Tag Archives: repair

Ampeg Super Jet SJ-12T repair…

I’ve seen a lot of the same complaints on various forums regarding these amps. Basically, the amp starts to get noisy and the occasional arcing can be heard through the speaker. The problem seems more exaggerated when using the tremolo circuit which is just a phase-shift LFO modulating the negative grid voltage. I’m not so sure that the blue 1/2 watt resistors inside the amp are not really metal film despite their appearance (**UPDATE** I’ve recently seen some evidence that arcing tends to happen more often in metal-film resistors. I’m not sure about how correct this is, but it would make sense in this amp). If you find yourself working on amp with these sort of issues, here is what I did to alleviate some of the nasty pops.

sj12t_01

On the main board, change R34 from 475k to a 1M 1/2 watt resistor. Also change R8 (pictured below) from 221k to 270k. This will lower the the overall voltage swing on the grid preventing so much stress to the power tubes.

sj12t_02

On the second board (the one that has the tube sockets), you’ll probably want to change out the metal-film resistors just to make sure they don’t start arcing. You can always tap them with a chopstick and listen for any pops in the speaker to determine if they are bad or not. I apologize for the lack of a picture, but there aren’t many components on the board. You should be able to find them all right in the middle grouped together.

R15 & R16: 100k
R17 & R18: 220k
R19 & R20L 22k

For this amp, the speaker was replaced with a 12″ Jensen C12N. The thing cleans up pretty nice and has a sound not unlike other true vintage Ampegs that I’ve heard.

Ampeg SVT-CL Repair

here’s a 300 watt Ampeg SVT-CL that i had on the bench today. a much different kind of amp from the 70s models that i’ve been more accustomed to working on in the past. this one had no sound or output and the fault light would stay on constantly. i assumed right away that it must be circuit related. turns out that it was actually just drift in an old set of tubes that got so bad it caused the amp to fault. i switched the tubes around so that they could get a different set of voltages and then adjusted the bias until it stopped faulting. also replaced the 12AX7 phase inverter since it was showing signs of age. i figured it would help to at least have one known good in one of the most important spots. after that, i gave it a couple hours of play time with some solid signal just to make sure it didn’t drift again. a new set of 6550s should square out everything for another year.