AC30 Top Boost Pulsating Tremolo??

[SYSDOA]

Hi,

This is my first post. I have a friend's AC30 Top Boost that he asked me to help with. I'm at a loss. The tremolo sounds lovely, but when the guitar is silenced, noise still comes from the amp and can be manipulated with the speed and intensity pots. I checked the pots for the tremolo section. They're a little weak, switched and still the same issue. It has the MIC tube rectifier still installed. I've switched a few to solid state. I'm not sure if that would have anything to do with anything.... help me.

[imbuedblue]

That is normal. The tremolo will still modulate the signal even if there no signal coming from the guitar. This includes any of the normal amp hum that occurs when it is powered on.

Try switching the tremolo off to compare the modulated and unmodulated noise levels.

[SYSDOA]

That is normal. The tremolo will still modulate the signal even if there no signal coming from the guitar. This includes any of the normal amp hum that occurs when it is powered on.

Try switching the tremolo off to compare the modulated and unmodulated noise levels.

Thanks for the quick reply. When the tremolo is disengaged, the amp silent. The other channels do vary depending on gain settings. So this is just the way it is?

[imbuedblue]

It may very well be normal. Which AC30 is this? What happens when you turn the intensity knob to zero?

Read this: Bias Modulating Tremolo Circuits

And check "Note 3" at the end.

[SYSDOA]

That's an excellent link, thank you. I believe this is a limited edition? Chinese build. It's definitely not the vintage reissue. When intensity is cut the pulsating noise goes away and the guitar plays, but without tremolo. I'll snag a matched pair today!

Thanks,

[Lee_ranaldo]

Some chinese AC30's have a MOSFET instead of a TUBE in the vibrato section
maybe that might be the source of the problem

[SYSDOA]

Some chinese AC30's have a MOSFET instead of a TUBE in the vibrato section
maybe that might be the source of the problem

I don't see a MOSFET in this model. I'm not sure as to what model it really is. I'm not able to specify a schematic for it. It has 4 EL84's, 2 ECC83's, 1 12AX7 and an unidentifiable rectifier tube (5A__ ?). I'm ordering new EL84's matched and the solid state rectifier for it.

[Lee_ranaldo]

I had a quite "similar" problem with a pearl flanger. I know there is no comparison between circuits but.

The Flanger had a constant ticking along with the LFO
I tracked the problem and the faulty component was a polarized cap leaking some DC voltage into the audio path.

I suggest that u should check the caps involved in the vibrato section if they have some DC leakage on them and also coupling caps.

If the amp has been under repair or has some mods maybe a new "non-stock" wiring is carrying that inducted noise to the power stage.

I'm gonna look my schematics too see what could be causing this. I hope a tube amp guru have more knowledge about this.