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    Are these good mods for a PAS 3x?

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    jrethorst


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    Post by jrethorst Thu Sep 21, 2017 11:14 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.
    Peter W.
    Peter W.


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    Post by Peter W. Fri Sep 22, 2017 5:35 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.
    j beede
    j beede


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    Post by j beede Fri Sep 22, 2017 11:00 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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    Post by Guest Fri Sep 22, 2017 11:30 pm

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    Peter W.
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    Post by Peter W. Sat Sep 23, 2017 7:55 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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    Post by Guest Sat Sep 23, 2017 11:00 am

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:41 pm; edited 2 times in total (Reason for editing : clarification.)
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    Post by Guest Sat Sep 23, 2017 1:01 pm

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    Peter W.
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    Post by Peter W. Sat Sep 23, 2017 7:28 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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    Post by Guest Sat Sep 23, 2017 8:07 pm

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    Last edited by PeterCapo on Thu Dec 03, 2020 11:42 pm; edited 1 time in total
    Peter W.
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    Post by Peter W. Sat Sep 23, 2017 9:14 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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    Post by Guest Sun Sep 24, 2017 11:37 am

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    Peter W.
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    Post by Peter W. Sun Sep 24, 2017 12:07 pm

    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.
    j beede
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    Post by j beede Sun Sep 24, 2017 12:28 pm

    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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    Post by Guest Sun Sep 24, 2017 1:30 pm

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    j beede
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    Post by j beede Sun Sep 24, 2017 11:01 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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