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Need help with Dynaco PAS2 Preamp rebuild

BmWr75

BmWr75

Top octave? pfffffftttttt
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Hello AKers,

I picked up a PAS2 a few months ago. It works, but doesn't sound very good. I read a while back that there are a few partsthat can be upgraded and freshen up this tired preamp. Can't find that info now though.

Any recommendations on common upgrades for this preamp are welcome. Links to info will be greatly appreciated.

Thanks, BmWr75
 
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As Grateful said the filament supply is a good place to start.
Here's my list of things done which improve (drastically) sonics on the PAS.
Filament supply replacement with 2200uF caps and 1n4007 diodes. Instructions can be found at Dynaco Doctor.
Replace the volume pot with 250k alps pot.
Replace the can cap---I usually fashion a replacement on breadboard using Panasonic Radials and poly bypass caps. This can be made to fit in behind the power switch. Or you can buy one from Triode that will handle the can cap and the filament supply together. Probably a good way to go if your not too experienced.
Replace and upgrade the coupling caps on the boards to MAX .47 (I've tried higher values with unsatisfactory results).
Remove the tone controls---this does amazing things for sound quality but you have to try it to believe it.
Spec out all resistors and replace as necessary.
Disconnect the power indicator lamp from the rectifier, this draws current from an already anemic supply.
I usually re-flow all solder connections and most of the time I rewire the entire unit.
Curcio's mod is good according to some, but what you end up with is no longer a PAS pre-amp and for my money (adding together the cost of a PAS and the Curcio mod), I'd buy a better pre-amp.
The PAS can be made to sound very good, but will never be world class. I've gotten the line stage to sound excellent...the phono-stage has its flaws by design, and not much can be done to truly improve it's weaknesses.
Just my 2 cents!
 
How does it sound bad? Hum, crackle, buzz etc. You might start by cleaning switches, controls and tube sockets. The PAS is very easy to work on.
 
audiodyssey said:
As Grateful said the filament supply is a good place to start.
Here's my list of things done which improve (drastically) sonics on the PAS.
Filament supply replacement with 2200uF caps and 1n4007 diodes. Instructions can be found at Dynaco Doctor.
Replace the volume pot with 250k alps pot.
Replace the can cap---I usually fashion a replacement on breadboard using Panasonic Radials and poly bypass caps. This can be made to fit in behind the power switch. Or you can buy one from Triode that will handle the can cap and the filament supply together. Probably a good way to go if your not too experienced.
Replace and upgrade the coupling caps on the boards to MAX .47 (I've tried higher values with unsatisfactory results).
Remove the tone controls---this does amazing things for sound quality but you have to try it to believe it.
Spec out all resistors and replace as necessary.
Disconnect the power indicator lamp from the rectifier, this draws current from an already anemic supply.
I usually re-flow all solder connections and most of the time I rewire the entire unit.
Curcio's mod is good according to some, but what you end up with is no longer a PAS pre-amp and for my money (adding together the cost of a PAS and the Curcio mod), I'd buy a better pre-amp.
The PAS can be made to sound very good, but will never be world class. I've gotten the line stage to sound excellent...the phono-stage has its flaws by design, and not much can be done to truly improve it's weaknesses.
Just my 2 cents!
Click to expand...

If you want a power light, use an led like Audio by VanAlstein does. I like green. If you want to really improve it and stay with the basic layout, try the Audio by VanAlstein Super Pas III kit. It also give you a great phono section.

Russellc
 
grateful said:
How does it sound bad? Hum, crackle, buzz etc. You might start by cleaning switches, controls and tube sockets. The PAS is very easy to work on.
Click to expand...

Grateful,

I'm glad you asked that question. Got me to thinking.....I only listened to this preamp through a Sonic T-amp and small set of Mirage Omnisat speakers. Not a fair test at all. I hooked it up to a Marantz 140 amp and set of JBL L112s. Had to play with the tone controls a little to get the treble and bass sounding decent. But, I'm glad to say it actually sounds pretty good hooked to decent system. No unwanted noise of any kind in either channel. :beerchug:

Thanks much for asking the question.
 
I've put in the power supply board and removed tone controls and lamp. First thing I did was to remove the bulb and I could tell right off in the bass department. I'm considering new caps for the signal boards but might just get a whole new up to date phono section for it. Good luck/
 
Saves a lot of power supply current. The lamp draws nearly 1/2 amp. Fillament voltage will rise 1/2 volt with the lamp out. Fillament supply is a marginal design
 
Hum. I wanted to take it out just so I could see the tubes glow better! lol Now I have a legit reason! The LED idea is good, I suppose.
 
Fast_Eddie said:
Hum. I wanted to take it out just so I could see the tubes glow better! lol Now I have a legit reason! The LED idea is good, I suppose.
Click to expand...

It works very well, and fits perfectly in the hole!:yes: draws little or nothing powerwise.

Russellc
 

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grateful said:
or spend $10 on parts at radio shack and rebuild the power supply. remove the lamp also
Click to expand...

Radio shack does not have large enough caps in sufficient high voltage to rebuild the power supply, at least in the ones around here. You could likely get parts there sufficient to do the filament supply. These are 250 and 450 volt Panasonic tsu caps.

You'll not find this at radio shack

Russellc
 

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grateful said:
Saves a lot of power supply current. The lamp draws nearly 1/2 amp. Fillament voltage will rise 1/2 volt with the lamp out. Fillament supply is a marginal design
Click to expand...

The #53 light bulb draws 120 milliamps, not quite 1/2 amp.

Replace the selenium rectifiers with a 1N4001 or higher value, that will bring your filament voltage up a bit.

Sal
 
Russellc said:
If you want a power light, use an led like Audio by VanAlstein does. I like green. If you want to really improve it and stay with the basic layout, try the Audio by VanAlstein Super Pas III kit. It also give you a great phono section.

Russellc
Click to expand...

I did the original Super PAS upgrade, and I use it occasionally. It still sounds quite good.
 
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