The Ampeg Jet J-12T is a 15-watt, class A, tube amplifier with a 12" speaker, tremelo, and spring reverb.  I purchased the sample reviewed here late in 1997 and have used it regularly ever since.  I've played out with it a few times, mic'd through the main board.  Every time somebody hears my Robin Savoy semi-hollowbody through this amp for the first time they remark how "great that guitar sounds" – alas, would that they were saying how great the guitarist sounds ;-).  The Savoy is a great-sounding guitar, but at least half the credit in any rig has to be given to the amp.

The amp has "guitar" and "accordian" inputs (I understand that this amp is quite popular with the harp crowd, BTW).  The controls are volume, tone, tremelo speed, tremelo intensity, and reverb "dimension."  The amp does not have an effects loop nor a jack for an external cab.

The "guts" are two 12AX7 tubes (Chinese) and two EL84 power tubes (I don't recall whether these were originally Sovtek or Chinese for reasons that will become clear later).  The tubes are mounted on a heavy circuit board inside the chassis and project through holes cut in the chassis.  This model is not, strictly speaking, "all tube."  The primary signal path (preamp and poweramp) are all tube.  However, the tremelo circuitry and the spring reverb driver are solid state.  If you really understand amps you know that isn't a problem (solid state circuitry doesn't "sterilize" an otherwise warm signal).

My first impression of this little amp was that it had great tone and way more volume than I would ever need (a class A amp with an efficient speaker is quite loud, even at 15 watts).  The tremelo and reverb are both very effective, and you can do the Dick Dale thing 'til the family screams for mercy.  Time (2-1/2 years) hasn't changed those first impressions much, but has revealed some things that could stand improvement.

I'd had the amp about two months when I blew the screen resistors and power tubes.  How?  Simply by driving it very hard with hot humbuckers and the volume on "11."  The local shop called Ampeg and they said, "repair it, but tell the guy not to crank the volume all the way up."  This is kind of a shame because this amp sounds really good on "11."  We replaced the tubes with Sovteks. Recently I was running it fairly hard and it started the same crackling sound that signalled the demise of the screen resistors last time.  I caught it in time but this is something I simply can't tolerate – meaning that I am probably going to try changing the resistors to larger wattage and bumping the cathode resistor value up a little to cool off the bias now that it is out of warranty.

The only other problem that I would categorize as definitely needing attention is a very bad buzz or rattle that is resonant around the low A and low G at high volumes.  I'd heard from others who reported this problem but thought they were blowing smoke because this one sounded fine. However, after about a year or so this annoying rattle started creeping in and has gotten progressively worse.  I haven't had time to track down the cause.  It may be something as simple as a loose cover on the reverb tank – at least I hope so because this baby just sounds too nice to replace.

As with most amps, it is impossible to achieve power-tube distortion at comfortable volume levels.  This thing gets just plain loud in a big hurry.  This isn't unexpected but with the simple controls leaves the player with a quandry (a common problem on small combos).  Since there is no master volume control it is not even possible to achieve preamp saturation at reasonable volume levels.  I added a master volume after the warranty expired and this has helped a great deal.  I used the Matchless style of master volume that "pulls down" the output of the phase inverter.  This is a little hard on the phase inverter tube but this way the preamp saturation occurs in the phase inverter and that seems to be quite a bit sweeter than saturating the second stage of the preamp itself – I suspect that's why the Matchless, a very good amp, is designed that way.  Incidentally, I didn't have to drill any holes in the chassis to accomplish this.  I mounted the new master volume control in the pilot-light hole and replaced the power switch with a piloted (lighted) push-on/push-off switch.

The Bottom Line

The Ampeg Jet II, J-12T is a very good sounding amp that unfortunately has a couple of fairly major problems at high volumes.  Mine, at least, blows itself up at high volume.  I may or may not be able to cure this fairly simply with heavier screen resistors and perhaps a bit cooler bias.  It also seems that it isn't uncommon for this amp to develop a bad rattle resonant around the low A or low G.  Again, this may or may not be easily curable.  I suspect that it is just a matter of tightening something up because the rattle crept in over a period of time.  If I am able to cure either problem, I'll post an update on the GuitarNuts amp page.

I probably won't ever purchase another guitar amp that doesn't have at least a full tone stack and a master volume, but of course such features kick the price up well above what I paid for the Ampeg.

John Atchley
June, 2000

Addendum – April 2001

A while back I tracked the rattle in my amp down to the cover on the reverb tank.  Just recently though I received the following helpful information via e-mail:

RE: I your review of the Ampeg JET II J12T, you mentioned a buzz or rattle with certain low frequencies. I had a similar problem with my amp (I have the 50 watt with 2 EL34's (A/B?)). I tracked it down to the power tubes. I believe that this had been recurring complaint. Ampeg modified/upgraded this amp (as well as the model you own) with a support/holder for the power tubes. It solves the problem. The modification is free and I'm sure Ampeg will help you out. They modified my amp at the factory (I live in St. Louis) - no charge and I did not have any warranty paperwork. They treated this issue in a very practical manner. They indicated that if the amp needs the modification, we'll do it for no charge. This modification will help. I tried spring tube holders with little improvement. I don't work for Ampeg, but the techs are very nice (heck, everyone is nice).

Good Luck


My amp already had the heavy duty bracket on the power tubes, but if yours doesn't you may want to take WK's advice.  If your amp does have the heavy bracket on the power tubes, you might check the cover on the reverb tank.