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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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

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    tygr1


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    Post by tygr1 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.
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    Post by Bob Latino 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
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    Post by tygr1 Sun Dec 14, 2014 5:52 pm

    Thanks Bob, that really brightened up my day, LOL.
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    Post by Guest Sun Dec 14, 2014 10:55 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 4:11 pm; edited 2 times in total
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    MexicoMike


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    Post by MexicoMike 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
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    Post by GP49 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
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    Post by tygr1 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.
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    Post by Guest Mon Dec 15, 2014 1:55 am

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

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


    Will do. Should have it later today.
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    Post by tygr1 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?



    Who runs a PAS 3x in front of their ST70? DSC_0001_zps00dc3fd5


    Who runs a PAS 3x in front of their ST70? DSC_0003_zpsb7cac54c


    Who runs a PAS 3x in front of their ST70? DSC_0004_zps60558dcd
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    MexicoMike


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

    Thanks Mike. I think I'd better check the knobs.
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    Post by MexicoMike 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
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    Post by tygr1 Mon Dec 15, 2014 7:45 pm

    I'm listening to it as I type. It actually sounds quite nice.
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    Post by Guest Mon Dec 15, 2014 7:56 pm

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    Post by tygr1 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.
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    Post by 10-E-C 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.

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    Post by j beede 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.
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    Post by Bob Latino 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

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

    Very interesting read, thanks Bob.
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    Post by MexicoMike 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
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    Post by GP49 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.
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    Post by dmag 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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