Dynaco PAS restoration issues.

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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.

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

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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...

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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.

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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.







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