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The Intermittent from hell

7/22/2003 11:00 PM
The Intermittent from hell
I've got the intermittent from hell in a 68 Vibrolux Reverb,it acts as though it's a loose tube socket or connection when it decides to appear. All of the connections are solid,the power tube and rectifier octals have all been replaced,the grounds are all solid. After this we tested it by putting signal through it and pounding the stuffing out of it with a large rubber mallet while it was being played at about 5 on the volume. It would not reproduce itself in the shop and made it through one gig fine but the next outing it started it's crap again with the the stage vibrations from the bass and kick.The power tubes have been replaced twice,a complete cap job all the voltages are were they should be and I can't get the S.O.B. to crap out on the bench !!!!!! It's in both the normal and vibrato channels.All of the pre-amp tubes have been swapped out with known quality tubes. Any help would be appreciated !!!!!!!!  
7/23/2003 4:05 AM

"""started it's crap again with the the stage vibrations from the bass and kick"""  
Could you explain a lttile bit of what its doing..Is it cutting out, looseing power?  
Sometimes its more heat related, screen grids or grid stoppers.. things around the sockets.  
Maybe give a little more info and see what we can come up with.  
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7/23/2003 5:53 AM

If you want to get it hot, throw a heavy towell over it or a small rug like a bath mat. DOn't leave it unattended for fire hazard, but this will let heat build up muh quicker. Then determine if heat is invovlved.  
Yep, I would like to know what it is doing too. Soundslike loose socket is not very descriptive.  
In an intermittant, the syptom is the most valuable thing you have. So once it starts, don't just hit it, be very systematic in how you touch it. Yes I have my own rubber mallet right next to me. But here you ought to touch each tube gently and push it sideways a bit. This small stress might isolate a bad socket. Even though you changed them, the resistors and stuff soldered to them might be bad, so flexing may help you discover that.  
Noise on the power supply will come out any channel.  
Isolate the problem. When it is making noise, do any controls affect the sound of it? RUn it without the reverb driver and the trem tube to see if they are involved. In fact, try removing all the preamp tubes leaving only the phase splitter - part of the power amp and not preamp anyway to be technical - and the rectifier and output tubes. This will tell you if the preamp was the problem.  
Get as many meters as you can and clip them to the power supplies. Hook your scope to something. Monitor as much stuff as possible and see what reacts when things happen.  
Remove the cables to the reverb pan and the footswitch cables. SOmething there migh tbe involved.
7/23/2003 8:54 AM

Sorry guys frustration had me rambling a bit.It gets a buzzy distortion and loss of signal strength  
.I left out that I did gently rock the tubes in the sockets. I have prodded every component this has yeilded no results.The controls had no effect on this other than the expected.I will try get her hot as suggested to see if it is thermal.The biggest problem at this time is to get the damned thing to do it while it is on the bench so I have some hope of isolating it and being more descriptive of the symptoms.For the past few days it's been rock solid and hiccup free.Thanks for the suggestions and kind responses I will update you as soon as I can get the problem to surface again.  
7/23/2003 8:58 AM

Whoops I hit it with the mallet to see if it would re-occur after the first repairs to emulate the stage vibrations... not to troubleshoot.  
Thanks Again  
7/23/2003 11:48 AM

Did you already think about pickup noise? Some guitar's pickups will transmit any sound that will resonat the strings ( air vibration due to high bass or kick levels, listen to Pantera's "good Friends and a Bottle of Pills" Far Beyond Driven, though in this case the drums were meant to trigger the gates used in the guitar rig)
7/23/2003 12:56 PM

Does it do it through both channels or only one channel in particular?  
I had one amp display a similar intermittent fault and it turned out to be the speaker jack (of all things!).  
As my old man used to say "all faults are simple - it's finding them that's the hard part". Sorry that doesn't help you any...  
Good luck brother,  
Pete :)
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