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    Are these good mods for a PAS 3x?

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    jrethorst

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    Are these good mods for a PAS 3x?

    Post by jrethorst on Thu Sep 21, 2017 10:14 pm

    I have the chance to buy one with:

    The power transformer is a Custom Coil PA-211, manufactured as a higher capacity direct replacement for the original Dynaco transformer.

    The tube rectifier has been replaced with two 1N4007 diodes and the original four-section power supply capacitor has been replaced with two NOS two-section capacitors of higher value. The plate power supply resistors have been replaced with 3 watt metal oxide resistors of the original values. Plate supply voltage is about 10 volts higher than original when running on a 120V AC line.

    The filament supply has been replaced with an adjustable integrated (LM-317) DC regulator. I have set the filament voltage to the original 22 volts, but the filament supply is capable of up to 24 volts.

    Most resistors have been replaced with metal film or carbon film types of the original value. Most capacitors (other than in the power supply) have been replaced with polypropylene or polystyrene types of original value. The output capacitor is a polyester film type.

    Are these well-chosen mods?

    Thanks,
    John R.
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    Peter W.

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    Re: Are these good mods for a PAS 3x?

    Post by Peter W. on Fri Sep 22, 2017 4:35 pm

    jrethorst wrote:I have the chance to buy one with:

    The power transformer is a Custom Coil PA-211, manufactured as a higher capacity direct replacement for the original Dynaco transformer.

    The tube rectifier has been replaced with two 1N4007 diodes and the original four-section power supply capacitor has been replaced with two NOS two-section capacitors of higher value. The plate power supply resistors have been replaced with 3 watt metal oxide resistors of the original values. Plate supply voltage is about 10 volts higher than original when running on a 120V AC line.

    The filament supply has been replaced with an adjustable integrated (LM-317) DC regulator. I have set the filament voltage to the original 22 volts, but the filament supply is capable of up to 24 volts.

    Most resistors have been replaced with metal film or carbon film types of the original value. Most capacitors (other than in the power supply) have been replaced with polypropylene or polystyrene types of original value. The output capacitor is a polyester film type.

    Are these well-chosen mods?

    Thanks,
    John R.

    OK - I am walking into the world of *opinion* here. The PAS3X is >the< PAS to have, so you are off to a good start.
    The OEM transformer is a bit of a weak link and is purported to sag under some conditions. BUT - replacing the 12X4 with silicon diodes is pretty much sufficient to overcome this problem. Replacing the OEM carbon resistors is also a good idea.
    I am not sure where anyone got the idea that small-signal tubes *LIKE* being starved, or that starvation somehow extends their effective life. No, they do not like being starved. No, a decent 12A*7 will not suffer if run at its rated voltage - and I have examples that are now-50 years old with tens of thousands of hours on them that test just fine on my big Hickok. From the factory, that 22V was under load. I would suspect that the open-value would be somewhat higher (C.F. Transformer sag). Writing for myself, my filament supply is also a regulated circuit, and is operating at 24V - and has been for years. I would like to see how the other parts of the filament supply are managed.
    If one is going to replace the power-supply filter caps, I would strongly suggest that they all be individual caps. Dual-caps are still multi-caps.

    What you have gotten is a better transformer (Good) and no 12X4 (also Good). And the individual components (resistors/caps) replaced with better stuff (Good). The rest of it can be cured with any of several after-market options, all less than $50, all-in, that neatly address filament, bias and B+, and multi-section caps in a tidy single-board solution. And, remember, you are starting with the X-version, the last and rarest of the series. For that reason alone, it may be a good deal - depending on cost. Lastly - there is NO comparison between a standard 2 or 3 and the 3X. Full Stop.
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    j beede

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    Re: Are these good mods for a PAS 3x?

    Post by j beede on Fri Sep 22, 2017 10:00 pm

    Beyond the flawed tone control pots, 1uF tone control coupling caps, and "blend" control resistor values there is virtually no difference between a PAS-2/2X/3/3X. Full Stop.

    The PAS-3X is my least favorite version of the PAS. The self-defeating tone controls featured in the 3X are on my top ten list of least favorite electronic components. If you remove or electrically defeat the tone controls then a PAS is a PAS in my view.

    I think a stock PAS is a rather poorly executed piece of consumer electronics. It has no shortage of shortcomings. Consider:

    -The plated chassis and top cover corrode
    -The RCA jacks corrode and fail mechanically
    -The slide switches are a common source of signal degradation and failure
    -The input selector switch is a rats nest of cross coupled misery
    -The lack of a properly buffered output makes the stock PAS unsuitable for driving modern amplifier inputs
    -The stock circuit boards are subject to pad and trace failures
    -The tone control pots' (3X) self-defeating feature is failure prone and a source of noise

    The good news is that all of these shortcomings can be addressed thanks to the likes of www.dynakitparts.com and others. The question that begs to be asked is: why? I can think of several used preamps that be had for about $100 that I would happily use in place of a PAS.
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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Fri Sep 22, 2017 10:30 pm

    The Audio Regenesis studies of Dynaco's PAS really nailed it.  Have a look on the left side of this webpage: http://www.audioregenesis.com/

    The PAS needs to be viewed in light of the fact that it's something like fifty+ years old.  After fifty+ years, I also am corroding, failing mechanically, miserable and noisy.

    At the time the PAS was designed c. 1960, PC boards were pretty innovative in this kind of equipment, and they were probably limited by available PC board technology, at a reasonable cost anyway.  I have to believe that practical availability of the more robust, epoxy PC boards was a few years off, and Dynaco did eventually switch to them.

    There was an interesting discussion over at AK regarding the PAS filaments.  The thought was that the filaments were run at a lower voltage to reduce noise, also.

    With regard to running "modern" power amplifiers with the PAS, the PAS2X and PAS3X in stock form work fine into an amplifier input impedance as low as 100KΩ.  If you do a proper bypass of the tone controls on any PAS, the low frequency response is down by less than 1dB at 20Hz into an amplifier load of 20KΩ – something you don’t hear mentioned very often.  Otherwise, even if you want to keep the tone controls, adjusting a resistor at the outputs could still make it work into lower impedances - have a look here: http://www.audioregenesis.com/documents/PAS%20Compatibility%20Issues.pdf

    As far as the OP’s question, there might have been enough changes so that it may not still be a true Dynaco PAS.  But, if the changes were properly engineered and implemented, it could be fine.  I’d want full documentation on what was done.
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    Peter W.

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    Re: Are these good mods for a PAS 3x?

    Post by Peter W. on Sat Sep 23, 2017 6:55 am

    There is no, and never was a PAS2X.
    The PAS3X was the last in the line, produced for a very short time, and created to address mating issues with solid-state amplifiers. It also followed the same tone-control system developed for the PAT-series. The tone controls are 100% out-of-circuit when in the null (center) position.

    Further to this, there is the well-established "X-Mod" for all PAS models that address impedance-matching to solid-state amps. This is based on the stock configuration of the 3x, but does NOT make a 2 into a 2X or a 3 into a 3X. Refining this a bit further, if your PAS requires this mod, it is not an X-model.

    Cutting to the chase, there is NO REASON to bypass the tone controls in an stock 3X. I believe that he X-mod is explained in several Dynaco manuals, such as for the ST120 and ST80 for those of you with older PAS models.
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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Sat Sep 23, 2017 10:00 am

    Peter W. wrote:There is no, and never was a PAS2X.
    The PAS-2X was real and differed from the PAS-3X only in cosmetics.  The PAS-2X and PAS-3X had the complete so-called X-mod within them.  The PAS-2 and PAS-3 did not come with the X-mod, but it could be added, later.  Again, the Audio Regenesis studies are spot on - highly recommended reading.  Here is a screen shot of the cover page of an original Dynaco operation (and assembly) manual for the PAS-2X and the electrically identical PAS-3X:



    Last edited by PeterCapo on Sat Sep 23, 2017 12:02 pm; edited 1 time in total (Reason for editing : clarification.)
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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Sat Sep 23, 2017 12:01 pm

    j beede wrote:The good news is that all of these shortcomings can be addressed thanks to the likes of  www.dynakitparts.com and others. The question that begs to be asked is: why? I can think of several used preamps that be had for about $100 that I would happily use in place of a PAS.
    A fair question.  I would say that a properly refreshed and properly functioning Dynaco tube preamp, having the original Dynaco circuit, has a very sweet sound along with other goodies like space, imaging, detail, etc., that capture the high-end vibe.  In my experience, the PAS does not have to be seen as audio pablum suited only for newbies to cut their teeth on.  If you treat it right, and match it to your power amp (not hard), the PAS can be a fine-sounding preamp.  Of course, there is no accounting for taste, and even a properly functioning PAS may not be to everyone’s preference.

    Peter W. wrote:Further to this, there is the well-established "X-Mod" for all PAS models that address impedance-matching to solid-state amps. This is based on the stock configuration of the 3x, but does NOT make a 2 into a 2X or a 3 into a 3X.
    Dynaco's TC-3X kit did, in fact, turn a PAS-2 into a PAS-2X and a PAS-3 into a PAS-3X.  Again, the PAS-2X and PAS-3X were electrically identical.  

    Peter W. wrote:Cutting to the chase, there is NO REASON to bypass the tone controls in an stock 3X. I believe that he X-mod is explained in several Dynaco manuals, such as for the ST120 and ST80 for those of you with older PAS models.
    Well… after decades of use or abuse, the potentiometers may not be functioning very well.  For instance, a shaft can freeze off-center or there might be other problems that prevent their proper operation when centered.  In such cases, you might have to do the bypass procedure on a 2X/3X after all.  It doesn't make sense on the face of it, but I recall at least a couple of reports over time where 2X/3X owners did the bypass procedure and felt it improved the sound.  What to do with the old pots in these things is something I have given some thought to.  I think spraying them should be an absolute last resort - have a read through to the last post, here: http://audiokarma.org/forums/index.php?threads/dynaco-pas-3x-and-the-tone-control-modification.772264/#post-10575372

    The manuals for the solid state Dynaco amplifiers did not so much "explain" the X-mod as much as they mentioned the two different PAS circuits along with discussing compatibility of the Dynaco tube preamps and the possibility of lifting (or changing, or something) a resistor at the preamps’ output to make them compatible, if need be.  The Audio Regenesis studies revealed that fine tuning a resistor at the PAS output makes it more compatible with solid state amps than commonly believed.  But, you have to understand how it works, which the Audio Regenesis studies explained.
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    Peter W.

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    Re: Are these good mods for a PAS 3x?

    Post by Peter W. on Sat Sep 23, 2017 6:28 pm

    For the record, anything with a Powelton Avenue address would NOT have the X-type controls. Those came about starting in 1967, long after Dynaco had left Powelton and move to Jefferson Avenue more-or-less in Strawberry Mansion.

    Yes, Dynaco sold an X-kit with said controls in the after-market (much as they sold the FM-stereo adapter for the FM1. And, using that kit, a PAS2X would be electrically equivalent to a PAS3X after installation. But no PAS2X kits or "factory built" (another story, as Dynaco had no factory at all until Jefferson) were ever sold as-such. Doing the impedance-matching mod (AKA the "X Mod" did not include the controls.

    The Powelton address consisted of a 3-car service bay, and some small attached offices. Much as the Hafler Pennsauken address consisted of a 2-car service bay and some small attached offices. I have been to both. The Powelton Ave. address is now part of Presbyterian-HUP, Jefferson and Pennsauken are still standing.
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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Sat Sep 23, 2017 7:07 pm

    Peter W. wrote:Yes, Dynaco sold an X-kit with said controls in the after-market (much as they sold the FM-stereo adapter for the FM1. And, using that kit, a PAS2X would be electrically equivalent to a PAS3X after installation. But no PAS2X kits or "factory built" (another story, as Dynaco had no factory at all until Jefferson) were ever sold as-such. Doing the impedance-matching mod (AKA the "X Mod" did not include the controls.

    If no PAS-2X units ever left the factory either assembled or as a kit, then I do not know how to explain the manual I posted an image of (Post n°6).  Why would Dynaco specify the PAS-2X on the cover and elsewhere in this manual if they weren't actually selling the PAS-2X as such?  And, this manual does not include instructions for converting a PAS-2 to a 2X, and there are no electrical distinctions in this manual between the 2X and 3X.

    I do not understand the apparent distinction you appear to be drawing between what you refer to as X-kit with the controls vs. impedance-matching mod AKA the X Mod that did not include the controls.  Whether you call it X-kit or X Mod, it would have to mean setting up the tone controls and output like the PAS-2X/3X had it.  The TC-3X kit came with the pots and the 1.0µF capacitor and, I think, a couple of resistors.  How can you upgrade from a PAS-2/3 to a PAS-2X/3X without all the 2X/3X parts, which were also the reason for the increased compatibility with lower amplifier input impedance?

    When you refer to the impedance-matching mod AKA the X Mod without the controls, do you mean the instructions in different Dynaco manuals for lifting a resistor at the PAS outputs?
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    Peter W.

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    Re: Are these good mods for a PAS 3x?

    Post by Peter W. on Sat Sep 23, 2017 8:14 pm

    Please note the interpolations.

    PeterCapo wrote:
    Peter W. wrote:Yes, Dynaco sold an X-kit with said controls in the after-market (much as they sold the FM-stereo adapter for the FM1. And, using that kit, a PAS2X would be electrically equivalent to a PAS3X after installation. But no PAS2X kits or "factory built" (another story, as Dynaco had no factory at all until Jefferson) were ever sold as-such. Doing the impedance-matching mod (AKA the "X Mod" did not include the controls.

    If no PAS-2X units ever left the factory either assembled or as a kit, then I do not know how to explain the manual I posted an image of (Post n°6).  Why would Dynaco specify the PAS-2X on the cover and elsewhere in this manual if they weren't actually selling the PAS-2X as such?  And, this manual does not include instructions for converting a PAS-2 to a 2X, and there are no electrical distinctions in this manual between the 2X and 3X.

    Dynaco did many things. From Powelton, no X ever was made or sold as an entirety. Those came *only* from the Jefferson Street location. That is not to state that Dynaco did not plan the X (with controls) whilst still at Powelton, but to state that as of 1967, the debut of the X as a kit, they were at Jefferson by some time.


    I do not understand the apparent distinction you appear to be drawing between what you refer to as X-kit with the controls vs. impedance-matching mod AKA the X Mod that did not include the controls.  Whether you call it X-kit or X Mod, it would have to mean setting up the tone controls and output like the PAS-2X/3X had it.  The TC-3X kit came with the pots and the 1.0µF capacitor and, I think, a couple of resistors.  How can you upgrade from a PAS-2/3 to a PAS-2X/3X without all the 2X/3X parts, which were also the reason for the increased compatibility with lower amplifier input impedance?

    Again, Dynaco made, and named an *impedance-matching* modification for any PAS which in the after-market became the "X-Mod". What it did *electrically* was duplicate the output impedance of the very latest PAS versions (3X) so that an early version of same could match their latter-day 120s and 80s. This DID NOT include the null-circuit tone- controls.


    When you refer to the impedance-matching mod AKA the X Mod without the controls, do you mean the instructions in different Dynaco manuals for lifting a resistor at the PAS outputs?

    Exactly that. And that became known as the X-mod. X-Kits, sold very briefly, did include the null-controls. Whether these were sold prior to the true 3X is shrouded in clouded history, but is quite likely. What is clear is that 3X from-git-go never came out of Powelton. Keep in mind that Dynaco sold all sorts of cosmetic updates for the FM1, PAS2, and even the 70 (latter day Dynaco vs. Script Dynakit)). No clue as to why, other than, perhaps,  a few bucks, but they did.
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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Sun Sep 24, 2017 10:37 am

    Well, I guess this all sounds a bit mysterious.  Let it be as it may, but among whom did "X-mod" come to not include the special tone controls?  Reason I ask is because I think terminology like X-kit, X-mod, X-this, X-that will confuse people who cruise Dynacoland across the Internet in 2017 and beyond.  For instance, if I see X-anything in the context of a discussion about the PAS, the first things I picture are the special pots (and the 1.0µF capacitor)...
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    Peter W.

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    Re: Are these good mods for a PAS 3x?

    Post by Peter W. on Sun Sep 24, 2017 11:07 am

    PeterCapo wrote:Well, I guess this all sounds a bit mysterious.  Let it be as it may, but among whom did "X-mod" come to not include the special tone controls?  Reason I ask is because I think terminology like X-kit, X-mod, X-this, X-that will confuse people who cruise Dynacoland across the Internet in 2017 and beyond.  For instance, if I see X-anything in the context of a discussion about the PAS, the first things I picture are the special pots (and the 1.0µF capacitor)...

    One example: Please refer to the bottom of page 6 of the ST120 manual.

    http://akdatabase.org/AKview/albums/userpics/10004/Dynaco%20ST120%20Assembly_Owner%26%2339%3Bs.pdf

    That will help with timing and explanations.
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    j beede

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    Re: Are these good mods for a PAS 3x?

    Post by j beede on Sun Sep 24, 2017 11:28 am

    Regarding the tone control potentiometers used in the PAS-3X... I assume that when you describe them as "special" you really mean flawed. The only good thing about them is the click/thump they produce when rotated away from the twelve o'clock position that tells me that I am dealing with a PAS-3X and not a PAS-2 or PAS-3.

    Regarding the rarity of the PAS-3X... Really? At the risk of prolonging pedantic Philadelphian pedagogies... My perfunctory guess would be that the PAS-3X is the most pervasive (tube) preamp on the planet.

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    PeterCapo

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    Re: Are these good mods for a PAS 3x?

    Post by PeterCapo on Sun Sep 24, 2017 12:30 pm

    j beede wrote:Regarding the tone control potentiometers used in the PAS-3X... I assume that when you describe them as "special" you really mean flawed. The only good thing about them is the click/thump they produce when rotated away from the twelve o'clock position that tells me that I am dealing with a PAS-3X and not a PAS-2 or PAS-3.

    Regarding the rarity of the PAS-3X... Really? At the risk of prolonging pedantic Philadelphian pedagogies... My perfunctory guess would be that the PAS-3X is the most pervasive (tube) preamp on the planet.


    PAS-2X/3X added the 1.0µF capacitor that should prevent clicks/thumps, or so I understand.  Perhaps the 1.0µF capacitors need to be replaced?  The originals were electrolytic, and after many years may have failed.

    By "special," I refer to their construction, which allows them to be out-of-circuit when centered, as opposed to the standard kind of potentiometer that the prior PAS-2/3 used.

    Otherwise, what was originally wrong with the 2X/3X pots?  You mentioned in Post n°3 that you think they are prone to failure and a source of noise.  Do you mean they are flawed after forty to fifty years of use and possible abuse?  For instance, over the decades, they may have likely been sprayed with different chemicals that may have compromised the carbon track, compromised the shaft lube and likely damaged the phenolic substrate that the carbon track is deposited on.  Or, perhaps a kit builder dropped solder blobs into them.  If they were spared such things, it’s remarkable that there are plenty of reports online of original PAS pots still working after half a century...

    Peter W. wrote: One example: Please refer to the bottom of page 6 of the ST120 manual.

    http://akdatabase.org/AKview/albums/userpics/10004/Dynaco%20ST120%20Assembly_Owner%26%2339%3Bs.pdf

    That will help with timing and explanations.

    Thank you for the reference.  Yes, I have seen this in the different Dynaco solid state manuals.  I just re-read page 6 at the link you provided.  It discusses the PAS-2X and PAS-3X, and the special tone controls.  It then goes on to describe the procedure for impedance matching in the absence of the special tone controls.  But, it does not use the term “X-mod” in association with the impedance matching procedure.  In fact, it nowhere mentions the specific term “X-mod.”

    It’s unfortunate, given the popularity of the original Dynacos, that there is not a central repository of Dynaco history somewhere.  Greg Dunn’s website approaches this and has been helpful, but I don’t think it has been updated for a while and there may be error in the existing information.  Gary Kaufman had compiled a good deal of Dynaco information on his website, but it has gone with the wind.  

    Quoting from the Dynaco Stereo 120 power amplifier manual at the link you provided, on page 6, under the section titled “USE WITH DYNACO PREAMPLIFIERS,” please note the following (bold and italics added for emphasis):

    The ‘X’ suffix on the tube-type Dynaco preamplifiers refers to units manufactured after 1966, or prior versions which have been modified to conform to the latest tone control circuitry with the addition of the TC-3X tone control modification kit.

    The TC-3X kit included the special pots.  Not to belabor the point, but at least for the sake of newcomers to classic Dynaco, my suggestion would be to limit the use of the term “X” to refer to the complete changes that distinguished the PAS-2/3 from the PAS-2X/3X, and just call impedance matching impedance matching.
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    j beede

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    Re: Are these good mods for a PAS 3x?

    Post by j beede on Sun Sep 24, 2017 10:01 pm

    The PAS-3X tone control pots that I have encountered produce a click or thump when they are rotated away from the twelve o'clock position. On one occasion I observed this transient push an amplifier into instability on the bench. Other than that the pots are fine. Wink

    The spectral content of a pseudo step function like a click or thump is not likely to be filtered by a 1µF capacitor. That cap's function is to block dc when driving power amps having direct coupled inputs.






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