NAD for JMP ends in failure

[gordman]

Hey Guys, been lurking for a while, keeping up to date on the 50th Anniversary 1-watters, and wanted to finally contribute with a few pictures of NAD with my JMP. Behold my latest pride and joy! Unfortunately, it died on me within about 20 minutes.

Everything started out fine - I connected it to a 16-ohm 1x12 cab (yes, used the 16-ohm output), powered it up, let it warm up for about 15 minutes, then plugged in my axe for the initial test. I had the low-power mode engaged, no boost, and I just jammed around for a while, enjoying what I was hearing. After about 20 minutes, it started sounding a bit weak and scratchy, but intermittently. I checked the usual things - cables, power cord, wireless transmitter, knobs and switches, etc. Problem remained. I shut off the JMP and connected my JTM-1 combo with the same cables, cab and guitar - no problem. Back to the JMP, still had the problem, then no sound at all. There were no pops heard nor smells of electrical things dying. I looked into the rear of the amp and lo and behold, the power tube next to the transformer was dark. Ok, I thought, bad tube - I can fix that. So I actually replaced all of the tubes with some spares from my tube box, powered it back up, and saw all four tubes glowing. Cool, ready to go. After a few minutes of letting the new tubes warm up, I plugged back into the amp, only to find that I still had no sound at all. There was a small hum from the back of the amp, but nothing from the speaker cab. Tubes are glowing, pilot light is on, but no sound. Tried low-power, high-power, boost on/off, turned all the knobs, still not a peep out of her. I checked the fuse just in case, but I assume that I wouldn't have a pilot light and glowing tubes with a bad fuse. Man, I never even got it out of low-power mode while it was making sound. Hey, I've owned enough gear to know that stuff happens, but obviously it's disappointing, especially as there aren't many of these around for a quick swap.

Anyone have any ideas? SteveD?

Thanks for any help!

[Wildone]

What kind of tubes did you put in it? Sounds like a bad tube issue so I am wondering if the tubes you had were the right ones for the amp.

Steve has said no 12Ax7s

No configuration of ECC83 (12AX7) will give the full power of 1 watt and they don't give a satisfactory option anyway IMO but for completeness here are the figures.

I think you need 12AT7 and 12AU7 tubes. Hope you get it worked out man, I would love to have one of those.

[SteveD]

Welcome to the forum gordman.

Sorry to hear you've had a jumpy start with your JMP but sometimes these things happen. Fortunately not very often.

12AX7s are not recommended for the power amp but they are stock in the preamp section.

This could be anything from a loose connector to a dry joint. If swapping the valves hasn't helped and all the fuses are ok including the heater supply fuse then it really needs to be looked at 'in the flesh' by someone with sufficient technical knowledge to trace the fault and repair it.

[gordman]

Thanks for the replies. I did indeed pay attention to the tube specs, so my replacements were a couple of new JJ ECC83s for the pre-amp to replace the stock Marshall ECC83s, and 2 Mullard AT7s in the power section. As a double-check, I put the 3 good stock tubes back in with a new JJ ECC81 next to the transformer to replace the failed power tube. In both cases all four tubes lit up, but no sound was produced.

Steve, with respect to fuses, I checked the little plastic pull-out drawer next to the mains. Both the active fuse and the spare looked good, but I even swapped them around to be sure. If the problem was there, though, I wouldn't expect any power beyond that point, right? You said "all" of the fuses - so the others are presumably inside the chassis? I'm not an amp tech, but I was solder-certified years ago as a Bose mfg tech, and I've done board-level repairs. I'm comfortable poking around a bit but humble enough to know when to defer to a pro.

[gordman]

Ah-ha! FS-1 (1 of 2 fuses on the main board) was blown. Off to Radio Shack tomorrow...things are looking up. Thanks for the idea, Steve!

[reverbbb]

@gordman:

Did the new FS-1 fuse fix the problem?

Would the bad tube be the cause of the blown fuse? As obvious as this might sound, I tend to want to know for sure in case this happens to me.