Dynaco PAS 3X and the Tone Control Modification

[BassKulcha]

I get that there seems to be some real performance benefit in bypassing the tone controls in the Dynaco PAS2 and PAS3, but what about the PAS 3X?

This PDF from Pacific Audio Regenesis clarifies the benefits to the PAS2/3, but I'm still confused as to the benefits to the PAS 3X...

"If you do not use them, bypassing the tone controls is, perhaps, the simplest modification you can make to your PAS 2/3 that results in a measurable, and often audible, improvement in performance. The primary benefits include increased drive capability and improved linearity. As well, the already very low distortion of the PAS 2/3 is further reduced at the low end of the audio band...

....Note that the PAS 3X incorporates special tone control potentiometers which are automatically bypassed when set to the center position."

Does this mean that the 3X is the standard to which the 2/3 mods try to achieve? Am I missing something obvious, here?

http://www.audioregenesis.com/documents/PAS Tone Control Bypass.pdf

 

[airtime]

If you retain them for tone control, then the audio signal is going through grimy, half century old carbon tracks. I’d want to get rid of them for the same reason we use upgraded volume pots, say, built with individual resistors, the idea being a supposedly cleaner signal path. But, that’s just IMO.

I agree that if any thing needs to go it's the volume pot. Followed by those tone pots. If you're willing to spend a few bucks these are the way to go. The bigger benefit is you can leave the design virtually stock. Which is "a good thing".

http://www.goldpt.com/prices.html

 

[peterh]

Why not test a bypass switch for the PAS altogether ? With a suitably high-level input ( CD ?) one could
compare the sound direct with the sound that goes through the PAS
All that is needed is a stereo signal selector and 2 rca splitters.
Connect the source to the splitter, one path goes to the pas, the other
to one input of the selector. The other selector input connects to the PAS output.
With the CD active adjust the volume on the PAS to be equal to the bypassed signal.
Have someone choose either source ( direct or through PAS) without showing
and try to hear differences, as a normal blind A/B test.

 

[BassKulcha]

If I correctly understand the Audio Regenesis study, I believe bypassing the tone controls equals the drive capability of the X-mod PAS with tone controls centered.

This is what I was assuming as well, thanks for the clarification—

I'm inclined to just clean the pots as best I can and then leave them in the detent position most of the time. I primarily listen to records (mostly mono LPs, though some early stereo as well—) so I may end up needing to tweak the tone controls every once in a while to get the sound I want. I have an entire Dynaco tube system that I'm slowly bringing back to life, with an ST-70 driving a pair of A-25 XLs (and a pair of A-50s when my wife isn't around ) I think I'm ok without the mods at this stage of my audio education...

Cheers!

 

[BassKulcha]

Others may disagree, but when we talk about "cleaning" old pots, I do not think spraying some chemical into them is the best idea. AFAIK, those old pots have the carbon resistance track deposited onto a phenolic substrate. Phenolics and chemicals do not go well together. Chemicals can take up residence in the phenolic and/or destabilize it. I opened an original bias pot from a Stereo 70 once and found that there was a bunch of carbon dust. IMO, cleaning these old pots may best be limited to alternately blowing them out with compressed air and vacuuming them out with some kind of implement.

Interesting—that's really useful information, thank you—

So...Deoxit might not be the best solution in this case? How would you suggest dealing with 50-year-old oxidation when the substrate is a phenolic, outside of compressed air and vacuuming (or is that the only stable solution?)

 

[BinaryMike]

Others may disagree, but when we talk about "cleaning" old pots, I do not think spraying some chemical into them is the best idea. AFAIK, those old pots have the carbon resistance track deposited onto a phenolic substrate. Phenolics and chemicals do not go well together. Chemicals can take up residence in the phenolic and/or destabilize it.

Caig claims "excellent" plastics compatibility. From the DeoxIT product information sheet:

"CAIG is committed to insuring product compatibility with new and existing materials and products. To accomplish this, we invest considerable resources and time working with hundreds of companies in many industries to insure that our products perform exactly as promised."

"These products are initially non-conductive liquids. When subjected to an electrical field, DeoxIT® D-Series and DeoxIT® Gold G-Series (ProGold) promote current flow. No short circuits will occur. In both states, DeoxIT® D-Series, DeoxIT® Gold G-Series (ProGold) and DeoxIT® Shield S-Series (PreservIT) have resistances high enough to prevent short circuits."

I have no affiliation with Caig Labs, but these claims ring true in my experience. Do you have evidence to the contrary?

 

[BassKulcha]

From the CAIG website:

http://store.caig.com/s.nl/ctype.KB/it.I/id.316/KB.215/.f

"Phenolic surfaces may absorb fluids depending on how porous they are. It is very important not to apply any polar (conductive) liquids onto the surface. For example, no solvent that has alcohol in it.

The alcohol may potentially be absorbed into the phenolic material and may not completely evaporate. Then the phenolic itself may become conductive, causing the switch to short.

The DeoxIT D5 (flushing, slow drying version) or DN5 (quick drying, non-drip version) would be fine for the CONTACT AREAS of the switch. If a little gets on the surrounding area, it should not be a problem, but you would NOT want to saturate the adjacent area."

At first glance, this would appear to confirm @petercapo 's post.

@CAIG Labs , is this the case?