Pas-3 Trouble

[pjkkaf]

I have had this modified Pas-3 preamp for several years without any issues. Recently there has been troubles. When the unit is touched or vibrated it sends a hollow "tubey" hi frequency ring through the speakers. Any trouble shooting suggestions would be appreciated.

 

[tubeampking]

Sounds like a tube went microphonic. Try swapping them out one at a time.

 

[Son-of-Vere]

Ditto. I had an original dynaco tube do that as you described. I could tap the rack it was sitting in and hear it.

 

[gadget73]

Could also be a dirty socket or bad solder joint. I've had both cause this sort of thing as well as tubes going funny. usually tubes don't suddenly develop this issue unless they have been jarred or something though.

 

[pjkkaf]

Thanks guys. I'll roll the tubes, hoping I have spares.

 

[Pio1980]

Take the cover off, turn everything on low, and tap around with a chopstick to isolate the source. If the noise occurs in Tuner or CD or other high level line input, it's probably something on the front circuit board. If only phono, it's probably on the board on the left.

 

[Retrovert]

As Gadget noted, corrosion sometimes does this.

The other thing to check for when poking around with a chopstick is a bad ceramic capacitor. The barium titanate used can be microphonic.

 

[Rich Davis]

The PAS has to be the most modified unit in history. Since most were sold as kits the insides look everything from beautiful to horrible. Feel free to go in and re-do everything you don't like. And they sell new boards, too.

My problem with the PAS series is one of the stages runs at 'starvation' to increase gain. Replacing the 470k plate resistor with one of the low noise ones quiets the unit down, and if you want to up the plate current from its tiny fraction of an mA, you will get a more "open" sound in the linestage.

The power trans is delicate, rated for only 10 mA. A shorted tube or a careless poke and it's all over. If the fuse is blown you can bet it's a goner. They're available, too, but not cheap.

You might find a cold solder joint or two with your poking around.

I would replace the RCA jack strips with new ones, too, if you want to keep and use the preamp. They make nice gold plated ones. While you're in there, you can replace all the ceramic disks with silver mica and I think you'll hear a little improvement.

As good and popular as the PAS was in its time, it's easy to get better sound from most of the newer units - but they're fun to play with and they teach you a lot.

 

[pjkkaf]

I replaced the 12ax7 tubes last night, problem solved! Thanks for the all the advice.