Wednesday, February 6, 2013
Straight Outta Yellville-The Ampeg Jet Reissues UPDATED
There are four models that I know of-the J12R and J12T, and the SJ12R and the SJ12T. The J12R has the standard volume/treble/bass/reverb layout, but the J12T has volume/tone/speed/strength/reverb control setup.The -R series amps cater to the overdrive types but do not interest me much.
It's tube modulated vibrato, so why anyone would not want that is beyond me. I snagged a J12T off of fleabay for a good price, upgraded the reverb tank with a Mod tank from AES, and then passed it on to a happy person to use in church. It's cathode biased, 15w, uses a pair of EL84s and is a pleasant companion to pass the afternoon with.
I decided I had to have its big brother the SJ12T -alleged to be 50w with a couple of EL34s but more like 30-and soon had one here with a few quirks. The rivet holding the terminal block on the speaker had broken so it was a small job to get rid of the remains and install a new pop rivet. It's a nice big Ampeg labeled Eminence speaker by the way.
More seriously, the reverb sounded stinky and thin, and the vibrato was an on again, off again proposition. In addition there were random crackles and spits and pops.
The cabinet is wide enough to accommodate a 17 inch reverb pan so in one went, and things got better.
Replacing the plate load resistors is pretty easy because they're all on the tube board and easy to get at. That helped with the spitting but the vibrato was still dodgy, and then it quit altogether.
Studying the schematic led to the indictment of the J112 JFET in the vibrato circuit, after I'd replaced everything else because it was open and I had the parts. Installing the JFET cured most of the vibrato problems but it still crackled when the vibrato strength was turned up. This seemed to be related to the first preamp tube which is also the vibrato oscillator. This morning I tried a 6072 preamp tube which sounded fine but the vibrato failed to function. As I'd just tried a JJ and a Sovtek 12AX7 and still had the crackling in time with the vibrato, I dug into the archives and found an oldish Bugle Boy which had come from my brown Concert.
It was just what the amp doctor ordered. Now, I can get all the vibrato I want without crackling.
UPDATE: This did not solve the problem. In a day or so it was back with intermittent operation and crackling when the depth was turned up past a quarter. What to do?
I pulled off the back cover and removed the molex connector that links the foot switch board from the chassis. I still had the problem but found that monkeying around with the cable could shut the vibrato off completely or induce random crackles. If the foot switch board and foot switch were connected the LEDs on the foot switch could be made to flicker or shut off entirely.
So today I had a few spare moments to dig into it and the molex connector looked like a likely suspect. I removed the main board and resoldered the molex connector to the board. I then gave the board a lookover and resoldered all the joints that looked dodgy-there were a few, particularlyu on the ribbon cables.
I powered it up and all seems to be well, so far.
Internally the amps are simple and easy to work on, the traces on the board seem pretty healthy, and they've got the look. There's no secret sauce, channel switching, high gain doodads and so on. At 400 volts on the plates they'll last forever, and the tone is nice and clean, not buzzy and fizzy, and enough volume for smallish clubs. I got mine for $300 in minty condition, and the Jet II came for $250. The price is right for a nice medium weight amp with a classic look.
One thing I do not like is the small blue pilot light so I figure a regular jewel style pilot light with a green lens will do the Ampeg thing just fine.
And if they ever explode there's plenty of room to build something more traditional.
If you have one and you're wondering where to get a schematic, if you email Loud Technologies-the current owner of the Ampeg name-and give them your serial number and let them know you own the amp, they will send you a schematic.