Weird sound in a recent Magnatone amp - any ideas?

H

HamishR

Well-known member
A friend came around recently with a relatively new Magnatone amp - well it's about 3-4 years old but it's one of the newer range. Apparently it's Magnatone's version of a Deluxe Reverb. I believe it was the Twilighter Mono 1x12. It has a great sound, runs pretty quiet and has a cool tremolo/vibrato thing going on. Two 6V6s, etc.

The strange part is that after it's been on for a few minutes it can get a strange deep feedback kinda sound. It doesn't overpower the guitar sound but is very annoying. Pressing on the back panel can make it go quieter, and I found tilting one foot of the amp off the floor could make it go away, only to reappear as soon as I let that foot touch the floor again. (My office is in an old house and the floorboards aren't perfectly flat!)

None of the tubes sound particularly microphonic and swapping out the 6V6s and the rectifier didn't make any difference. Apart from the weird noise the amp sounds very good and can go for some time without making the sound. We can't work out where the sound is coming from. Any ideas?
 
The usual suspect is the first preamp tube or the reverb.
I would use a second person who grabs the tube with a glove on and see if low vibrations of a single tube causes this. I had good results with that method. Since most of the times this happens to precious vintage preamp tubes, you can buy silicone rings
to damping them.
I use them on a Mullard preamp tube and even on a precious pair of RCA 7591a output tubes.
 
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We did try tapping the tubes but I will suggest it to my friend. To me it really did sound like reverb feeding back but it happened even when the reverb was completely off.

Thanks! You have give me a few ideas to try!
 
Mono Twilighter? What’s the point?…

On a serious note: check out Psionoc Audio on YouTube. He’s an amp tech and does live q&a’s on Saturday’s most weeks. Pop on a tad early and ask him in the chat window…
 
I suspect mechanical failure based on this being "positional" in nature. broken solder joints, worn out tube sockets, that sort of thing. edge cases would be like a capacitor that's unsecured (no hot glue) that's mechanically stressed. you can tap those like tapping a tube to check for microphonics
 
I second what @ICTRock mentioned.

I have also had microphonic ceramic caps create feedback like sounds. It'll compound with what little microphonics exist in any good preamp tube if its in the vicinity. But when you mention the conditions that make it go away I think it must be a bad connection.

I wouldn't suggest opening it if you aren't comfortable with the voltages present and are able to safely check and discharge filter caps. And I wouldn't reccomend poking around with it powered up without something like a dim bulb tester. It does not make the amp safer for you to work on but will indicate a fault and protect the amp if you create a dead short. Wooden chopsticks from my favorite noodle bar are my prefered instrument for poking around spicy circuits.
 
Also if the schematic in this EL34 world thread is correct then a JFET is used to mute the Reverb return. So if there is an issue with the soldering of that part then that could cause the issue if it really sounds like the reverb.

Does the vibrato speed/intensity affect it in anyway?
 
HamishR said:
A friend came around recently with a relatively new Magnatone amp - well it's about 3-4 years old but it's one of the newer range. Apparently it's Magnatone's version of a Deluxe Reverb. I believe it was the Twilighter Mono 1x12. It has a great sound, runs pretty quiet and has a cool tremolo/vibrato thing going on. Two 6V6s, etc.

The strange part is that after it's been on for a few minutes it can get a strange deep feedback kinda sound. It doesn't overpower the guitar sound but is very annoying. Pressing on the back panel can make it go quieter, and I found tilting one foot of the amp off the floor could make it go away, only to reappear as soon as I let that foot touch the floor again. (My office is in an old house and the floorboards aren't perfectly flat!)

None of the tubes sound particularly microphonic and swapping out the 6V6s and the rectifier didn't make any difference. Apart from the weird noise the amp sounds very good and can go for some time without making the sound. We can't work out where the sound is coming from. Any ideas?
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Got a sound sample? If it's a microphonic tube, the vibration might change what it's picking up. Changing position might do that.
 
Thanks guys - I don't have the amp here but I'll talk to the owner and sort something out. I'm pretty comfortable inside amps having built quite a few! And I love Psionic Audio vids!
 
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