a keeper?

Question 2: A little obscene perhaps. I want to pair it for the time being with my 303s. The input impedance is only 22K. Is that totally perverse or is it doable? Will I end up with no bass? No treble?

I would love some expert input, and this evening I have to stick my nose in Morgan Jones' book.

This gentleman wanted to make my journey to him as beautiful as possible. He asked me to come at a certain time for the "best light over the lake". I love doing business with people like that. I want to pay what he asks (100 euro) but I can't overpay either. Its not to much, is it? It is original and seems OK, but e.g. two knobs are missing.

[GP49]

Question 2: A little obscene perhaps. I want to pair it for the time being with my 303s. The input impedance is only 22K. Is that totally perverse or is it doable? Will I end up with no bass? No treble?

Not compatible.  The input impedance of 22KΩ is WAY too low for a PAS-2, even if all the pulldown resistors on the output are removed from the circuit.  There will be a loss of bass.

ANY PAS-2 in its original stock condition will need some rework.  Almost certainly the selenium rectifier for the filaments is probably weak; the symptoms are slow warmup and poor definition.  Modern silicon diodes cure that, immediately.  There could be hum, usually due to a weak power supply capacitor.  The RIAA phono equalization is not optimally accurate, though many find it "euphonic."  Carbon resistors may be noisy and may have drifted off value. 

With proper rework, the PAS-2 can still be a highly credible high-end preamp.

If it's cheap enough, and if it still has its original DYNACO tubes, you might want it for the tubes alone.  Original Dynaco tubes were rebranded TELEFUNKEN 12AX7.  Knobs for Dynaco preamps are available new (there is a Dynaco users' parts aftermarket) but NOT for the PAS-2, which used the old, brass front plate and brown knobs.  My own rebuilt, upgraded PAS-2x has non-stock black knobs since I could not find enough stock knobs to complete it.

[Eustacius]

Thank you, Gene. I'll stick to my plan with a passive for now.

[GP49]

His price of €100 is not unreasonable especially if the case and front panel are in good cosmetic condition.  On lesser-condition units the original
clear coat on the upper brass section of the PAS-2 front panel often wears through, especially around the selector switch and volume control;
while the brown lower section remains in good condition; but the clear coat is easily removed and the "brushed" finish on the brass can be
easily duplicated with steel wool, prior to spraying a new coat of clear.  The PAS-2 with its embossed lettering is more easily restored (if the
brown painted lower panel is OK)  than the later PAS-3 with silkscreened lettering.   

However, most consider the PAS-3 to be more attractive in appearance.

The "x-suffix" versions PAS-2x and PAS-3x are preferable but not as commonly found.  They have special bass and treble pots that have
"true-flat" positions that take the tone controls out of circuit.  The pots are not replaceable by anything in production today, and for the
knowledgeable, can be used to interface the Dynaco tone control circuit with many of the linestage upgrades.  The sad part is that some
Dynaco modifiers will take a PAS-2x or PAS-3x, upgrade it to minimalist standards (i.e., no tone controls at all) and THROW AWAY
those rare potentiometers.

[decca4]

Hi Christoffer,
 
As usual Gene has a lot of info. I was not aware of that there was a PAS-2 and also a PAS-2X. Gene is there any other difference between the two besides the pots?

The PAS 2X is in my mind a very nice little pre-amp. In my view it looks so much cooler than the PAS-3X that has the same circuitry as far as I know, but a different face plate (at the time considered more modern). At the time the face plate of the PAS-3X could be bought as an update.  My PAS-2X was bought together with the ST-35 power amp, in the late 60th at a total price of €50. I still have them and certainly don't intend to part with any them. The ST-35 is still with todays standards a very fine amp given the right speakers. However, the PAS-2X certainly needs some serious attention and updating when it comes to parts. In the early 70th I made my first mod on the PAS 2X, with the help of a kit from Old Colony (part of Audio Amateur). I had absolutely no idea what I was doing when changing the resistors and capacitors. Back then I don't think that I could tell the difference between the two. You can imagine my apprehension when I first turned it on after the rebuild. Much to my surprise it worked and I was very very happy. As I recall it there was some later articles in Audio Amateur suggesting modifications that made it close to an Audio Research SP-3 with an active RIA I think. Please forgive me for my nostalgic trip.

In the manual for the PAS-3X it is said that it should not be used with a power amp with less than 100 kΩ input impedance. The output impedance of the unit is as high as 1 kΩ according to the same manual. One thing that I remember and that made a difference was when I increased the voltage in the filament supply. At that time we had ca 220 volts in the mains and the unit was meant to have 240 volts. This discrepancy led to a filament voltage of 5.6-8 instead of the 6.3 that it should have had. I added some windings to the transformer that supplied the filament. This resulted in much better clarity and bass.

Should you by it? Well, that entirely depends on you willingness to spend the time and money on the update it and if you are attracted by its looks.

If you buy it my recipe would be to put some new caps in the power supply and at the same time make them larger. Then I would put in some modern coupling capacitors and also at least double the value of those. I also suggest that you check the voltage of the filament and ad some windings to the transformer if the voltage is to low. The RIA should also be updated with modern components without any change of values. If you can, you should measure the new components to see to that they are the correct values. The components in the RIA are critical for the end result.  After that I would listen. If it sounds fine then use it as it is. If it is to noisy you would probably have to take out all the resistors and replace those with metal ones. Different manufacturers resistors sound different and have an effect on the sound so if you have the energy you should find out more about this subject. Gene already mentioned the selenium rectifier and I agree.

The PAS-2/PAS-2X is a bit rare these days and does not show up as often as the PAS-3X at least not where you and I live so if you like the looks then go ahead. You could make a calculation of the cost to restore the unit and present that to the seller, maybe he would be willing to back of on the price a bit. Still I don't think that the price is excessive even if it needs a major restoration.

If you cannot find the circuitry on the net I can make a copy and send it to you

[GP49]

As usual Gene has a lot of info. I was not aware of that there was a PAS-2 and also a PAS-2X. Gene is there any other difference between the two besides the pots?

No difference electronically.  The knobs differ...the PAS-2x has setscrew knobs, the PAS-2 has splined knobs.  The reason is the tone controls;
the knobs need to be set very carefully to ensure flat response at the "zero" position, and splined knobs do not have that degree of precision.

PAS-2, PAS-3 schematic:

http://i9.photobucket.com/albums/a78/Bob01605/DynacoPAS2_PAS3schematic.jpg

PAS-2 assembly manual (includes schematic):

http://www.tubes4hifi.com/Dyna%20PAS2%20manual.pdf

PAS-3x assembly manual (as decca4 noted, only difference is the front panel and knobs):

http://www.tubes4hifi.com/Dynaco%20PAS-3%20manual.pdf

[Jolyon]

I had a PAS 2 ages ago; can't remember what happened to it.  But, I still have some brown knobs (3 large with set screws and three small without) and an old brass faceplate that'll need restoring.  If you get the amp and want to restore it you can have these for the cost of the postage.

Yrs,

Jolly

[decca4]

No difference electronically.  The knobs differ...the PAS-2x has setscrew knobs, the PAS-2 has splined knobs.  The reason is the tone controls;
the knobs need to be set very carefully to ensure flat response at the "zero" position, and splined knobs do not have that degree of precision.

Gene, are the knobs visually the same so that you have to pull one off to see if the knobs are splined?

[GP49]

Gene, are the knobs visually the same so that you have to pull one off to see if the knobs are splined?

You can look for the setscrew.  It's in a little hole on the side of the knob.

AND I was reminded in a private message that production variations did occur, so what I said should
not be accepted as "absolute!"

[billyz]

 If you did not need the tone controls you could turn the second half of V2 into a cathode follower , that would surely drive your amp.

[Eustacius]

Tank you guys!! I secretly hoped that Gene would answer. Reading his posts is like taking a course.  

I would never throw anything away and I would not conduct non reversible experiments with "antiques" (as opposed to "old stuff").

Thanks for your offers. I still haven't decided but I think I will ask him to cut down a little. In the case that I go for it, it will turn into a lengthy project instead of just fixing it and putting it in the system.

I am attracted to the looks and it is (viz. seems) in good cosmetical shape. The brown stripe seems to be without scratches.

I had heard of the Dynaco-Telefunken tubes and I forgot to ask him about that. He was very concerned that I had to walk to his workshop in the slush that we have, and he talked a lot about this. I certainly shall check the tubes if I buy it.

If I am in doubt, it is only because I am between jobs and I am a little careful... If things go well with tests, health check ups, and interviews ad nauseam, I shall in April be a driver in the Underground of Stockholm. Then I can very slowly begin my business and formal training as cabinetmaker! Yippie!

Thank you Stefan for the support! I would go for the electrolytics and the rectifier the minute I got it in my hands. I'm a bit concerned for that rectifier... I should certainly change the RIAA components.

Speaking of the tone controls. Researching the web people, seem to be bypassing them, also on the PAS-yx. I don't know...

Thank you Jolly! You'll know if I pm you.





Billy, that sounds interesting. I need to learn some stuff here. "Cathode follower" is but a word for me. As I said -- perhaps I should read that Morgan Jones... That'll certainly revive -- or not -- those fossilized pieces of knowledge from college.

Billy, If you ever come to Stockholm, I'll take you here:

[Eustacius]

Not original tubes/valves and many problems.

I realized that I was paying for a nice box. I would have built a new one with upgrades. A little disappointed...

Thanks for the info guys.

[billyz]

 Looks like my kind of place .   Would love to visit .   

 The cathode follower is a simple circuit to implement. AS your tone control circuit is last  just before the output jack to the power amp it is already in the ideal location.

 The cathode follower gives a Low impedance and hi current output , which is well suited to difficult loads and longer distance runs.  Not affected by Capacitance etc.

 You can goggle the circuit topology . Here is a link to get you started.
 
 http://www.tubecad.com/october99/