Ampeg J12T cuts out after heating up

[dporto]

Hi,
I've got an Ampeg Jet J12T that sounds fine for about 20 minutes or so at gig type volumes. It's pretty obvious that something's heating up and cutting out. My question is this: Where's the best place to start? Unfortunately, I'm pretty sure I've got to totally disassemble the amp...ie: remove both circuit boards, pots etc... Thanks in advance for any suggestions

[Enzo]

I run a repair shop, and I would say I almost never have to take an amp completely apart.

Is this an old one or a reissue/current model?

Ball up your fist and whack it on the top. ANy effect? If so, something is loose.

When it cuts out, is there ANY sound coming out, or complete silence? That is, is the music still there but WAAAAYYY down, or no music at all but some background sounds? Or not even background sounds? (Little hum or hiss, whatever.)

If it is not something like an effects loop jack, then I get out the voltmeter and monitor what is happening at each tube.

[dporto]

Enzo,
Thanks for the reply. It's a re-issue - part of the "Blue Diamond" series of amps. I think this one's from '96 or so. Unfortunately for me, all the solder connections are on the underside of the pcb's and can't be reached with a soldering iron without removing the boards from the enclosure. I was just wondering whether there is a common failure to these amps which I could check for. Your help is much appreciated.

[dporto]

Ok, so I finally got around to taking this thing apart. The only obvious break I found was a ground lug on one corner of the pcb. Though I didn't think this was the cause of my problem, I was sort of hoping it was... No such luck.
After firing up the amp at moderate volume (about 4 on the volume dial-fairly loud) it ran fine for about 10 minutes then started to cut out. I say "started" because it didn't die all at once - it kind of sputtered a bit. It also came back when I turned off a "boost" pedal. If I turned on the boost again it would cut out again. After a minute or so, it cut out for good. When I say "cut out" I mean the volume is mostly gone, but there is still signal noticeable, albeit at a very low volume.
In addition to the playing test, I shut the amp down for a few minutes and then turned it on again. It worked fine for about 5 minutes. At this point I stopped playing and let the amp idle for about half an hour when I came back and started playing again it was still fine. After about another 5 minutes of playing it quit again. The power tubes have been changed but the pre-amp tubes haven't.
Once again, any ideas/help is appreciated!

[Don Moose]

This "boost" pedal - part of the amp, or an external stompbox?

If it's part of the amp, there may be something like the well-understood Fender HRDeluxe low-voltage-rail issue.

Is there a loop on this thing? Does the amp still cut out if you're using it as just a power amp? just a pre-amp?

Hope this helps!

[dporto]

Hi Don,
Thanks for the reply. External stomp box. No boost on the amp, no effects loop - real straight forward single channel amp with tremolo and reverb. FWIW, the tremolo and reverb still work when the amp "cuts out" (ie: the volume drops to almost zero, but there is still signal present as I described in the previous post). It seems to me that due to the symptoms, the problem must reside in the "power amp" circuit. I haven't been able to find any obvious loose or broken connections(besides that one ground lug), and and the tubes are new (2- JJ el84's) ... ???

[dporto]

^bump^ Anyone?

[52 Bill]

Does this amp have an FX loop? If so try cleaning the jacks.

Have you checked the speaker and connections?

Did you try Enzo's "fist test" or the chopstick test?

If the signal comes and goes, then the problem is mechanical in nature. Something is opening up and not making connection somewhere. The real problem comes when the breaking connection is located inside of a resistor or capacitor. Try to isolate the problem in any way that you can, and by process of elimination you may find your problem.

[dporto]

Hi Bill,
Thanks for the reply. Had you read my previous posts, you'd have seen that the amp has no FX loop... And I have indeed tried the fist and chopstick tests... "The real problem comes when the breaking connection is located inside of a resistor or capacitor" I Think that this is the most plausible scenario as I haven't been able to identify anything either visually or by pushing and poking things. I think I'll probably need a schematic and specs on voltage points so I can pinpoint where the problem is occuring. Thanks again!

[pdf64]

Schematic
http://www.schematicheaven.com/ampeg...12t_jet_ri.pdf
If you can get it to go into cut out mode with the pcb accessible, it should be possible to check most of the voltages. Bear in mind it could be something oddball like intermittant shorted turns on the OT primary. The insulation breaking down following the effects of high voltage and temperature might result in spluttering then major vol loss. But the dc voltages will look ok. And as it 'gets better' when it cools, it will be difficult to test for. But that's your worst case scenario. Good luck - Peter

[dporto]

PDF,
Thanks so much, that's exactly what I'm looking for! Yhea, it's almost like clockwork after about 5 min. or so. Hopefully, withe the test points, I'll be able to figure out where it's failing. I'll let you know what happens...

[dporto]

PDF,
I wish I had gotten your response about 5 minutes sooner...I had just purchased the schematic from MUSICPARTS.COM for $15.00 - Can you say RIPOFF ? These guys are charging for schematics that are free at Schematic Heaven... And they refuse to refund my $...Oh well, live and learn... All I can say is PLEASE DONT BUY FROM THESE GUYS UNLESS YOU'RE POSITIVE THAT WHAT YOU NEED ISN'T AVAILABLE ELSEWHERE FOR LESS!

[52 Bill]

Now that I've seen the schematic, I recall fixing a couple of these where R24 470R 5w had opened up. Check the voltage at TP10 when it cuts out.

[dporto]

OMG! I just spent the last 20 min on a reply and it disapeared when hit the submit reply button (said I wasn't logged on, but I was...) In any case,
suffice it to say that generally speaking the output voltages (DC) are all about 10vdc higher than spec'd but they are all present. the only anomalies I got were on the +speaker lead TP8. While the amp was "working"(ie:loud) I got positive acv (+ 2 to 10vac) that would fluctuate depending on how hard I strummed the guitar. After the amp "cut out" I could only get readings in the 1/100ths or 1/1000ths vac. The voltage at TP10 remained at 322vdc regardless of output volume. The leads me to believe that PDF's diagnosis of a possible intermittent short in the OT is correct. The next question is whether voltages of +10vdc over spec'd would cause this, and if so what's the fix... Thanks again for evryones help

[pdf64]

Don't worry about the dc B+ voltages being a few volts different, as it is totally dependant on the line supply voltage at your wall socket, which can have a +/-10% variation. That will be reflected as a similar variation in the B+.
A word about safety - you are at risk of a fatal electric shock if you are touching your guitar strings/metalwork with one hand, and have to other close to uninsulated line or B+ conductors/terminals, one slip and you could be dead. Use a ground/earth current detecting circuit breaker (like for garden power tools). You can download software which turns your pc soundcard into a signal generator, which would make life much easier and safer than strumming the guitar, that's what the 50mV p-p 1 kHz input signal means.
Before confirming the OT as the problem, what do TP1 ,2,3 measure when it's working ok and when it's cut out?
Peter