Trouble with my "new" PAS-3 Preamp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 17th May 2006, 01:01 AM   #1
Dylan is offline Dylan  United States
diyAudio Member
Dylan's Avatar
Join Date: Aug 2004
Location: Oregon, USA
Default Trouble with my "new" PAS-3 Preamp

Hello, I've been lurking here for quite a while, but haven't posted much at all. So here's a long one!

First off, I'm fairly new to tube hi-fi, and I recently bought a Dynaco PAS-3 preamp along with the SDS Labs filter capacitor/heater supply upgrade board. I figured this would be a nice affordable preamp to start with.

I gave it a test shortly after it arrived, and found that the left channel played while the right channel was nearly nonexistent (No, I didn't turn the balance all the way left! :P). Checking voltages on the tube pins showed that they were all over the place, so today I carefully installed the SDS cap board hoping that it would fix this.

I fired it up and nothing exploded!

However, the left channel is now heavily distorted and the right still won't work! Back I went with my multimeter to the tube pins. All were very close to what they should be, except for two pins on the right channel 12AX7 of Board PC-6.

Pin 6 should be 200V but reads 311V
Pin 8 should be 1.25V but reads 0.21V

The other tube's pins on the same board check out fine.

Troubleshooting began, and this is where my knowledge isn't so strong.

First, I inspected the board for shorted traces, bad joints, or solder crumbs and clippings from installing the SDS board. No problems. I pulled up one side of each of the resistors connected to the tube and tested them. All were within tolerances. All caps are ceramics and new orange drops, and all tubes had tested good for emission and shorts.

My SDS board installation was ruled out because it is giving me the proper voltage, and all the other tubes' pins check out fine.

I've hit a wall. I don't know if these voltages are causing my initial problem, but they certainly are well out of spec. I'd appreciate any input on this, I really want to hear it play.

I have a schematic of the pre-amp that has the voltages at all tube pins which I've been using for troubleshooting, so I can refer to it if anyone has suggestions.

Thanks for any help!
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Jung/Gallo "LT" preamp and op-amps... FidPup Solid State 2 1st March 2015 12:34 PM
Bad Tube in my "new" Hammond C2 Organ preamp? HELP w/ diagnosis!! jeffstev Tubes / Valves 10 24th November 2011 05:59 PM
What makes an amplifier "bright", "warm", or "neutral"? JohnS Solid State 51 13th December 2009 06:42 PM
D.I.Y. "Duplex" 2sk369 jfet mm phono preamp doggy Analogue Source 6 26th September 2006 08:23 AM
FS: Unfinished phono preamp w/ "audiophile" parts needtubes Swap Meet 1 5th July 2005 09:50 PM

New To Site? Need Help?

All times are GMT. The time now is 07:39 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2