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What was done to this Dyna PAS (1)?

Benz

Benz

Yeah Baby!
Greetings, I just picked up a nice Dyna PAS + Dynakit ST-70 combo (my first tube gear, finally). Both units have been restored but I was wondering if the experts here could help me figure out what was done to the PAS as the previous owner wasn't a tech guy and only knew it had been restored when he bought it. From the looks of it, most of the internals have been replaced... the thing looks very modern inside, and brand new. It sounds really good, except for the volume pot which has a balance issue when going from say 9 o'clock to 11 (goes from right to center). Anyway, here are some pics. Looking forward to learning more about what's been done to it. Cheers
 

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Board sitting where the tube rectifier used to live appears to be a solid state HV supply, diodes and caps on the board. Rear area where the heater supply used to be looks like a regulated heater supply. Not really sure why there is a need to regulate that, but hey whatever. Walls around the line stage probably to reduce hum or EMI or alien incursions.

rest of it looks like fresh parts on the old circuit board. Generally a good idea, the resistors and caps drift after half a century and need some refresh.

Volume tracking problem is likely just a junk volume pot. Mine was similarly bad, and a lot of people have the same issue. Fix is a replacement volume pot.
 
Hey thanks for the quick reply, you've actually given me some homework to do as I'm still learning lots about this preamp.
 
I assume this is what a stock unit is supposed to look like?
 

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Looks like it has the "3X" mod output mod with 1uF film caps in the outputs to prevent DC pass through to the amplifier. The pic in post #4 does look like a stock unit--PAS2 or PAS3. A true 3X will have the tone pots which disengage in the center/neutral position. Nice looking work.
 
Ok, I suspected as much. That's definitely nice to have. Meanwhile I'm reading up on the pros and cons of going with a SS rectifier vs tube rectifier (based on gadget,s comments, this is what was done to the PAS)... boy what a can of worms that whole question is. It reminds me of the neverending "what kind of engine oil is best" debates...
 
yup, pic in #4 is a stock one. The green stuff in the middle is the DC heater supply, stuff lower right corner is the high voltage with the tube rectifier and the original multi-cap can.


endless arguments about what is better, mine is solid state because the 12x4 was missing and most of the power supply boards that give you the caps also have the option of installing diodes. I could have bought a 12x4 and just used the board for the caps but I had the diodes on hand already.
 
Odd that whoever rebuilt it didn't upgrade the volume pot - looks original to me. If that's all that's wrong it's an easy fix.
 
gadget73 said:
yup, pic in #4 is a stock one. The green stuff in the middle is the DC heater supply, stuff lower right corner is the high voltage with the tube rectifier and the original multi-cap can.


endless arguments about what is better, mine is solid state because the 12x4 was missing and most of the power supply boards that give you the caps also have the option of installing diodes. I could have bought a 12x4 and just used the board for the caps but I had the diodes on hand already.
Click to expand...

Great, it's helpful to understand exactly "what replaced what". From my brief readings, if implemented correctly a SS rectifier can help bass and otherwise not detract from the tubey sound. Plus the PAS feeds a ST-70 which is tube rectified so I imagine most of the character remains. The combo sure sounds very, very good to me. I have to admit that I was taken aback by how fast the bass sounds. This may help explain it.

daveshel said:
Odd that whoever rebuilt it didn't upgrade the volume pot - looks original to me. If that's all that's wrong it's an easy fix.
Click to expand...

Yes, it's the only thing that bugs me and I will consider replacing the pot. Are originals still available or will I have to use a different type? I probably won't do it right away but it'd be good to know the best component to use so I can note it for later.
 
The original one wasn't anything special. You can get one like it from dynakitparts.com but I'd go for a better one, maybe a Noble. I used a stepped attenuator on my last build but I can't recall where I got that.
 
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daveshel said:
The original one wasn't anything special. You can get one like it from dynakitparts.com but I'd go for a better one, maybe a Noble. I used a stepped attenuator on my last build but I can't recall where I got that.
Click to expand...
Thanks for the reference, it doesn't look like they have PAS volume pots in stock but I may have messed up the search. Found this on ebay, appears to be a reasonable OEM-like replacement but pricey: https://www.ebay.com/itm/164996959993 . I'll investigate non-OEM options as you suggest.
 
So far as I can tell, based on answers here and some research, it looks like my franken-PAS has had the following done:
  • solid state rectifier and high voltage power supply board
  • regulated heater supply
  • tone bypass mod
  • most/all caps replaced
  • "3X" mod output mod with 1uF film caps in the outputs to prevent DC pass through to the amplifier
  • protection fuse
  • "isolation" dividers (for lack of a better term)
  • new PA-522 transformer
I'm also seeing two extra red/orange flat caps on the underside that don't match pictures of examples I've seen so perhaps those have been added in as well, for which purpose I don't know.

One thing is certain, someone spent a pretty penny upgrading / modifying this unit. Probably can't truly be called a PAS anymore!
 
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It is still a PAS if it's in a PAS chassis.
They were kits originally and got all tricked out by the builders... That's always been the point. A kit that gets built to whatever standard the builder is ok with.
As for a volume control, I spend about $10-$15 on a good carbon volume pot.
You can spend a lot more than that if you want to, but to get a good quality part should be about $15. Advertising/ marketing wank will add to that figure..

Alps blue at Mouser last time I checked was ~14 and change...
 
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If you want to retain the loudness yes. Those wires were already hacked out of mine when I got it so I bought the more common 3 terminal pot.
 
It's the loudness tap that makes the eBay pot expensive (not that anyone should actually spend that much). You'll probably need to use a non-tapped pot and leave the Loudness switch non-functional. Looks like someone cared about this preamp enough to maintain it well. Parts that commonly fail have all been replaced, except the switches and controls.

Jack
 
Only two of my tube amps have tube rectifiers. Even my tube preamps have SS rectifiers. There are pro's and con's for using either. But when it comes to hifi, is voltage sag really something you want? If your worried about a slow or soft start, it can be done with SS rectifiers, although needing some extra bits installed.
 
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