systemtruck
Well-known member
Hello all,
I’ve modified an old tube oscillator to become a guitar amp. Also recapped all electrolytics and audio pathway caps and large values.
It sounds terrific except there is a distinct 120Hz audio hum at a low volume. No other noise whatsoever, and no mechanical hum, just a nice clear 120 Hz in audio at various stages throughout circuit. It’s pretty low in volume, but it ruins silent moments.
This original unit is from 1955.
I tried hooking up a lantern battery to briefly test using DC heaters instead of AC, and that did nothing. I’ve also chop sticked quite a bit with zero results.
The noise is so pure that it doesn’t seem like it’s proximity to the power transformer that’s the issue. Although, it is indeed close. But, there is no buzz or hiss at all, it’s just a perfect 120 Hz. In my experience listening to transformer proximity hum, it’s usually much messier sounding and has some high buzz along with the mains sound.
So I’m assuming it’s coming from the rectified DC signal which is the only thing i can think of that would present a perfect 120Hz signal. But it doesn’t make sense to me that the signal would be so strongly AC instead of DC since the caps are all new. So I’m a little confused about that. I tried a different 5Y3 i had on hand, which seems to work fine in other equipment, but i don’t have a tube tester on hand. The results were identical. So I’m guessing it’s not the 5Y3.
So that leads me to grounding… in all of the wiring they simply used all of the tube sockets as ground points, which are of course fixed to the chassis in various places. The first three filter caps are also connected directly to the chassis. They are in a can, which was manually wired to signal ground near rectifier but also to the chassis with an extra lead.
So there are at least like 8-10 different places that the signal ground goes into the chassis.
Would a 120Hz hum make sense as a symptom from this? Or would it mean that there is also an issue with the rectification itself? I have a couple other different rectifier tubes but am thinking they put out higher voltages. The schematic here says 5AR4, which I have too, but the unit contains a 5Y3 and is screen printed 5Y3 on chassis next to tube.
I’ve labeled 4 example spots where signal ground is connected to chassis, and attached the full schematic below those…
I’ve modified an old tube oscillator to become a guitar amp. Also recapped all electrolytics and audio pathway caps and large values.
It sounds terrific except there is a distinct 120Hz audio hum at a low volume. No other noise whatsoever, and no mechanical hum, just a nice clear 120 Hz in audio at various stages throughout circuit. It’s pretty low in volume, but it ruins silent moments.
This original unit is from 1955.
I tried hooking up a lantern battery to briefly test using DC heaters instead of AC, and that did nothing. I’ve also chop sticked quite a bit with zero results.
The noise is so pure that it doesn’t seem like it’s proximity to the power transformer that’s the issue. Although, it is indeed close. But, there is no buzz or hiss at all, it’s just a perfect 120 Hz. In my experience listening to transformer proximity hum, it’s usually much messier sounding and has some high buzz along with the mains sound.
So I’m assuming it’s coming from the rectified DC signal which is the only thing i can think of that would present a perfect 120Hz signal. But it doesn’t make sense to me that the signal would be so strongly AC instead of DC since the caps are all new. So I’m a little confused about that. I tried a different 5Y3 i had on hand, which seems to work fine in other equipment, but i don’t have a tube tester on hand. The results were identical. So I’m guessing it’s not the 5Y3.
So that leads me to grounding… in all of the wiring they simply used all of the tube sockets as ground points, which are of course fixed to the chassis in various places. The first three filter caps are also connected directly to the chassis. They are in a can, which was manually wired to signal ground near rectifier but also to the chassis with an extra lead.
So there are at least like 8-10 different places that the signal ground goes into the chassis.
Would a 120Hz hum make sense as a symptom from this? Or would it mean that there is also an issue with the rectification itself? I have a couple other different rectifier tubes but am thinking they put out higher voltages. The schematic here says 5AR4, which I have too, but the unit contains a 5Y3 and is screen printed 5Y3 on chassis next to tube.
I’ve labeled 4 example spots where signal ground is connected to chassis, and attached the full schematic below those…