Dynaco PAS 3 Can Capacitor

bedrock602 · « **on:** 11 Mar 2020, 06:39 pm »

Hello to all. I'm new here but a longtime audio collector and tinkerer with absolutely no electronics training (and little knowledge of the subject

)

I recently acquired a Dynaco PAS 3 that was built as a kit. I brought it up slowly on a variac but none of the 12AX7's lit up, only the 12X4. I tested all of the tubes on my B&K tube tester and they all tested strong with no shorts. I also pulled the 2000 mfd capacitors out of circuit, one tested good, the other not so good. After doing some googling, it seems as though the selenium rectifier should also be replaced.

While I am in there, I'd like to replace the 4-section can capacitor which is 30/20/20/10 mfd @ 450V. The only recommended replacement I can find is 20/20/20/20 @ 475V from dynakitparts.com. Is this a good option?

I want to keep the preamp as designed, with no mods other than replacing the selenium rectifier with the 1N4006 diodes as recommended by Curcio Audio. I will also rebuild it from the ground up since the wiring and soldering of this kit was poorly done.

Thanks in advance.

dB Cooper · « **Reply #1 on:** 11 Mar 2020, 08:39 pm »

The cap you mention is probably OK especially if you parallel a suitable cap across the first stage.

Frank van Alstine also has extensive experience with this preamp and some simple cheap mods such as replacing the stock pilot light (which actually consumes as much energy as two of the tubes) with an LED, lessening the burden on the power supply and undersized power transformer. You're probably already aware that many mount the power former outside the back panel for reduced hum.

I can send a copy of his pilot light mod if you like.

bedrock602 · « **Reply #2 on:** 11 Mar 2020, 09:05 pm »

Thanks for the reply. Yes, please send the schematic of the pilot light mod.

I've read of mounting the transformer on the back of the chassis. I'm guessing that doing so would free up some space inside the chassis for new individual capacitors with the correct values to replace the can cap.

Mark Korda · « **Reply #3 on:** 11 Mar 2020, 09:44 pm »

Hi Bedrock, Go to You Tube and type in Dyna PAS D Labs. There is a guy who step by step installs a PAS board that fits right over where your selenium rectifier used to be. It also includes your electrolytic can assembly, it replaces both and gives you more capacitance.db Cooper described the small transformer which is explained in the video. I just bought one for my PAS from SDS electronics, Sheldon Stokes ,who has a great web site and a site for that board. I have all sorts of links and emails that could help you but my regular computer is down and this is a tablet I'm typing on my brother gave me. There is another Dyna site which sells Sheldons board if he does not have one. If I get my desk top computer going I'll forward these sites but right now you might have to Google them. I don't know how to forward with this tablet,(kordamark@gmail.com). Bedrock if you email me when I get the bigger computer going I'll forward everything you will need to make your PAS sing better than before.......Sincerely....Mark Korda

dB Cooper · « **Reply #4 on:** 11 Mar 2020, 11:11 pm »

Sheldon Stokes' web page Dyna stuff is at www.quadesl.com. The links are hard to find at first due to the design of the page but are on the upper right.

FullRangeMan · « **Reply #5 on:** 12 Mar 2020, 01:13 am »

Welcome

bedrock602 · « **Reply #6 on:** 12 Mar 2020, 04:29 pm »

Quote from: Mark Korda on 11 Mar 2020, 09:44 pm

Hi Bedrock, Go to You Tube and type in Dyna PAS D Labs. There is a guy who step by step installs a PAS board that fits right over where your selenium rectifier used to be. It also includes your electrolytic can assembly, it replaces both and gives you more capacitance.db Cooper described the small transformer which is explained in the video. I just bought one for my PAS from SDS electronics, Sheldon Stokes ,who has a great web site and a site for that board. I have all sorts of links and emails that could help you but my regular computer is down and this is a tablet I'm typing on my brother gave me. There is another Dyna site which sells Sheldons board if he does not have one. If I get my desk top computer going I'll forward these sites but right now you might have to Google them. I don't know how to forward with this tablet,(kordamark@gmail.com). Bedrock if you email me when I get the bigger computer going I'll forward everything you will need to make your PAS sing better than before.......Sincerely....Mark Korda

Thanks for the D-Lab suggestion, I've watched a number of his videos, they are very informative. I'm considering the SDS board

bedrock602 · « **Reply #7 on:** 12 Mar 2020, 04:38 pm »

Quote from: dB Cooper on 11 Mar 2020, 11:11 pm

Sheldon Stokes' web page Dyna stuff is at www.quadesl.com. The links are hard to find at first due to the design of the page but are on the upper right.

Thanks for the link. I like the simplicity of the mod, combining the can cap with the others. I'm curious about the capacitor values. The original can cap is 30/20/20/10 mfd but the SDS board has four 47 mfd caps. Also, the two caps that replace the 2000 mfd's are only 1800 mfd. Page 11 of the PAS 3 manual has a voltage chart, does this still apply with the new cap values of the SDS board?