thelastdeadmouse Posted May 23, 2009 Share Posted May 23, 2009 The Valve Junior Head I bought has recently stopped working, and before I let my brother the electrical engineer inside to play I wanted to make sure its the version I think it is and I'm providing him the correct schematic. Bought summer 2007, 14 digit serial number, no "IO" on the power switch. That makes a v.3, which is the second version of the Head, which uses the following schematic? http://i166.photobucket.com/albums/u103/cgil155/EVJr_schematic_V2b.jpg If I'm incorrect please let me know, I haven't keep up with this type of info since fall 07. Also, assuming I've blown a tube, is it recommended I just replace the stock one or is an upgrade in order? Link to comment Share on other sites More sharing options...
cGil Posted May 23, 2009 Share Posted May 23, 2009 That schematic has the v3 parts values, but we're only talking about a couple of components that changed (R1, R8, & R9), so it'll do fine for modding version 2 or 3 VJr's. Your amp is definitely a version 3 VJr if the label on the OT says it's 5.2k. And if it has that black pc board inside. If so, all the mo bettah for you and your tone. Ask your tech to lower the el84's plate & screen voltages, and get bias under control. The amp will continue to chew through power tubes if you don't. Meanwhile, tubes are like light bulbs. They only blow when you need 'em, so get a bunch. JJ el84's generally sound better than EH or Sovteks at a comparable price, and they're more rugged so they should last a little longer at the excessively high plate, screen, and cathode voltages. How much longer? Who knows. So like I said, get a bunch of spares. Quality control for tubes isn't so perfect unless you pay the extra premium for balanced quads that are pre-tested by folks like TAD, Ruby, or Groove Tubes. And even then you'll find duds! Anyway, with random JJ tubes, occasionally you'll find that one rare, fresh out of the box tube that's DOA, and others that may not last a week. Still, it's generally a lot cheaper in the long run than buying the premium pre-tested tubes, even you don't send the duds back for a replacement. Get your amp tweaked properly, and a good JJ el84 should last for a couple of years if not longer. I certainly wouldn't recommend risking a valuable NOS el84 in it until it's tweaked. Gil... Link to comment Share on other sites More sharing options...
thelastdeadmouse Posted May 23, 2009 Author Share Posted May 23, 2009 I feel really silly asking this, but how on Earth do I get the back panel off? I've got the seven screw on the back panel out, and I'm afraid if I pull any harder I'm going to break it. Do I have to remove screws from the back corner's metal caps or is it just being stubborn? Link to comment Share on other sites More sharing options...
cGil Posted May 23, 2009 Share Posted May 23, 2009 It's just the tolex glue being stubborn. They probably screw the back panel down while the glue's still wet on the cab! Cheeper than the extra steps of clamping and cleaning off the excess glue. Anyway, just use a putty knife to free the back panel as best you can and maybe a flat blade screwdriver with a little leverage against the chassis to wedge it out the first time. The cab corners are just for protection. The four chassis screws are under the black plastic caps stuck in the top of the cab. Gil... Link to comment Share on other sites More sharing options...
Eracer_Team Posted May 23, 2009 Share Posted May 23, 2009 Don't think I read it in Gil's post.. the orange wires have plastic fuse holders. The brass inside those holders are the amp's weakness. once the back is off and with a speaker connected, turn the amp on.. do the tubes glow (you should only see one that has a spring retainer on it, the other is inside a can) if it glows then more trouble shooting is needed but I bet you won't see the 1 tube glow. So turn the amp off, twist open the fuse holders and scrape the brass ends inside the fuse holder, put it all back together and test again.. I bet the tubes will light again and you will once again have sound. Link to comment Share on other sites More sharing options...
thelastdeadmouse Posted May 24, 2009 Author Share Posted May 24, 2009 After a half hour with a putty knife and me wincing with every groan and creak of the amp, I got the back off. The inside of that back panel was covered in glue, no wonder it was so hard to get off. The tube inside is glowing, but no sound. The electrical engineer that will be helping me has done some work with sonar systems, so he theoretically has a solid grasp of what's going on in that amp, but has no practical experience working with this or any other musician tube amp. Is there a usual culprit for no sound with a glowing tube that we should look for, or better yet a thread or page somewhere where one can look up their particular issue? I'm sure with enough time wandering around with a voltmeter we can find the issue, but it never hurts to refer to the wisdom of those more experienced than yourself. Link to comment Share on other sites More sharing options...
cGil Posted May 24, 2009 Share Posted May 24, 2009 As to the fuse problem, Eraser_Team's right. Varnish on the fuse holder's contact pads have been a problem for VJr's since they first came out. Your tubes may be glowing, but they need at least 5.7v to work. 6.3v is the norm, with a 10% variance. Below that, and your tube's cathode gets strippedl. . Here's some handy info on tube life versus filament voltage: http://www.tone-lizard.com/images/Tremaine.gif If you've cleaned the fuse holder's contact pads (not the fuse) and the amp still won't work, ask your buddy to do a "pop" test to help localize the problem. Basically, all this means is plug in a speaker and tubes that are known to be functional, turn the amp on and crank it all the way up; then start measuring voltages. You won't need anything plugged into the guitar input for this. Start at the power tube, and listen to the speaker for the pops and crackles of the meter probe making contact with signal line components. These pops should get louder as you work your way forward from the power tube on though the preamp stages. When the pops stop, you're close to the problem area. You'll find the pop test and other tried and true troubleshooting techniques here: http://www.pacificrecone.com/JackDarrBook.html Gil... Link to comment Share on other sites More sharing options...
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