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AC30 screaming at me

9/19/2005 7:53 PM
David	AC30 screaming at me Hey all, I've got an AC30 RI from 1999 (Made in England, thank god!), that's causing all sorts of pain. She was like this when I got her, so I don't know what exactly caused the sickness, but here's a description of the symptoms: When I first powered up, there was very little sound, and what came out was all distorted and farty, and there was a sqeal that came and went. I replaced the power tubes, and the lo-fi distorted sound went away, but the squeal was still there. I've also replaced the rectifier, but the squeal still plagues me. The nature of the squealing noise is not changed by turning knobs at all. It squeals regardless of whether or not anything is plugged in. I tried pulling the power tubes (first the outside pair, then the inside pair). The squeal goes away when I pull the el84 closest to the rectifier. Another strange symptom of this disease - when the amp is upside down with the circuit board pulled partway out, the squeal goes away. When I reinstall the board and flip the amp right side up (tubes & trannies up), there is a slight click or popping sound and then the squeal starts again. Any suggestions would be very much appreciated. Thanks, David

9/19/2005 7:53 PM

David

AC30 screaming at me

Hey all,

I've got an AC30 RI from 1999 (Made in England, thank god!), that's causing all sorts of pain. She was like this when I got her, so I don't know what exactly caused the sickness, but here's a description of the symptoms:

When I first powered up, there was very little sound, and what came out was all distorted and farty, and there was a sqeal that came and went. I replaced the power tubes, and the lo-fi distorted sound went away, but the squeal was still there. I've also replaced the rectifier, but the squeal still plagues me.

The nature of the squealing noise is not changed by turning knobs at all. It squeals regardless of whether or not anything is plugged in.

I tried pulling the power tubes (first the outside pair, then the inside pair). The squeal goes away when I pull the el84 closest to the rectifier.

Another strange symptom of this disease - when the amp is upside down with the circuit board pulled partway out, the squeal goes away. When I reinstall the board and flip the amp right side up (tubes & trannies up), there is a slight click or popping sound and then the squeal starts again.

Any suggestions would be very much appreciated.

Thanks,
David

9/19/2005 9:16 PM
Don Symes	Any signs of scorches or arcing on the inside of the chasis? (PCB amp or tagboard and wires?) Any unusually long leads or tall solder mounds on the chassis side of the board? With the board in place, but the amp still inverted, how does it do? (you may need to come up with some cinderblocks or something to hold it up) And, as always, I refer you to R.G.Keen's Tube Amp Debugging Page .

9/20/2005 12:19 AM
David	Hi Don, Its PCB, so that sucks to trace. And of course they decided it was a good idea to use PCB mounted preamp sockets. Luckily, I don't see any scorching on the chassis. The only leads that may be out of the ordinary are the leads from the power tubes to the OT. The white lead, which connects the OT to the pair of EL84s furthest from the rectifier, seems to have some brownish discoloration on it. Looks like it may be a little bit scorched. The solder jobs on the board are pretty clean, but the power tube sockets look to have a little bit of discoloration around some of the pins. It looks as though it may have been caused by a too-hot soldering iron, though, and the solder on the socket pins would seem to agree. It's late night here, so I can't test it with the board in place and the amp inverted, but I will try that and get back to you. Thanks

9/20/2005 12:19 AM

David

Hi Don,

Its PCB, so that sucks to trace. And of course they decided it was a good idea to use PCB mounted preamp sockets. Luckily, I don't see any scorching on the chassis.

The only leads that may be out of the ordinary are the leads from the power tubes to the OT. The white lead, which connects the OT to the pair of EL84s furthest from the rectifier, seems to have some brownish discoloration on it. Looks like it may be a little bit scorched.

The solder jobs on the board are pretty clean, but the power tube sockets look to have a little bit of discoloration around some of the pins. It looks as though it may have been caused by a too-hot soldering iron, though, and the solder on the socket pins would seem to agree.

It's late night here, so I can't test it with the board in place and the amp inverted, but I will try that and get back to you.

Thanks

9/20/2005 2:39 PM
David	Well, my hypothesis about the squeal only coming through when the amp was right side up seems wrong. I flipped it over (tubes & trannies down), and played it for a while - it sounded fine. So I flipped it back over (tubes & trannies up) and kept playing for a few minutes - again it sounded fine. So I left it on standby for maybe 30 minutes and came back to it. It was still tubes & trannies up at this point. As soon as I took it off standby the squeal was back. I put it back on standby, flipped it over, took it off again and the squeal was still there with the board down. I did notice something else that may or may not be of significance. When I tap any of the pot shafts while the amp is on, I can hear the tapping noise through the speakers. Also, when I switch the amp into standby, I can hear the switch "click" through the speakers, too - seems like more than just the normal pop of a switch being thrown.

9/20/2005 2:39 PM

David

Well, my hypothesis about the squeal only coming through when the amp was right side up seems wrong. I flipped it over (tubes & trannies down), and played it for a while - it sounded fine. So I flipped it back over (tubes & trannies up) and kept playing for a few minutes - again it sounded fine.

So I left it on standby for maybe 30 minutes and came back to it. It was still tubes & trannies up at this point. As soon as I took it off standby the squeal was back. I put it back on standby, flipped it over, took it off again and the squeal was still there with the board down.

I did notice something else that may or may not be of significance. When I tap any of the pot shafts while the amp is on, I can hear the tapping noise through the speakers. Also, when I switch the amp into standby, I can hear the switch "click" through the speakers, too - seems like more than just the normal pop of a switch being thrown.

9/20/2005 5:10 PM
Don Symes	AHA! (maybe) Classic symptom of a microphonic component. If this amp has an EF86 preamp tube, start there. Then, try removing all the preamp tubes but not the PI, and work your way back toward the input, adding one tube at a time until you find a culprit. Be prepared to find more than one bad tube. If you have only the output tubes installed, do you get that tapping response? It's entirely possible you'll find, not a microphnic tube, but a microphonic cap, choke or resistor - pretty much in descending order of likelihood. Good luck! (and don't touch even COLD tubes with bare fingers!)

9/20/2005 5:10 PM

Don Symes

AHA! (maybe) Classic symptom of a microphonic component.

If this amp has an EF86 preamp tube, start there.

Then, try removing all the preamp tubes but not the PI, and work your way back toward the input, adding one tube at a time until you find a culprit.

Be prepared to find more than one bad tube.

If you have only the output tubes installed, do you get that tapping response?

It's entirely possible you'll find, not a microphnic tube, but a microphonic cap, choke or resistor - pretty much in descending order of likelihood.

Good luck! (and don't touch even COLD tubes with bare fingers!)

9/20/2005 11:13 PM
David	How can I tell if a cap, choke, or resistor is microphonic? All I've got for testing purposes is a DMM. 'Will do!' on the "not touching cold tubes with bare fingers" thing, but why is that a bad idea? Just asking for my own information - for now, I definitely will take your word for it.

9/21/2005 5:01 AM

Don Symes

quote:
"How can I tell if a cap, choke, or resistor is microphonic?"

Tapping individual components with a non-conductive tool (this is call the 'chopstick test').

quote:
"touching cold tubes with bare fingers" thing, but why is that a bad idea?"

One, tubes do break, and slicing up your fingertips really sucks. Second, the oils and dirt from your fingers get corrosive when heated. There is a fear ... call it folklore ... that the corrosion could eat a pinhole in the glass and 'air' your tube. Third, it looks nasty

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