My "new" Dynaco PAS-2 preamp sounds horrible!

[jpm-boston]

I just bought an unmodded Dynaco PAS-2 for my office system at work from a used stereo store in Boston. I've purchased equipment from this guy before and have always been satisfied. However, this Dynaco sounds tinny, like an AM radio. I used to have one of these models before so I know how to hook it up with my Rotel amplifier, but this one sounds like crap. I can take it back and get my money back, but does anyone know what would make it sound so crappy?

[CardinalFang]

If nobody can help you here, try AudioKarma.org:
http://www.audiokarma.org/forums/index.php

[Tony Caldwell]

I just bought an unmodded Dynaco PAS-2 for my office system at work from a used stereo store in Boston. I've purchased equipment from this guy before and have always been satisfied. However, this Dynaco sounds tinny, like an AM radio. I used to have one of these models before so I know how to hook it up with my Rotel amplifier, but this one sounds like crap. I can take it back and get my money back, but does anyone know what would make it sound so crappy?

Take it back and have them test it out. Something is definitely wrong if it sounds like AM radio.

Good luck!

[TONEPUB]

Agreed. Could be really dud tubes, could be a capacitor problem.

That is a very rich sounding preamp....

Hope you get it fixed so you can enjoy it!

[indy mike]

Might have a weak selenium rectifier (common problem on PAS preamps - usually replaced with diodes), as well as the above mentioned bad/weak caps and tubes.

[www.records]

I can't help, but if it was mine I would be searching and asking questions here-

http://www.audioasylum.com/cgi/tv.mpl?s=1&v=e&forum=dynaco

[indy mike]

Joe Curcio helps troubleshoot Dynaco gear at the Asylum; you can also ask a few questions at his site: http://www.curcioaudio.com/dynadr_3.htm

[jpm-boston]

Thanks to all for your comments and suggestions. I'm taking it back today to get it checked out. The gentleman I purchased it from said that he can either fix it for me free of charge or refund my money.

[indy mike]

Thanks to all for your comments and suggestions. I'm taking it back today to get it checked out. The gentleman I purchased it from said that he can either fix it for me free of charge or refund my money.

That's good to hear - either way you come out ok on the purchase. :)

[TONEPUB]

Good luck!

[morinix]

Bad capacitors - that's my vote

[Sckott]

Gut says:

If you're buying older tube electronics online, it doesn't matter if you've got a guarantee or not. In the near future, you might need to do some tweeking yourself. Bad tube sockets, noisy caps, brittle power cords?

The pre is potentially 30-40 years old. Much like replacing the roof every 20-30 years, this stuff needs attention and even some careful replacement parts if it hasn't already.

Being a tube enthusiast and not wanting or being able to solder is an expensive and lonely affair. Be prepared to have someone else handle it for you.

[XMIAudioTech]

Might have a weak selenium rectifier (common problem on PAS preamps - usually replaced with diodes)


I do believe that the PAS uses a 12X4 tube for its rectifier, and a pair of silicon diodes for the heater supply, unless the really old ones had seleniums in the heater circuit...

-Aaron

[indy mike]

I do believe that the PAS uses a 12X4 tube for its rectifier, and a pair of silicon diodes for the heater supply, unless the really old ones had seleniums in the heater circuit...

-Aaron

The selenium rectifier sits on top of a pair of 2000 mfd caps leading to the transformer. It's part of the heater supply as you mentioned. Rebuild sites usually recommend ditching the old selenium part and replacing it with diodes. Here's a link to Joe Curcio's site detailing the steps used to switch out the selenium part: http://www.curcioaudio.com/PAS2-3X%20Fil%20Rect%20Replacement.pdf

I repair old Lionel trains, and they used selenium rectifiers as part of the horn/whistle activating circuit in their transformers. After 40 or 50 years they've typically gone bad and need to be replaced with heavy duty diodes, too. :)

[KenJ]

I had a PAS 2 and matching Dynaco amp recently. I found the vintage, stock PAS pre-amp not to my liking...to a lesser degree same for the stock amp. The amp, however, with new circuit design and parts sounded awesome...they have good iron which is all I used. I didn't feel the PAS pre-amp had much to offer even for restoration. I also felt like this old gear was moments from failing...too old...needs a good restoration like a car.

I think I would rather pursue a value low priced new tube pre than try to restore the PAS (perhaps a Bottlehead kit)?


For value in tubes I really like the new integrated $600 or so Onix/Melody SP3 and there are other integrateds that aren't too much.


I've also used some restored classic 40 year old SCOTT boxes....a tweaked and restored interated Scott amp would be of interest but running them stock and vintage right out of the attic is sure to have static controls and potential failure.

[Hegeman's Ghost]

My primary preamp is a modified PAS 2. Mods included replacing the selenium rectifier, increasing the power supply capacitance and removing the tone control circuit completely. I also replaced all of the coupling capacitors and checked the values of all resistors.

Power supply up grade kits are available at www.triageaudio.com or at www.triodeelectronics.com.

Before the upgrade I was underwhelmed by the performance of the PAS 2.....after the mods, WOW!

Below are two pictures from two different preamps which show the chassis before and after installation of the power supply mod. This mod gives you the option of using solid state rectification or maintaining the stock tube rectifier. It also includes a fuse, which believe it or not, the original preamp does not have!

HG :cool:

[jpm-boston]

I think I figured out what the issue was. I brought the Dynaco unit back from the repair shop and hooked it up to my Rotel RB-1080 amplifier and Linn Katan speakers and it still sounded like crap, and this was after it was examined and tested. I then tried hooking it up by putting a Musical Fidelity X-10v3 tube buffer between the Dynaco Pas 2 and The Rotel amplifier. After I did this it sounded great. I think the issue was maybe an impedance mismatch? Why would the tube buffer solve this?

[jpm-boston]

Does it make sense to have to add a tube buffer to enjoy a used 40 to 50 yr. old pre amp whose time in the sun has long since passed?

It probably doesn't make sense - but it does sound pretty good - an upgrade over the NAD 162 preamp that I was using. I only tried the tube buffer (I was using it for my cd player) when I couldn't figure out why the preamp sounded so tinny. I think it was definitely an impedance mismatch.