Other Amps

N(B)AD - new (broken?) amp day

1

Well I wanted to come on and wax poetic about my new toy but it's giving me some issues. Hoping one of the many well-versed amp gurus on here can point me in the right direction.

I just picked up an Ampeg Jet II reissue (J-12R) with the stock speaker replaced with an older MojoTone. I loved the sound of this when I checked it out, and all was good (although I didn't think to try out the Accordion input). And I think the seller was genuine and not trying to rip me off.

Brought it home in the trunk of my car and only mishap was that I wedged it into the corner of my trunk standing upright and when I got home it was laying on its back, so it had a minor fall.

I plugged it in (guitar input) seemed to work for 5 min, then starting going all static-y and when the static got worse, the volume drastically dropped. I unplugged the guitar thinking maybe a bad cable, but the static continued even unplugged, and still pulsated in volume.

I unplugged power, took off back plate, everything looks good and intact. The preamp tubes were very slightly off-kilter so I seated them firmly (but all the pins still seemed well engaged). After this, it seems the guitar input is mostly behaving, but if I plug in to the accordion input it's still pulsating the volume (randomly, not rhythmically). Also, this is the version without tremolo but with reverb, so it's not like just a wonky trem circuit.

Should I take this straight to an amp tech? Can I do more harm if I keep running it and trying to troubleshoot? Should I swap out tubes and if so, are these power tubes cathode biased and I can just swap them, or would I need to adjust the bias?

Thanks for any help you can offer - my mood took a sharp turn from thrilled to deflated, and hoping to right this ship!

(When it was working, it's a pretty amazingly full, rich, clean sound!)

2

Since it worked before you let it fall over in the trunk,I wouldnt assume a fall in a moving vehicle to be a "minor fall",It can slam pretty hard if it fell when hitting the brakes,my first suspect would be a bad connection or a wonky tube.Dont know how much you know about tube circuits or troubleshooting,but you can open the back with the amp on and try wiggling each tube to see if the noise happens.If you have spare tubes,by all means,try that first.You can also try the "chopstick test".Get a chop stick or a similar non-conductive instrument and,with the amp on and volume turned up,poke at different components to see if you have a broken solder joint.Do these tests with a guitar plugged in and the volume on the guitar turned down,amp volume up.If you are not familiar with the dangers of high voltage present in amps just take it to a tech,you can get a nasty zap if you are not careful.If you find a "problem' component and dont know what to do about it next,you will end up taking it to a tech anyway,so I would just cut to the chase and see a tech first.

3

I agree. take it to a tech. getting "zapped" won't be fun.

4

I have had similar problem with my Blues Deluxe RI. It was due to an internal short between two electrodes in a tube; it also caused a resistor to be burnt. From what you describe, I would suspect the accordion preamp tube. Tubes are fragiles (that's why semiconductors have been called "solid state"), especially when they are hot, so it is a good practice to way for them to cool before moving an amplifier.

5

That's why tubes are made easy to replace. They're fragile, yet some tubes were made strong enough to be fired from cannons---used in WWII proximity fuses for anti aircraft rounds. The tapping idea is a standard trouble shooting practice. You can use a Sharpie as well---anything non-conductive will do.

6

Swapping tubes out is the easiest thing you can try doing. Try it first Always try the easy things first

7

I'm not super amp savvy - I take it to a tech and let them diagnose it before I go buying sets of tubes. If it's not the tubes, you'll still end up taking it in to the tech.

8

I'm not an amp expert by any means, but trust I can poke around in there with a chopstick without getting zapped. It's just that I've read (I think) that running with a faulty power tube can ruin an output transformer. I just don't want to cause more damage in the process of troubleshooting.

Here's my plan of attack in order (unless someone strongly advices against it): All functional tests as per Stokes advice: guitar plugged in, guitar volume down, amp volume up.

  • remove and inspect all tubes for signs of physical damage

  • swap preamp tubes, I have some and know they are good

  • swap power tubes. I have a spare set the seller gave me, but they were tucked in the cab when it tipped over so may have also been rattled. I won't rebias anything, but won't run them for long in this state.

  • If no improvements, I'll put original tubes back in, tap the tubes for microphonic, then poke around at the connectors or anything I think would be more likely to get banged up (e.g. heavier components, pots, etc, but not every little resistor).

And at this point if no solution I'll bring it to the tech.

Thanks for the advice.

9

If your power tubes are red plating,running so hot the plates turn orange/red, I would advise not running it with those tubes.Turn the amp on and watch the power tubes,if you see the plates turn red in one or more,turn it off.If they dont red plate,plug the guitar in and play it as you watch the tubes,if they start to red plate,take it to a tech,do not install the spares,if the redplating is being caused by something in the circuit beyond the tubes,it would likely fry the spare tubes as well.If the power tube plates are ok,use a good preamp tube to sub for each preamp tube,one at a time and see if the problem goes away with one of the subs.

10

Amp only has two pre amp 12ax7s, and two el84s for the power section...

without seeing it, my gut suspects that the instrument input jacks are probably soldered directly to the circuit board..and with normal plugging and unplugging, the solder has cracked...causing buzzing and volume drops...very common with circuit board amps these days!

other things to have checked...

they used 1/4 watt screen resisitors on the power tubes, a 1watt or 2 watt would be better

also those amps sometimes have trouble with the filter caps gone awry

try the tube exchange..if that doesn't cure it..take it in to a repairman and tell him to go thru my checkpoints

luck

cheers

11

I completely missed the fact that the amp is a re-issue,musta been skimming.Although I've never worked on one of these re-issues,neatone is right about the input jacks.I knew the 1/4 watt plate resistor problem exists in some of the early '59 Bassman re-issues, if they used them in this amp it is a likely source of the problem as well.


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