Other Amps

5E3 problem

1

This has got me really stumped. Months ago I built a 5E3 with some nice MM trannies, Mojo chassis, board and cab, Sozo and F+T caps, Celestion Gold, all the stuff I usually would use. I use a little choke instead of the big resistor between the first filter caps, and use the Ampeg style PI, also known as the PaulC mod.

But whenever I play this particular amp I get a harsh, static-y crackle on the note attack. I have checked for dry joints, bad caps, bad tubes, broken tube sockets, arcing tube lugs - every single thing I can think of. I have replaced every tube, checked every socket...

So this weekend I totally rebuilt the amp. I used new resistors, caps, octal tube sockets, and even a new board. I am very anal about my builds. I fill each eyelet with solder but not over-fill, and I'm super careful with wire routing.

So when I was finished the rebuild I tested the amp through my tweed Super's cab which has a Celestion gold 12" in it. All good! No crackles, very low noise floor, low hum, sounds absolutely killer. So in a good mood I put it back into the Mojo 5E3 cab with the Celestion Gold 12" speaker in it, and fire it up. Immediately the crackly attack is back.

What on earth could be causing this? One of the 6V6s is quite close to the speaker but it's nowhere near actually touching and that has never been a problem before in any of my other 5E3s anyway.

Hmmm... I guess my next job is to test the speaker and it's wiring with a different amp. Has anyone had something like this before? It's kind of annoying.

2

Just plugged another amp into the speaker in the 5E3 cab - perfectly fine. So I doubt it's the speaker or the speaker wiring.

If I turn the amp on with volume up but nothing plugged in I can get it to do the crackle by tapping the chassis, pretty much anywhere but most strongly if I tap by the on/off switch and standby switch. I have used a chopstick to tap everything and as long as the tapping impacts the chassis I can get the crackling. So I would imagine that the cracking is caused by the speaker vibrating the chassis. But which part is crackling?

Tapping directly on either switch cause a great deal of crackling and even a buzz which lasts a second or so. Could the switches be faulty? They are the standard Carling Fender-style on/off switches. Both switches cause the crackling when tapped - I have never experienced this before. I've used dozens of these switches - why would the two in this amp be faulty? And only when the chassis is vibrated - the amp works fine when not in the cab.

Any ideas??

3

Just plugged another amp into the speaker in the 5E3 cab - perfectly fine. So I doubt it's the speaker or the speaker wiring.

If I turn the amp on with volume up but nothing plugged in I can get it to do the crackle by tapping the chassis, pretty much anywhere but most strongly if I tap by the on/off switch and standby switch. I have used a chopstick to tap everything and as long as the tapping impacts the chassis I can get the crackling. So I would imagine that the cracking is caused by the speaker vibrating the chassis. But which part is crackling?

Tapping directly on either switch cause a great deal of crackling and even a buzz which lasts a second or so. Could the switches be faulty? They are the standard Carling Fender-style on/off switches. Both switches cause the crackling when tapped - I have never experienced this before. I've used dozens of these switches - why would the two in this amp be faulty? And only when the chassis is vibrated - the amp works fine when not in the cab.

Any ideas??

– JimmyR

Typically those sounds are indicative of a bad ground somewhere in the signal chain.

Also, don't ignore the quality of the connections on the filaments/pilot light. Not in the signal path, but can cause all sorts of funny microphonic stuff if there's a cold joint.

4

The standby switch may have arc'd and be causing incomplete contact. The switches are a cheap swap. If you are using a tube recto you might just skip the stby switch altogether.

5

Do you have a foil shield on the rear cover?

6

What I would do is isolate the 5E3 mechanically from the cab by running a long speaker lead to determine if the cause is the speaker/cab shaking the chassis.

But yea I would think replacing the switches would be a cheap experiment. You could always use them later on another build if they aren't faulty.

7

Well I kinda have done the experiment with the vibration - When I played the amp out of the cab it was perfectly fine. I thought I had sorted it. It was only when the amp went back into the cab that the problem resurfaced.

Kap'n I have had a problem with the filaments before, so that was one of the first things I checked. And I have used new sockets and resoldered any other connections to do with the filaments, so I'm quite confident about those - thanks! And cold joints were my first thought. But I have checked and now rebuilt the entire amp so I don't think it's a cold joint. I am so anal that it's rare for me to have that as a problem!

I think it must be a switch. I have ordered some new ones but I guess I can steal one from another amp in the meantime to check. I've heard a similar sound from a dirty input socket before too, but this is a more aggressive crackling sound. I have played the amp while watching for arcing with the back off and can find no sign of arcing at tube sockets or the actual tubes themselves. When I solder the tube sockets I always bend the pins outwards and make sure to have the bare minimum of wire exposed to avoid arcing being an issue.

Powdog I have wondered if I really need the standby. It's unlikely that I will be using the amp for gigs. I just like having it there as it uses the hole which was made for a ground switch.

Thanks for the ideas guys - I really appreciate them all!

8

I usually put a voice switch into the ground hole, just to give the 5E3 a little more latitude.

9

Hello Jimmy, I had a similar problem with my 5e3 build. In my case it was the input Jack resonating resulting in bad contact. Probably this is not the case here as the sound is present when nothing is plugged in, but you never know....

10

Check whether the baffle screws are tight.

11

You don't really need a standby on this amp. None of the originals had them. Mine does just out of habit, and I needed something to fill the hole.

12

Don't overlook the proximity of the speaker to the 6V6 tube. Had to flip the baffle on this Orpheus because it did the same thing.

13

man we got some serious amp fixin dudes on the scene

14

I'm with Jer - I've had amps with almost no gaps between speaker, tranny, and tubes and they were fine. I've had other amps with wild magnetic coupling/oscillation issues.

15

I've heard about--but not witnessed--microphonic transformers before. Could it be the choke? Do you have a spare, or a resistor you could swap in? Just to eliminate the possibility.

IDK, this one's a noggin scratcher

16

Try reversing the phase of the speaker.

17

Thanks fellas - I'll try some stuff based on your suggestions and report back.

18

Wow. Well I can't say 100% that it's fixed now because my son is asleep in the next room, but I have turned the amp on and no amount of tapping anywhere on the chassis will cause an arcing, nasty crackle noise. I have plugged in a lead to make sure signal was going through and yup, it sure is. In an hours or so when my son wakes up I will test by playing through it.

The fix? Billy's "Try reversing the phase of the speaker." To be honest I didn't expect it to do anything. But it was the easiest option and I believe in following up every option.

Ok Billy - how the hell does that work??

19

Yup - All fixed. Had a decent play with it and my 6120 and it sounds wonderful. And I needn't have rebuilt it! I still have no idea why just swapping the speaker wires around would fix it with the symptoms I had but everything works perfectly now. Before just lightly tapping the switches would make hideous sounds - now it's as quiet as a very quiet mouse.

Thanks Billy!

20

Could it be that the phase doesn't actually matter, but his suggestion got you to physically disconnect and reconnect the speaker terminals, thereby scraping off some schmutz which was the true cause of the bad connection?

21

I don't think so. It's a newish speaker and the connections were very solid and clean. But nothing is impossible.

22

I would love to know more about that, Billy! I always thought speakers were AC devices, but I am eager to learn this stuff.

23

An amp I built in the mid-seventies did the same thing. It drove me nuts for a couple of days.

24

Billy, What is the science behind it? I am very interested.

25

I'll include a full explanation in my posthumously published memoirs.


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