Your question is technical, so please humor a rather techncal answer. The origial Pas volume pot had eight lugs. The additional two lugs go to the .1uf caps and are the loudness take off. If you do not require loudness, either of your six lugs stereo pots may work.
Why do I say "may" work. The original volume control is 250K. Other values will have a deleterious effect on the already dodgy RIAA eq of the phono stage. If you don't use phono, then you can easily go with a 100K ohm pot. There is also a mod, which can be found on the Dynaco Doctor website which corrects the inaccuracies in the Dyna RIAA curve and is designed to be flat into a 100K ohm load, so this would work as well.
You can also get a PDF copy of the original instruction manual for free at the same site and this should help you understand the circuit. Earlier PAS 2 preamps had .02uF coupling caps on the line stage and phono. No good. These should be replaced with .47uF film caps and the output coupling cap on the phono stage should be jumpered, it doesn't need to be there. You may also want to consider replacement of the Selenium rectifier, as it is a fire hazard and toxic when it burns, as well as bypassing the tone controls, blend and balance control. Oh, scratch filter as well.
Thanks for the help. I have done the tone control defeat. I have been to the Dynaco Doctor site and it is quite helpful, but I didn't see anything on the volume pot. I will go back and look again.
My Pas 2 is the later model that looks just like the Pas 3 and all the caps have been replaced. I didn't do the work so I am not sure what they were replaced with.
Any and all help is very appreciated.
thanks so much.
I went back to Dynaco Doctor and I couldn't find anything about the volume pot.If you have a link,could you post it up?
Here is a link to a diagram. The copy of the owners manual on the site also clearly shows the wiring, if you can read a schematic:
I figured it out. Thanks Viridian.
I disconnected the loudness circuit,and installed a new 250k pot that came with my new Pas 2. The pot was way too sensitive,so I put some 10k resistors between the output and ground to refigure the taper.It is still too sensitive but it is much better.I will get some 32k resistors which should put it in the sweet spot for pot taper.
I am also thinking of installing a stepped attenuator but I want to figure out the best pot value to use before I do that.
I am thinking that 100k is probably where I should be.
I have a power supply upgrade coming too.
Since replacing the pot the sound is much more balanced,with much better imaging.The tone seems less colored while still maintaining the tube transparency.
A very nice sounding little preamp.
Great job! I hope that you are enjoying it. I installed the SDS power supply board in my PAS and it made a world of difference for the better. I just used cheap mylars as the bypass caps on the power supply, I may switch them out for something more ambitious.
Viridian: I was thinking I would just purchase on of the upgraded power supply mods from Audio Regenesis. Here is there link:
Not expensive and a easy conversion and makes the amp much safer and with better power supply.
I am also going to try my 25k stepped attenuator.I will let you know how it goes in the next day or so.
I used this board and am quite happy with the outcome, if you prefer there is an option to delete the tube rectifier and use diodes (I did not do this). There is also film bypass caps:
I imagine that any replacement board is going to kick the snot out of the stock setup.
Do you use the phono? If so you will have to change values if you go with a 25K pot. I would be interested in what the measured input impedance is with this value, if you can measure it. Please post back, or contact me off the board.
Viridian: Do you think I should use a 100k pot? I am not using the loudness feature.I have been looking at the math and I am unsure.
The low level input is 50k and the high level input is 250k and the output is 1k.
I have been reading up on pots and attenuators and I will admit that I am a little confused.I was never very good at math and the equations (as expressed in the white paper I am reading) are beyond me.I understand the concepts but am having a problem putting numbers to formulas.
If you or anyone has experience with this or a recommendation I would be very grateful.
rank beginner hack
I would, and have, gone with a 100K pot. The problem being, if you use the phono stage, you will have to replace several of the caps and resistors on the board with different values to return the RIAA curve to being flat. This is not a bad idea, as the original design pretty much sucks and even with the upgrades, you will find that it is probably not up to the quality of the line stage.
BTW, here is a link to a simple graphic by Joe Curcio that shows the mod. Again, once modded, this will only be flat into a 100K volume control:
Finally, as Ralph pointed out, moving the time constants to the board will make the first three inputs all phono inputs, which is kinda cool. No more "tape head" and "special".
Thanks guys. I will change my order on the attenuator.
This mod doent' look too bad to perform.
Do you guys recommend any particular caps?
Ralph would be the expert on caps, for sure. I can say that I don't like the sound of orange drops, but there are really nice caps in all price ranges. For $1 to $2 there are the polypropylene and tin foil Daytons from Parts Express, which are surprisingly nice. Some like the Solens as well. For $5-$6 the Russian K40 caps are fantastic but a bit larger than many other caps. The sky is pretty much the limit from there if you have the budget.
Oh, I am sorry, for the caps that define the RIAA time constants you will need small value caps. The caps that I recommended above are large value coupling caps. I wanted polystyrene for these small values and had a heck of a time finding a suitable vendor. Google "Just Radios", they are a husband and wife team in Canada, the prices were fantastic, the polystyrene caps sound great and service was awesome. Be careful though, polystyrene caps are more sensitive to heat than other compositions so a variable temperature iron or a still, quick hand will help here.
I bought a kit for my hafler and most of the caps were Daytons. The amp sounds better than new.
I am curious about the russian oil and paper.
There is not that much on these boards, so I imagine it is pretty cheap to recap.
How about the resistors?
Resistors are dirt cheap, most use metal film, but some prefer carbon. There are many Dayton branded caps, but unless you need high values, the only ones that I would use are the tin foil jobs. The Russian K40, not K42, can be purchased direct from Mother Russia on ebay, just check the dimensions first.
Regarding the Dynaco Doctor image: is this mod to bring the RIAA curve closer to spec?
As for the caps: what voltage should the phono board caps be?
I am thinking of doing the Dayton foil caps.Good price and easy (1 day) delivery.
I looked up Orange drops online and I must not have them because mine are redder than what they show. I can't see anyone's name on them.Also I would say that this pre doesn't have a harsh bone in it's body.
I noticed on the Dyno Doc's image what looks like a 470pf shunt on the 2200pf (flat looking)caps.Is this right?
No, the 470 is not a shunt on the 2200, the value that was needed is not common, so Joe is just building the 2670pf that he needs for the circuit. The purpose of the mod is two fold. First, the original circuit was +-3db or so, this tightens it to +-.3db so it is much more accurate, second it replaces the old parts on the board that have probably drifted, old carbon comp resistors often drifting upward in value and caps leaking. It also replaces the old parts with much more high performance modern parts and it necessitates that the volume pot be switched to 100K. That's more than two I guess. The RIAA caps are low voltage.
BTW, the Dayton caps only come in the values needed for the coupling caps, not the RIAA caps.
I noticed that when I started looking at the values.
This Pas has been rebuilt, but it was done before I bought it. The 250k pot is too sensitive and the pre sounds very good. I am wondering if the guy that did the mods changed the values toward the good. Should I start over and make sure that everything is ok? This is where I am leaning.
I feel like this little Preamp has alot to offer for a beginner hacker.
I really appreciate your help V.
It's fun to tinker but, if you like the sound, and like the orange drops (mylar caps come in various shades of red/orange all referred to as orange drops) why screw with it? I wouldn't. It is more than possible that you could make it worse.
Why not get a stock Stereo 70 to go with your PAS and beat on that? Or even another stock PAS and hack on that? The benefits of starting with stock is, first you know where you are, and second, you are not paying for someone else's work that you are just going to re-do.
But, should you prefer not to follow that bit of advice, e-mail me a photo of the phono stage and maybe I can figure out what has been done.
Then again, completely redoing the phono stage per Joe's suggestion, and changing out the volume pot to a 100K unit is very inexpensive.
I'm still hoping that Ralph checks back into this thread with some of his brillant ideas. I'm just a hacker.
Viridian: a St70,though very interesting to me,would never work with my speakers (Acoustat 2s).
Correct me if I am wrong,but Joe's circuit removes some of the caps to be replaced by wire? He seems to have a handle on the RIAA curve. I will send you some picts of my Pas board.
In the meantime, I definetly plan on changing out the rectifier for a new power supply cap board,and change out the pilot light for a diode. I feel like the volume pot is way off from where it should be. Honestly I really don't have a grip on that yet.Will having a 100k pot make it less sensitive?
I love it that Ralph jumps in on these strings. It proves to me that he has genuine interest in our hobby and makes me more interested in his products than others.(not to mention the great write ups his amps get)
Wow yes, the 70 is not an appropriate amp for the 2s; that is for sure. Although, back in the day, my dealer was running 2+2s off of conrad-johnson MV-50s in mono, which are pretty much the rich man's Stereo 70. It sounded great, but did not play loud enough for me.
I really don't know if the 100K pot would make a difference in gain, but I suspect not.
When you do the cap board, don't forget to put in a power line fuse. It's strange, and dangerous, that they did not put one in. I think that the PAT4 has one, but I'm not at home to check my PAT.
Yes, one of the caps in each channel is deleted in the Curcio RIAA fix; hey, that's one less cap to buy!
Good luck with the build.
Thanks V. I have used a 25k stepped attenuator and built myself a little one input passive preamp (no autoformer)and it sounds pretty nice using the Pas2 as a phono stage only. The weird this is;I don't have to turn on the tape monitor,it just works.
Is this normal?
Yes, the tape out is always live. Completely normal.