PAS tweaks

[nyazzip]

since i got my st-35 assembled, i have been feeding it with either my trusty cheapo sony c.1989 CD player, using the headphone out, which happens to have a level contol, or, feeding it with my "iPod" FM tuner. sounds pretty great thus far- really.
but, i just finally got a dynakit PAS 2. honestly, and amazingly, i can't yet say i can detect much difference between the CD phone out to amp in, versus CD line out to PAS to amp.
my only initial complaints thus far about the PAS are: the volume control is whack at low volumes in regards to left-right tracking; its a bit scratchy(roary?) when i move it; the overall soldering is pretty crude(LOTS of metal); and the RCA outputs are corroded like they've been sitting on the dock of the bay, wastin time.
also, for some reason, my "iPod" FM tuner does not work well thru the PAS...apparently, its headphone out level is much lower than my CD player's line out level(or headphone out level). thats all i can think. but i'm working on getting an actual FM tuner.
my reasons for buying the preamp were basically to have a source-switcher/master volume controller, and to see what all the hype is about. i do some home recording, and using a tube mic preamp makes a big difference, so i thought hmmm.....
my "sources" will be: CD player, FM tuner, and Focusrite Saffire D/A/A/D box connected to my PC, sending either streaming internet audio or monitoring my home recordings via a 1/4" stereo headphone jack. haven't tried that connection yet.
the PAS 2 tone controls(more bass) are a bonus. i am forced to listen late at night at extreme low volume, so the "loudness" feature is groovy as well.
my intentions were to do a bunch of "modding" and rebuild that is oh so fashionable right now, but really, i think the preamp sounds ok like it is straight outta 1960.
at this point i think i might just get some new tubes, find a decent "plug-n-play" stereo volume control(...if possible- but would i need to alter PAS topology with modern volume pots?), and get some shiny RCA ins/outs.
i don't really care if the selenium rectifier fries some day and smells bad....i have bigger issues in my life worrying about a possible bad smell in the house for a day. everything looks ok except for some evidence of light roast on the wafery-things(i think they are tone related. sorry, i told you before i know nothing) in the RIAA section(haven't tried a record player yet but the section works with the "iPod", and as expected, it is bass-heavy).
.......so: are all the rebuilds really going to improve stuff, or is it dumb to sink upwards of $400 into this relic? opinions?
regards
k
and hey- is there any way to search keywords for a forum topic? such as "PAS"? i feel bad for posting so often, especially as i have only questions, no answers.....

and hey hey- i did the "strapping the power switch with ceramic capacitor" mod on the DIYTube ST-35 and it worked wonders....

[lorne]

I would take a very conservative approach to fixing up a PAS. I've had one since the late 80's. It's currently in pieces. It’s going back together as a dedicated phono EQ section.

The easiest mod you can make right now is to pull out the 'power on' bulb. The power supply is so marginal that the bulb is sucking on any reserve.

Next: get some spray designed for cleaning pots. And get another designed especially for switches. The PAS is famous for their pots and switches being troublesome. You may coax them back into life with a good cleaning. Just keep repeating — spray then turning the switch or pot.

Another approach is to build yourself a passive attenuator and skip out the pre-amp. If you need a switcher, you can build a separate box, or include it along with the attenuator. Then you will have what is usually called a passive pre-amp.

Assuming you wanna keep the PAS on line: you can make the PAS a sweet little machine that will get you started up the tube road. At first, don't spend big bucks — just my opinion. Later you can go to heroic lengths, and after playing with the stock unit, you will know a lot more about what you are doing. Eventually, you will probably be faced with increasing the power supply. For now, just get cleaning — and pull that bulb. And if the tubes are original, they will be Telefunken tubes — treat them as if they were the Holy Grail — because they are! Hope that helps a bit ... Lorne

[TomMcNally]

Personally, as one who has the following Dynaco museum pieces:

a pair of MK-III's
an ST-70
an ST-35
a PAS
an SCA-35
and an FM-3 tuner

The PAS in the only one I didn't restore.
It's a cheezy box not worth the trouble.
Sure - all of the shortcomings can be addressed,
like the RCA jacks that are too close together, the
low budget pots, the dirty rotary switches, the
tiny power transformer, bad circuit boards ... etc

When you are done, you have spent a ton of money
for little benefit ... you're better off starting from scratch,
or marrying a nice phono preamp like one of Hagerman's
with some nice switching, a line stage, and a good
attenuator.

OR ... for much less money - do what quite a few of us
renegades on the forum have done ... get a nice used
Adcom high end solid state preamp for a fraction of
the original price on eBay. Some of them have phono
preamps, most have remote control of inputs and volume,
some have good tuners ... many models.

There is no advantage in my thinking, of running an all
digital source through a couple of 12AX7's on their way
to your tube amp ... you don't need the gain, and all
you are doing with a PAS is adding some crap in the
line.

... tom

[crispycircuit]

Enjoy it for what it is... The only worthy investment is the SDS power supply board. I was very pleased and surprised at the improvement in sound. It's great to have PAS around for the tone controls, loudness, and tubey phono section. These options are great for mini speakers, background music, or that nostalgic tube sound. It's really is a wonderful preamp and holds it's own in any system. It does a lot of things right...

[mesherm]

I have two PAS-3Xs. One is a factory built unit that someone sold on ebay after drilling out the rivets on the line amp board and rmoving all the components. I simply replaced the line board with a new PCB with new parts and soldered it back in place. I also added a SDS power supply board. The other 3X was non-functional so I completely rebuilt it. New PT, new RCAs, new PCBs, etc. The only things I kept were the chassis, switches and pots. It was a LOT of work as I wanted to keep all the original tone control functions (the now unobtainable pots).
Even with the SDS board the factory unit has a low level hum probably from the selector switch and factory wiring.
At one time I also had an Adcom preamp so I could compare the two and I must say the Adcom sounded every bit as good.
A few PAS recommendations.
Use Caig Deoxit Red on the jacks, pots, switches, and selector switch. I applied a few drops inside the pots and then blew canned air into the pot while rotating the shaft back and forth to spread it around inside. That cleaned up the pots nicely.
Disconnect those AC outlets in the back. They are accidents waiting to happen.
Even if you don't use the phono section you need tubes installed for both PCBs to function because the filament circuit grounds through the phono board.
Replace the power supply caps and the Selenium rectifier.
If your a glutton for punishment you can replace the rotary switch and rewire the input section but its a PITA.

[lorne]

There is no advantage in my thinking, of running an all
digital source through a couple of 12AX7's on their way
to your tube amp ... you don't need the gain,

Exactly ... these days line source has enough gain in virtually all cases — excepting phono EQ. All that remains to be serviced are these factors (a) attenuation; source selection; EQ (in the case of phono). A and B can be done through extremely, even primitive, passive circuits with astonishing success in many cases. Some well publicized caveats apply. In the case of 'C', Tom has nailed the solution squarely. But, I'll reiterate the point I made above: you can make a very nice phono EQ amp from the PAS for very little money other than what it takes to repopulate the EQ board. Just run the signal directly out the 'Tape Out' plugs. This may not apply to you, as you have only mentioned CD as a source.

You could use the PAS as a switcher, but you would be a lot better off to get a good switch and make one. I assume that since you built your ST-35, you can do this. (BTW - congrats! I want one too!) And there is help all around — I'm sure. (I'm 80% through building one as I write).

[Slartibartfast]

I have a PAS that was an ebay purchase. It worked in all original condition, but the tone control pots are shot. I replaced the tired pre and phono boards with a new set (all original circuitry). I also replaced the selenium rectifier with a C Chong board and removed the indicator light.

All tone controls have been removed, no balance or blend switch. The loudness switch is gone as well.


The selector switch has been replaced with a modern one. The volume control is new as well. I also made some changes to the phono board to turn those low level inputs into line inputs, save the phono input.


I have also made changes recommended by Joe Curio that helps the pre amp.

I am happy with the sound. It was fun doing all the mods. The only hum I have is when the phono input is selected and the volume is at the 12 o'clock position, with no signal. One day I will go in and use shielded wiring for the phono in and out.

[lorne]

Slartibartfast: I think that you have described the limits of what our friend 'K' should do with his. What you have done does not cost a wad of money, and you get an effective instrument as well as a lot of hands on practice.

In my experience, as well as by reputation, a bone stock PAS should have no hum. It may have had issues due to the build/budget constraints, but hum was not one of them. Perhaps you should be hunting up the cause off-chassis. I had some hum and ended up doing a lot of work on my TT. ... Lorne

[Slartibartfast]

Slartibartfast: I think that you have described the limits of what our friend 'K' should do with his. What you have done does not cost a wad of money, and you get an effective instrument as well as a lot of hands on practice.

The two boards, seltector switch, and volume control were certainly cheaper than what I paid for the PAS. Yellow_Light_Colorz_PDT_02

In my experience, as well as by reputation, a bone stock PAS should have no hum. It may have had issues due to the build/budget constraints, but hum was not one of them. Perhaps you should be hunting up the cause off-chassis. I had some hum and ended up doing a lot of work on my TT. ... Lorne

Well I do have a SS Dynaco amp. I will have to play the TT through it to see if there is any hum. One thing I do know is that I could do a better job of routing the wiring away from the tubes on the phono board. I moved the wires right next to the tube and the hum became very pronounced. A better layout and using shielded wiring should improve the problem.

[lorne]

And, if you have not done it already, you should reflow all the solder that was not involved in your board changeouts.

[Slartibartfast]

And, if you have not done it already, you should reflow all the solder that was not involved in your board changeouts.

With the amount of modding and gutting I have done, I don't think there is a solder joint left untouched. Yellow_Light_Colorz_PDT_03

Everything from the inputs to the pre out, has been reflowed.

[Sal Brisindi]

Tom,
If you don't want that old crappy PAS-2 useless no good preamp, just send it my way... Yellow_Light_Colorz_PDT_02

Sal

[CpuZapper]

Well for me the Pas 3x was in very good shape and all original when it arrived. Little to no pot noise but the sound of the unit was so disappointing I though I had made a monumental mistake... I hated the sound and it has excellent telefunken 12ax7's all test like new and a Dynaco black plate 12x4 rectifier tube. Keep in mind I have no ultra HiFi solid state stuff and my home theater amp might make it to what I will call midfi, just so you have a point of reference.

To get this unit up to my listening standards I did the following:
1. Built a new power supply along the SDS schematic with snubbers on the diodes. Lamp should be removed.
2. Remove all the tone pots and switches like blend and loudness. If you want to keep the tone controls I think this unit needs drastic mods much like Norman Koren has posted on his site.
3. Replace volume pot, 100k step attenuator.
4. Replace selector and RCA jacks in and out, all the wiring, I used stranded Teflon silver clad copper. I like the detail this wire provides over the stock solid core.
5. All the caps, some resistors (so far).
6. Minor Mods to phono and line stage board, mods can be found at Joe Curcio's site.
7. Tubes, Out of my 12ax7 collection, Telefunkens are nice, Mullards a little nicer but Tungsram take the top spot. Great tone, detailed and dead quiet. The Tungsram was an eye opener in the Pas.

Now would I do this again, no. If you have one and money is not a factor why not make it sound as good as you want, but if you don't have one yet I think a kit project or making a pre-amp from scratch are better options.

How does it sound. I think it now sounds very good but I don't have enough experience with HiFi tube stuff to say it is HiFi but I think it fits in the Midfi bracket. I hope to complete a Curcio line stage soon and a Aikido line stage soon after. Maybe then I will know just how the cleaned up Pas stacks up.

I hope some of this info was helpful but what ever you do HAVE FUN and the experience is priceless!!!

[Slartibartfast]

To get this unit up to my listening standards I did the following:
1. Built a new power supply along the SDS schematic with snubbers on the diodes. Lamp should be removed.
2. Remove all the tone pots and switches like blend and loudness. If you want to keep the tone controls I think this unit needs drastic mods much like Norman Koren has posted on his site.
3. Replace volume pot, 100k step attenuator.
4. Replace selector and RCA jacks in and out, all the wiring, I used stranded Teflon silver clad copper. I like the detail this wire provides over the stock solid core.
5. All the caps, some resistors (so far).
6. Minor Mods to phono and line stage board, mods can be found at Joe Curcio's site.
7. Tubes, Out of my 12ax7 collection, Telefunkens are nice, Mullards a little nicer but Tungsram take the top spot. Great tone, detailed and dead quiet. The Tungsram was an eye opener in the Pas.

Now would I do this again, no. If you have one and money is not a factor why not make it sound as good as you want, but if you don't have one yet I think a kit project or making a pre-amp from scratch are better options.

You can accomplish what you did with a lot less work by buying a set of new circuit boards from Antique HiFi for $105. Then perform the Curio mods on those boards ( these are minor mods, removing certain caps, changing values on others).

That is what I did along with tone control removal and I am pleased with the sound, and as I said in another post, it was fun to do. Yellow_Light_Colorz_PDT_11

[CpuZapper]

[/quote]

You can accomplish what you did with a lot less work by buying a set of new circuit boards from Antique HiFi for $105. Then perform the Curio mods on those boards ( these are minor mods, removing certain caps, changing values on others).

That is what I did along with tone control removal and I am pleased with the sound, and as I said in another post, it was fun to do. Yellow_Light_Colorz_PDT_11[/quote]

The Work on the Pas was probably the biggest benefit, it gave me first hand knowledge of just how each of the mods changed the sound of the Pas. I have been playing with the PAS for a number of years now, testing many different items in the unit including vintage electronics pas boards but the problem is it is still the original circuit and overall I think this is the limiting factor. Keep in mind I'm not an engineer and most of my info has been gleaned from years of reading and some hands on time with the Pas.

I have read allot on the Pas, few people think the Pas stock circuit is all that good and some will say a waste of time from the get go. I will say it does sound good all cleaned up but most people who have compared other kits like Curcio PAS which is not a Pas at all, VTA SP6, Hagerman or Aikido to name a few all say they sound better than the Pas. I picked up the Aikido 9pin on sale (less than the vintage board) that includes the board, tubes and components to populate it for multiple tubes, very reasonable. I have to say that the quality of this board is the best I have seen so far in a kit.

If you have a Pas go for it, if you want to play and see how good you can make a Pas sound, go for it!!! But if money is tight and you want the best sound your dollar can buy I think the money you need to spend on a Pas is "not" a wise investment. When I get my other line stages done I will be doing some A/B comparisons and will know which one is the best bang for the buck, for me.

Until I know for myself I have to think there is better than the Pas out there or my quest is over. I hope that what ever road people take it turns out to be as enjoyable as mine has been.

Happy Soldering.