JTM45 experts, I need your help!

[Mike.]

I bought a custom-built handwired JTM45 about 6 months ago and only recently bought a cab for it. It is an orange PPC212 open-back cab, loaded with a h55 and a h75.

The problem is that when I hit certain notes, the amp makes a static, ripping noise. It also makes a static noise right after I've played something and the amp is quiet. Does this mean that certain frequencies are vibrating certain amp components (meaning that they are loose), or is the amp vibrating a bad solder joint inside the speaker cab?

The volume also used to jump to a higher level and stay there when I played certain notes. I would have to hit the standby, lower the volume to 0 and put it back to where it initially was.

It's a great sounding amp, but these problems really are preventing me from enjoying it right now.

[davemccarthy707]

Its hard to diagnose an amp over the internet. If I was a betting man I would guess maybe a power tube socket. The low notes cause it to vibrate and then arcing occurs. Best to get it looked at. It could be any number of things.

[WaltC]

and your suspicions about bad internal solder joints is worth checking out as well...

[GearHeadFred]

Difficult problems to help you with remotely.. If it's a custom built layout (not following an established layout), it could be a parasitic oscillation.. Have you tried the easy things like swapping the pre-amp tubes? Some brands of current production 12ax7's don't like to be used in the cathode follower position of that circuit.

Definitely get it fixed because they are AWESOME amps when working correctly!

[tfunster]

Are you sure it's the amp?

Sounds like the noise my strat would make after I did a hack job on the wiring--a "ripping static noise"

[Mike.]

Are you sure it's the amp?

Sounds like the noise my strat would make after I did a hack job on the wiring--a "ripping static noise"

It's been happening with both of my guitars ( strat and es-339).

I don't have any 12ax7s at the moment, but I'm starting to think that it's just a bad solder joint.

And I've searched but could not find a proper definition of parasitic oscillation when it comes to guitar amps. What does it mean, and how do you fix it?

[GearHeadFred]

And I've searched but could not find a proper definition of parasitic oscillation when it comes to guitar amps. What does it mean, and how do you fix it?

It's when the amp "self oscillates".. it often happens at frequencies above the audible range, so the effect is parasitic in that it robs the amp of power and response. It's usually a result of bad lead dress.. Improper (or lack of) pairing/twisting/location of wires.. Wires that are too long.. That type of thing.. That's why I asked about the layout scheme.

An oscilloscope is usually required to identify and correct this.

If it's a loose solder joint, you should be able to bring on the problem by tapping on the circuit board. Be sure to use a wood stick (not a pencil - graphite conducts!) to do this.