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    Dynaco PAS restoration issues.

    PeterCapo
    PeterCapo

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    Post by PeterCapo on Sat Jul 04, 2020 9:18 am

    peterh wrote:I contacted the seller "re-engineer" and asked if the potentiometers in the
    pas potentiometer kit. Did they have correct values .
    The answer was not to my satisfaction :

    The question you're really asking is:
    Are they the same resistance as the
    originals.
    The answer is:
    If they were, there would be no need for the
    kit.
    Thanks for stopping by...C


    In other words he refuses to tell what he sells.
    Maybe better to skip this seller.

    Adnick wrote:I’ve used several of the Dynacodesign tone kits in PAS builds, they work and replicate they original circuit very well.....

    Does it appear as if the Dynaco Designs tone control circuit uses the same value potentiometers and associated parts as the original Dynaco circuit?  Not sure in what sense it "replicates" the original circuit, other than simply providing a tone control function.  If it uses different value parts, I don't see how it can truly replicate the effect of the original.  How "well" the Dynaco Designs tone control circuit works is a reasonable question.

    It's good to have options.  However, given that the performance of the PAS line stage depends on the value of the pots and their associated tone control parts, I'd really like to see complete line stage performance specifications published for an aftermarket tone control circuit change like this one.  It should then be vetted by an outside lab in order to understand how it alters the performance of the PAS line stage.

    For anyone who might be interested, here's an engineering study of the original PAS line stage that includes discussion of the original PAS tone control function http://www.audioregenesis.com/documents/PAS_Line.pdf

    With regard to changing the value of tone control circuit parts in the PAS line stage, notice comments in the last paragraph of the following post by Audio Regenesis https://audiokarma.org/forums/index.php?threads/dynaco-pas-tone-control-%E2%80%9Ctuning%E2%80%9D.919539/#post-13906489

    Quoting at this time from the post by Audio Regenesis at the link immediately above: "Even with non X versions, altering the values of the pots necessarily must alter the performance since the they, especially the bass pot, are an integral part of the unusual TC implementation. Any changes will also alter the very specific output loading requirements of the circuit as well as (likely) degrading other performance parameters."


    Last edited by PeterCapo on Mon Jul 20, 2020 10:14 am; edited 2 times in total (Reason for editing : add quotation, clarification)

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    rjpjnk

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    Post by rjpjnk on Sat Jul 04, 2020 10:20 am

    Agreed. That unusual tone control is both part of the negative feedback circuit and the output loading. Any change to the components (pots, resistors, or caps) is going to make it a different amp with different characteristics. It may sound great, but it will be different.

    Audio Regenesis did a Spice analysis of the circuit in that article. Someone familiar with Spice could put the values for the modified circuit in and see what it looks like. That would be relatively simple.

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    Adnick

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    Post by Adnick on Sat Jul 04, 2020 12:44 pm

    Used the term replicate and very well, very intentionally ;-) Could have used a term like, representative facsimile......Because, most people couldn’t tell the difference, with one stock PAS playing juxtaposition one with Dynacodesign tone controls.

    Said nothing about them being exact physical, technical, nor circuit copies, they are not.

    Seems I had a prophetic vision of the responses 0-:

    No it’s not the original, but if your original pots are dis-functional then it’s a viable option.

    In fact the capacitor selection for the treble is very similar to the Audio Regenesis suggested modification(?) ( Not sure who came up with the treble mods, but Audio Regenesis has recommended them as an option.)

    Again, my argument wasn’t to replace stock parts because the Dynacodesign parts were the same and didn’t change anything...

    However, if one has to turn the volume down for tone adjustments, then it’s time to fix the pots.


    Andy

    PeterCapo
    PeterCapo

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    Post by PeterCapo on Sat Jul 04, 2020 1:30 pm

    Adnick wrote: ... Said nothing about them being exact physical, technical, nor circuit copies, they are not. ... Again, my argument wasn’t to replace stock parts because the Dynacodesign parts were the same and didn’t change anything... ...
    That's what "replicate" means, Andy.  If you meant something else, others can still only go by the words you actually write.

    Adnick wrote: ... In fact the capacitor selection for the treble is very similar to the Audio Regenesis suggested modification(?)  ( Not sure who came up with the treble mods, but Audio Regenesis has recommended them as an option.) ...
    In the thread on another forum to which you refer, Audio Regenesis referred to values of .018uF for bass and .001uF/.01uF for treble as "a good starting point."  But, "a good starting point" to what?  Firstly, those values were in reference to the original tone control pots, not replacement pots of a different value.  Secondly, in that same post, he nevertheless issued his caution (that I quoted in Post n°51 of the thread we are in right now) about changing the values of the tone control circuit parts.  Unless I missed it, he didn't mention any exceptions to his caution.

    Adnick wrote: ... No it’s not the original, but if your original pots are dis-functional then it’s a viable option. ...  
    Well, it's an option.  The viability would, as I mentioned previously, preferably be borne out by some independent lab testing. This is a reasonable caveat emptor.
    peterh
    peterh

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    Post by peterh on Sat Jul 04, 2020 1:49 pm

    One could mention that the original pots ( talking about the non-X models ) can be
    substituted by standard 250k and 500k pots with some reduction of range.
    How ?
    take the treble pot, it's 400k according to documentation.
    replace with a 250k lin and add 75k at each end between the lugs on the put and the connections
    to the PAS. The total resistance across the pot is 400k, the negatve side is that the viper won't
    reach the ends of this 400k, it will only reach across the 250k.

    Same with the bass pots, here the original is 750k. Use a 500k pot, add 125k resistor at each
    end ( outer lugs on pot)

    250k and 500k LIN is more available, by using this technique nothing is changed in the way
    the PAS works, the only difference is somewhat reduced working range of the tonecontrols

    I have no solution for the -X pots however...
    PeterCapo
    PeterCapo

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    Post by PeterCapo on Sat Jul 04, 2020 4:01 pm

    peterh wrote:One could mention that the original pots ( talking about the non-X models ) can be
    substituted by standard 250k and 500k pots with some reduction of range.
    How ?
    take the treble pot, it's 400k according to documentation.
    replace with a 250k lin and add 75k at each end between the lugs on the put and the connections
    to the PAS. The total resistance across the pot is 400k, the negatve side is that the viper won't
    reach the ends of this 400k, it will only reach across the 250k.

    Same with the bass pots, here the original is 750k. Use a 500k pot, add 125k resistor at each
    end ( outer lugs on pot)

    250k and 500k LIN is more available, by using this technique nothing is changed in the way
    the PAS works, the only difference is somewhat reduced working range of the tonecontrols

    I have no solution for the -X pots however...

    Using that exact technique, I see what you are saying.  The question it brings up to me is how to know when the pots give flat response in this case.

    Reason I bring this up is because the original non-X pots, and they way they were oriented into the front panel, required a certain asymmetrical rotation of the knobs relative to the center mark on the face-plate in order to have flat response when the pot's indicator is aligned with the center mark.

    The Dynaco manual explained how to orient the tone control knobs so that you'd get flat response at the center positions even though the pots rotated asymmetrically relative to the center marks.  But, using a different value pot with fixed resistors added to both ends might alter this?  If this is the case, then it might only be possible to orient the knobs for flat response with the use of lab equipment.  Come to think of it, the same question would apply to other replacement tone control circuits, too.

    But perhaps adding the same value resistor to both ends of the pot wouldn't change the orientation of the knob for flat response. Not sure, but it is something that would need to be sorted out.
    peterh
    peterh

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    Post by peterh on Sat Jul 04, 2020 5:16 pm

    PeterCapo wrote:
    peterh wrote:One could mention that the original pots ( talking about the non-X models ) can be
    substituted by standard 250k and 500k pots with some reduction of range.
    How ?
    take the treble pot, it's 400k according to documentation.
    replace with a 250k lin and add 75k at each end between the lugs on the put and the connections
    to the PAS. The total resistance across the pot is 400k, the negatve side is that the viper won't
    reach the ends of this 400k, it will only reach across the 250k.

    Same with the bass pots, here the original is 750k. Use a 500k pot, add 125k resistor at each
    end ( outer lugs on pot)

    250k and 500k LIN is more available, by using this technique nothing is changed in the way
    the PAS works, the only difference is somewhat reduced working range of the tonecontrols

    I have no solution for the -X pots however...

    Using that exact technique, I see what you are saying.  The question it brings up to me is how to know when the pots give flat response in this case.

    Reason I bring this up is because the original non-X pots, and they way they were oriented into the front panel, required a certain asymmetrical rotation of the knobs relative to the center mark on the face-plate in order to have flat response when the pot's indicator is aligned with the center mark.

    The Dynaco manual explained how to orient the tone control knobs so that you'd get flat response at the center positions even though the pots rotated asymmetrically relative to the center marks.  But, using a different value pot with fixed resistors added to both ends might alter this?  If this is the case, then it might only be possible to orient the knobs for flat response with the use of lab equipment.  Come to think of it, the same question would apply to other replacement tone control circuits, too.

    But perhaps adding the same value resistor to both ends of the pot wouldn't change the orientation of the knob for flat response.  Not sure, but it is something that would need to be sorted out.
    A good point ! Didn't think of that.

    If you peter is in possession of a non-X a little exercise with a dvdm could solve and
    document this.
    Have the pots adjusted as per the manual. Then de-solder the outer taps and
    measure resistance between each outer tap and the viper. Note which side each
    measurment refers to ( to the left or right when looking from the front).

    These values would give resistor values to add at the left and right taps.
    The remaining issue is "where to document this". A good starting point might be
    to ask bob for a locked subject here.

    PeterCapo
    PeterCapo

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    Post by PeterCapo on Sat Jul 04, 2020 5:27 pm

    I hope to do this at some point.  But for now it will have to remain a thought experiment for me, as I need to repair my plumbing and my car, and several other things... too bad that life takes time away from fun stuff. Sorry.
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    Adnick

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    Post by Adnick on Sat Jul 04, 2020 7:36 pm

    Replicating the sound, sorry if that needs explanation, it’s clear the circuit values were not the replicated.

    Have you ever installed the Dynacodesign tone controls and/or compared them to stock?

    What’s your argument?  That the  Dynacodesign tone control replacements don’t work?

    Or is it that Dynacodesign tone  are so far from the stock sound that you don’t recommend them?

    Saying the Dynacodesig parts are questionable is unfair without personal experience...BTW, Dynacodesign is an Affiliate of Dynakitparts.


    Wish I had recorded my testing session, see photo, that’s one PAS with original tone controls and the other has the Dynacodesign tone controls...with turning knobs I couldn’t tell which was which playing through A35s powered by MKIIIs

    Unfortunately when one turned the tone controls on the PAS with original controls, the differences were painfully clear....unfortunately I’ve taken it apart. Dynaco PAS restoration issues. - Page 3 5b870f10







    Reference 250 vs 500k treble pots:

    Take a Dynaco 400k pot and test it against a new, 500k pot, there is very little difference in values....


    Have at least 6 good Dynaco 400k pots here and each one will hit 485-495k and even jump above that while being turned, and 2 of them are3x pots.  Believe in the discussion referenced Audio Regenesis makes a similar point about using 500k treble pots.

    Regards,
    Andy
    PeterCapo
    PeterCapo

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    Post by PeterCapo on Sat Jul 04, 2020 9:13 pm

    Adnick wrote: ... What’s your argument?  ...

    As always, what I have posted are my opinions as well as questions that in my opinion can reasonably be raised on the subject.  If you missed them, and if you are interested in knowing my opinions and my questions, then all you'd have to do is to go back and read posts I have already made.  If not, then you (and anyone else) are free to disregard my posts.

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