Bizarre issue. I have an Ampeg VT-120 I've posted about before, which I've taken on as a pet project. I like the amp a LOT where it is now.
It's had this constant but irregular crackle in the background since I got it. I didn't notice it much at first because it had so many other issues, but it's still there. I've replaced every electrolytic cap in the amp, every plate resistor, every coupling cap, every power supply dropping resistor (for other reasons, not for this problem), and the standby and power switches -- so I know it's not those. Swapping tubes does nothing. I can't isolate it to a particular tube stage, it's like it's coming from everywhere in the preamp. The only way to make it stop is to take out all the preamp tubes except the phase inverter. Even if the last tube before the phase inverter is out, if there are other preamp tubes before that still in, it will make the noise (albeit at a lower volume level).
I can turn up the crackle with either the gain controls OR the reverb control (even with no pan connected). The only thing that all these areas have in common is their ground path -- the bottom of the reverb pot, the gain controls, and the input circuit are all grounded to the neg terminal of first stage's filter and are AC-coupled to the chassis through C55 (which I've tried removing and altering the value, no change). The other parts of the reverb recovery stage are grounded elsewhere. Just the reverb pot is grounded to the input stage. I tried moving it but the amp hummed.
The WEIRD thing is -- it only makes this noise when the chassis is in the combo cabinet, RIGHT SIDE UP. It does not make the noise when the combo is turned on its side, does not make the noise when the chassis is right side up OR upside down OUTSIDE of the combo cab. ONLY inside the combo cab, right side up. There is no shielding or metal of any kind in the cab, just wood and tolex.
I think I'm going insane. Any ideas? I've attached an audio file of the noise recorded from the FX send.
vt120-noise.mp3
It's had this constant but irregular crackle in the background since I got it. I didn't notice it much at first because it had so many other issues, but it's still there. I've replaced every electrolytic cap in the amp, every plate resistor, every coupling cap, every power supply dropping resistor (for other reasons, not for this problem), and the standby and power switches -- so I know it's not those. Swapping tubes does nothing. I can't isolate it to a particular tube stage, it's like it's coming from everywhere in the preamp. The only way to make it stop is to take out all the preamp tubes except the phase inverter. Even if the last tube before the phase inverter is out, if there are other preamp tubes before that still in, it will make the noise (albeit at a lower volume level).
I can turn up the crackle with either the gain controls OR the reverb control (even with no pan connected). The only thing that all these areas have in common is their ground path -- the bottom of the reverb pot, the gain controls, and the input circuit are all grounded to the neg terminal of first stage's filter and are AC-coupled to the chassis through C55 (which I've tried removing and altering the value, no change). The other parts of the reverb recovery stage are grounded elsewhere. Just the reverb pot is grounded to the input stage. I tried moving it but the amp hummed.
The WEIRD thing is -- it only makes this noise when the chassis is in the combo cabinet, RIGHT SIDE UP. It does not make the noise when the combo is turned on its side, does not make the noise when the chassis is right side up OR upside down OUTSIDE of the combo cab. ONLY inside the combo cab, right side up. There is no shielding or metal of any kind in the cab, just wood and tolex.
I think I'm going insane. Any ideas? I've attached an audio file of the noise recorded from the FX send.
vt120-noise.mp3
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