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    Who runs a PAS 3x in front of their ST70?

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    tygr1

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    Join date : 2014-12-08

    Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Sun Dec 14, 2014 4:04 pm

    Does it make the ST70 sound better or worse? I'm running my source (Sony ES CD player for now) straight into my ST70 (which I finally got to sound right) and it sounds exceptionally good. What can I expect when I get my Pas 3x and get it plugged in?

    Thanks in advance for any comments/opinions.

    Bob Latino
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    Re: Who runs a PAS 3x in front of their ST70?

    Post by Bob Latino on Sun Dec 14, 2014 5:37 pm

    tygr1 wrote:Does it make the ST70 sound better or worse? I'm running my source (Sony ES CD player for now) straight into my ST70 (which I finally got to sound right) and it sounds exceptionally good. What can I expect when I get my Pas 3x and get it plugged in?

    Thanks in advance for any comments/opinions.

    The PAS-3X or really any PAS preamp will hold even a stock ST-70 back a little with respect to overall sound quality. Don't get me wrong, I love old Dynaco tube gear but the PAS preamp is only mediocre by today's standards. To upgrade a PAS preamp to get the type of sound quality you need to keep up with any of Dynaco's tube amps (or upgraded ST-70's like our VTA ST-70) can be costly. When you consider what you have to pay for and old PAS preamp and then the cost to upgraded the PAS, IMHO - it's not worth it .. You are better off buying one of the VTA SP preamps. The sound quality of any of the VTA SP preamps is much higher than even a modded PAS.

    Bob

    tygr1

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Sun Dec 14, 2014 5:52 pm

    Thanks Bob, that really brightened up my day, LOL.

    PeterCapo

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by PeterCapo on Sun Dec 14, 2014 10:55 pm

    tygr1, you’re getting a PAS?  Can you say anything about what kind of condition it is in?

    I think there are a couple of points to consider.  When I ran a disc player straight into an amp with no preamp in-between, the sound had a very pure quality that I don’t think I’ve ever been able to get back with any of the preamps I’ve since put into the signal chain.  The closest thing would probably be a really good passive preamp, which I considered getting at one time (the Placette Audio passive).

    A choice to add a preamp is often driven by a desire for control functions.  An active preamp is also said to be livelier than some kinds of passive arrangements, which I found to be true.  Plus, there just might be some active preamps that you might appreciate for their particular kind of sonic character, even if it is not as pure as running your source directly into the amp.

    With regard to the PAS, anyone who reads my posts knows I am a fan of the original Dynacos.  However, to be honest, I have a couple of samples of original, unrestored, decades-old, well-used PAS preamps that leave a great deal to be desired in terms of sound quality.

    That said, I also have a third sample of PAS that I completely tore-down and rebuilt to the original circuit design but with a beefed-up power supply (tubes4hifi cap board) and other parts upgrades but with most of the control functions removed.  The results are really very good, very sweet – probably the most dramatic improvement I have ever heard.  It is not embarrassed by my other preamps (Parasound JC 2 and Cary SLP-98L) or most gear I listen to at local hi-fi shops.

    So, how much you will like your PAS depends on what kind of condition it is in and if the necessary adaptations are made so that you get a good match to your power amp.  IIRC, the Z-in of the VTA is something like 200K or 250K, is that right?  If your PAS is all original, you might have to make an adjustment so it mates properly to your VTA Stereo 70.  Besides this, it will sound as good as the condition it is in.  If it is not in the best condition, it can be improved, but it will take some research.  By selecting just the right things to refresh in your PAS, it could still make financial sense.


    Last edited by PeterCapo on Sun Dec 14, 2014 11:08 pm; edited 1 time in total

    MexicoMike

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    Join date : 2014-11-21

    Re: Who runs a PAS 3x in front of their ST70?

    Post by MexicoMike on Sun Dec 14, 2014 11:07 pm

    I'm quite happy with my 3x/oem ST70. Both have been recapped; the ST is oem except for the diode replacing the selenium rectifier and the 3x is oem other than the 1st level tubes4hifi power supply. They are playing into a pair of Thiel CS2s and sound very good. However, I can't claim that I've worked the amp/preamp all that hard since the only input I have used with them is iTunes. Wink

    GP49

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by GP49 on Sun Dec 14, 2014 11:25 pm

    Over the years (and there have been many) I have tinkered with the PAS preamps, I have learned that the Number One thing one must do is to check the filament voltage.   By now, probably all of the selenium rectifiers have gone high-resistance.  As early as 1968(!) I was given a PAX-2x that took two minutes to start producing output.  Once it did, it sounded murky, flabby, slow...and its filament voltage was about 60% of normal due to a weak selenium rectifier.  Bridging across it (in effect, using it for a terminal strip) with silicon diodes, which were cheap even then, restored full filament voltage, normal warmup time, and the sound of a good, functioning PAS.  

    46 years later, that same PAX-2x chassis is still in my system.  It can't really be called a PAS-2x any more, though it still LOOKS like one.  Almost all of it has been changed (including interfacing the "center-flat" PAS-2x tone controls with the new circuitry) but, ironically, the same silicon diodes are still in there, as is the old selenium rectifier "terminal strip!"

    Yes, it would have been less work to build a new preamp from the ground up.  Wouldn't have taken 46 years, for one thing!


    Last edited by GP49 on Tue Dec 16, 2014 12:02 pm; edited 1 time in total

    tygr1

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 1:32 am

    Thanks for the encouraging words guys! The one I'm getting appears to be all stock and I had already planned to change out the power supply board and some of the caps in it. I think it will be a fun project pre.

    PeterCapo

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by PeterCapo on Mon Dec 15, 2014 1:55 am

    Post some images if you can, eh?

    tygr1

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 8:22 am

    PeterCapo wrote:Post some images if you can, eh?


    Will do. Should have it later today.

    tygr1

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    Join date : 2014-12-08

    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 5:59 pm

    Here it is. Looks bone stock to me. Does the 3x have detents at the tone pot center position, because this one does not?










    MexicoMike

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    Join date : 2014-11-21

    Re: Who runs a PAS 3x in front of their ST70?

    Post by MexicoMike on Mon Dec 15, 2014 6:58 pm

    Looks like a really nice stock preamp. The 3X pots don't have a detent.  They are supposedly out of the circuit when they are centered.  Of course, how you mount the knobs may mean "centered" is not "off."  Wink

    tygr1

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    Join date : 2014-12-08

    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 7:03 pm

    Thanks Mike. I think I'd better check the knobs.

    MexicoMike

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by MexicoMike on Mon Dec 15, 2014 7:38 pm

    Frankly yours looks so nice and unmolsted, it's almost a shame to mess it up...er, I mean.. upgrade it! Smile

    Did you just acquire that PAS? Have you powered it up with a variac or dim bulb to see if it's working OK? Of course you could just plug it in/turn it on/watch for smoke...you know the "REAL MAN" method. Smile

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 7:45 pm

    I'm listening to it as I type. It actually sounds quite nice.

    PeterCapo

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    Join date : 2008-12-05

    Re: Who runs a PAS 3x in front of their ST70?

    Post by PeterCapo on Mon Dec 15, 2014 7:56 pm

    It really does look like a very nice original sample!  The "POWER" and other lettering on the front panel looks intact, which suggests it may have low miles.  Kind of like finding a '65 Chevy in a barn under a tarp, or something.  It is a 3X, as you can see the 1uf electrolytics connected between the front panel and PC5.  

    As far as centering the tone controls, if you can manage it, see if you can look right into the slot under/behind the solder lugs.  As you rotate the knob, you will see the wiper assembly rotating past.  If you study how it moves, you might be able to ascertain when it is centered, visually.  Also, the bass pots are shorted when centered, and the treble pots’ wipers are open when centered.  You can try checking for this, but since they are wired into the circuit, it might not work.

    Congratulations on your find.  Do you find that it sounds "sweet?"

    tygr1

    Posts : 35
    Join date : 2014-12-08

    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Mon Dec 15, 2014 8:46 pm

    It does sound rather sweet to my ears. It has the original Tele 12AX7s and the lettering on them is near perfect. I think it may be a "low mileage" barn find.

    10-E-C

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by 10-E-C on Tue Dec 16, 2014 12:19 am

    tygr1 wrote:It does sound rather sweet to my ears. It has the original Tele 12AX7s and the lettering on them is near perfect. I think it may be a "low mileage" barn find.


    I have a PAS-3 all original with the tele tubes also, I've not touched it in the years I've owned it. I want to, but will probability keep it stock for now.

    TM

    j beede

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by j beede on Tue Dec 16, 2014 3:31 am

    The easiest way to tell if you have a PAS-3X is to listen closely while you turn the bass or treble knobs away from the 12:00 position. If you hear a click or thump through your speakers you have a PAS-3X. The thump is due to the wiper recontacting the resistive material inside the pots.

    Bob Latino
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    Re: Who runs a PAS 3x in front of their ST70?

    Post by Bob Latino on Tue Dec 16, 2014 8:12 am

    If you are interested in rebuilding a PAS-3X > One of the nicest complete rebuilds of a PAS-3 was done by the late Bill Thomas in 2008. Bill posted a nice commentary on his rebuild with a lot of photos on the Audio Circle. It is at the link below ...

    Bill Thomas PAS-3X rebuild

    Bob

    tygr1

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    Join date : 2014-12-08

    Re: Who runs a PAS 3x in front of their ST70?

    Post by tygr1 on Tue Dec 16, 2014 8:36 am

    Very interesting read, thanks Bob.

    MexicoMike

    Posts : 28
    Join date : 2014-11-21

    Re: Who runs a PAS 3x in front of their ST70?

    Post by MexicoMike on Tue Dec 16, 2014 9:15 am

    j beede wrote:The easiest way to tell if you have a PAS-3X is to listen closely while you turn the bass or treble knobs away from the 12:00 position. If you hear a click or thump through your speakers you have a PAS-3X. The thump is due to the wiper recontacting the resistive material inside the pots.

    One of my treble pots does that, the other does not though they are the same part. Neither of the bass pots do that. I ASSUMED that since only one pot did it, that it was not working properly, doesn't seem like the thump in the speakers would be a good thing design-wise, but maybe that one is the only one working correctly! Wink

    GP49

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by GP49 on Tue Dec 16, 2014 2:59 pm

    The PAS-3x tone control knobs will rotate visibly less, stop-to-stop, than the balance control knob.  

    The rotation of the PAS-3 tone control knobs is the same as that of the balance control knob.

    dmag

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    Re: Who runs a PAS 3x in front of their ST70?

    Post by dmag on Sat Jan 31, 2015 4:33 pm

    I think I would be hesitant to mod a PAS in that condition. It is in much too nice original condition. Bypass the selenium rectifier as has been said, and a new quad cap.
    As mentioned in Bill's article the bass pot part number is 147754. The treble is147404.

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