I have a cheap hybrid guitar amp. Beth I get A108. It has a 12ax7 input and SS output.
It is NOISY. So noisy that I can even use headphones with it. 60hz it sounds like. Louder with volume.
I can’t find a ground loop problem, and I tried an alternative power for the tube heaters without any change.
The transformer has what looks like a center tapped main winding. Two yellow, one brown. The brown (center) is tapped to the SS output, referenced to one of the yellows, for 13v and a full wave rectifier. The yellows look to give 26v to a half wave rectifier, I assume somehow powers the tube.
I don’t have a schematic, and probably won’t be able to map it from the PCB. I just hope to make the noise tolerable and am looking for suggestions. I can’t help but think it is the power supply.
Or maybe the amp just sucks.
Any help is appreciated!
It is NOISY. So noisy that I can even use headphones with it. 60hz it sounds like. Louder with volume.
I can’t find a ground loop problem, and I tried an alternative power for the tube heaters without any change.
The transformer has what looks like a center tapped main winding. Two yellow, one brown. The brown (center) is tapped to the SS output, referenced to one of the yellows, for 13v and a full wave rectifier. The yellows look to give 26v to a half wave rectifier, I assume somehow powers the tube.
I don’t have a schematic, and probably won’t be able to map it from the PCB. I just hope to make the noise tolerable and am looking for suggestions. I can’t help but think it is the power supply.
Or maybe the amp just sucks.
Any help is appreciated!
I did, but got apple translated...
Berhinger A108
Not sure if the photos will help much.
Thanks for responding!
OK, the amp is an AC108, not A108. You can see that on the front panel in your photos.
Are you saying the volume control turns the hum up and down? Does the gain
Is there DC voltage on the speaker? switch affect the level of hum?
In any case, I'd first check for large ripple on the power supplies.
Remove the tube, does hum remain?
Are you saying the volume control turns the hum up and down? Does the gain
Is there DC voltage on the speaker? switch affect the level of hum?
In any case, I'd first check for large ripple on the power supplies.
Remove the tube, does hum remain?
Thanks! I appreciate your time.
Without the tube there is less hum and no longer volume dependent.
Gain and volume increase the hum when tube present.
I don’t find any DC on the speaker, but I only have voltmeters, no scope.
I powered it though and isolated power supply and still had the same hum.
I need a scope, but don’t have one and haven’t used one in 30 year or more.
My suspicion is the tube power supply, and crappy transformer. I will have to try to bypass it with an alternative supply, but I am not sure I have a suitable transformer. Also, since the two power circuits are linked with the transformer, I am not sure exactly how to go about it without a similar set of windings. My skill set is limited and rusty.
Work backward. Check to see the output am is working properly. The chip amp with the heat sink should tell you what part it is. From there you figure out if it is working.
Where is the noise coming from?
You have no test equipment. Is the chip bolted to the heat sink working properly? Is the noise coming from the power supply? Is it coming from the circuitry befor the chip amp? How are you going to check?
You have no test equipment. Is the chip bolted to the heat sink working properly? Is the noise coming from the power supply? Is it coming from the circuitry befor the chip amp? How are you going to check?
"Crappy transformer"? What does that mean? I doubt the transformer is making the preamp hum. It makes the voltage or it doesn't.
Use your meter, got ripple or don't. also could be as simple as a ground connection near the input.
Use your meter, got ripple or don't. also could be as simple as a ground connection near the input.
I will try to ask more specific questions when I have time to try a little more troubleshooting.
I don’t really understand the power supply circuit and the PCP is pretty opaque to tracing.
Thanks!
Power supply is just basic, only remotely unusual part is the voltage doubler making the tube "high voltage."
I figure that was how they did it. Could that be a problem? Seems more likely a bad ground loop at other bad connection. I will double my search.
I think there are two ground loop problems and one must be in the input to the tube, since it is amplified there.
What about basic troubleshooting? Rinse switches and pots, look for poor contacts and solder joints etc?
Did a bunch of that, but I agree with Turk182, so clearly I need to go through it again.
Thanks for the thoughts!
have a magnifying glass? visual inspection is a good thing but assuming that just because things "look" good the problem isn't there, i've surprised myself how many "good" to the naked eye solder joints turned out to have microscopic cracks.
the other i would do is clip one meter lead to the input ground and check every other ground point possible.
have you managed to find a schematic?
the other i would do is clip one meter lead to the input ground and check every other ground point possible.
have you managed to find a schematic?
Thanks. No schematics available and the PCP is not easy to trace. I will try these ideas when I get time to get back to it.
I finally had an opportunity to mess it up...I mean work with it.
In my testing, I blew up the SS chip by slipping and shorting. I replaced that and it still has the exact same noise. So at least I know it wasn’t the chip.[emoji2361]
It otherwise seems the same. It is not grounded to chassis or the power ground. If I do ground it. Nothing changes. All the connections seem sound otherwise and I find no bad solder joints with a magnifying glass.
I think I will just use it without the headphones and put up with it until I find something else. Thanks for the input!
In my testing, I blew up the SS chip by slipping and shorting. I replaced that and it still has the exact same noise. So at least I know it wasn’t the chip.[emoji2361]
It otherwise seems the same. It is not grounded to chassis or the power ground. If I do ground it. Nothing changes. All the connections seem sound otherwise and I find no bad solder joints with a magnifying glass.
I think I will just use it without the headphones and put up with it until I find something else. Thanks for the input!