superjet sj 12-t makes

[marksmusic]

Hi all,

I have an Ampeg SJ-12T Reissue. It works awesome, but came to me with a messed up trem circuit. I replaced the caps in the oscillator circuit as well as the PI coupling caps and the caps between the osc and the resistors at the power tubes. Cleaned pots,etc.

The tremolo works now but at intensities above 6 or so I get a weird crackling/clicks as it sweeps. I get blue flashes inside the tubes in time with the trem so I tried new EL34s and the results are the same. Also at high settings the pilot light will dim in time with the trem and noise which seems odd to me. A throbbing, clicking metronome type sound (even with no instrument attached)

Bias is set around 31ma with the trem bypassed (a bit cold I suppose). With my bias meter in the sockets I get a swing from 30 up to 110+ at high intensities which seems way off base to me.

Checked the "voltage divider" resistors that feed the trem voltage to the grids as well as the gridstoppers themselves and the screen resistors. All check out well within the printed tolerances. Also swapped in new preamp tubes, no changes.

I had to fill out some nondisclosure form with LOUD, so I'm not allowed to post the schematic.

Any ideas? What have I overlooked?

p.s. you can turn down speed and still it makes that clicking sound. It will however stop when speed and intensity are turned off

Thanks, Al

[audiopete]

Hi there is a Fender mod of adding a cap to kill the tick or could be simple as proper lead dress (although I think you are all PC board) - more info here and here -

http://www.braught.com/Schematics/FenVibMod9.gif
FDP - Forum

More info in this thread - http://music-electronics-forum.com/t26384/

Amps that use a neon bulb as a power indicator also can tick.


Good luck.

Pay no attention to the blue glow in the tubes - perfectly normal in any amp and they will pulse with your guitar playing - Blue Glow in Tubes, Tube Getters, and other Tube Info

[marksmusic]

Thanks for the help... however this amp has that bias modulating tremelo circuit. Any thoughts....?
Thanks again.

[Jazz P Bass]

Not sure about the ticking sound.
Good output tubes & correct biasing appears essential.
You might want to give this link a read.
Link: Bias Modulating Tremolo Circuits

[audiopete]

Since you can't post the schem can you describe the trem circuit a little better? Where does it modulate bias?

EDIT - I looked at the owners manual and it is power tube bias (as you post says) so sorry bout that. Try swapping the trem tube plate resistor and/or 2 bias resistors to the grids (270K maybe?). Crackles and pops can be bad resistors. Uprate them in wattage if you can.

[52 Bill]

It sounds like you've done everything right and checked all that can be checked, which leaves you to solve the problem. If the power tube bias is being pulled down so low that the output tubes are drawing almost 4 times the idle current I would imagine that there is a lot of stress being placed on the power supply and related components.

I know that you checked them for resistance, but it could be a power supply resistor that can't handle the extra current draw, or I suppose it could be the primary winding of the output transformer. My guess would be to swap a few parts to see if you can find whatever it is that's causing the noise. What happens to the B+ when it does this? Have the filter caps been checked?

The alternative would be to treat the symptom instead of curing the disease and limit the depth of the effect by reducing the trem signal to the output bias.

[marksmusic]

Thanks everyone for your replies so far. I will let you know what I find when I get back to the amp anf do some more investigating...

[Bonke]

Did you find a solution? I have one with the same issues on my bench. It is hard to find the right schematic.
Henrik

[Jazz P Bass]

Schematic attached.