JVM 205 Problem - Distortion / crackle / scratchy sound when playing notes

[Dawis67]

Unless the amp is faulty, it has no effect.

So what you are saying is that I can still use the chassis as the ground for multimeter measurements as well as discharging of capacitors using a resistor to chassis?

I suggest to check that your house has ground fault interrupt GFI / residual current detector RCD breakers. If it doesn’t, use a wireless connection between your guitar and whatever else. Use a GFI / RCD adapter on your amp’s mains cable.
And get your house rewired asap.

Not my house, only renting. Our "fuse" cabinet does have what looks like RCD breakers (at least according to google images), and some digital meters. Overall modern looking so likely complies with whatever Norwegian guidelines on that are.

On a more general note, probably something I should have mentioned earlier. The amp has also lately become noisier. The 60hz humm is much louder than before (actually can't really recall hearing it before the issue started). There has always been the noise from not grounding the guitar by touching bridge/strings, but now that noise has become even louder and more resonating. When I touch the bridge, a lot of the noise goes away, but the 60hz humm is still there and considerably loud. Volume of the humm can be controlled by amps volume knobs. Didn't mention it as I initially thought it was the guitar grounding or guitar cables fault, but after checking that the connections of both are fine, I now think it might be a part of the overall amp problem.

 

[Pete Farrington]

So what you are saying is that I can still use the chassis as the ground for multimeter measurements as well as discharging of capacitors using a resistor to chassis?

Yes, there’s no need for an electrical connection to planet earth.

my house isn't wired with ground in the sockets

How did you find that out, eg remove the outlet from the wall?

Not my house, only renting.

I suggest to check with your landlord. In the UK, all rental properties must have their electrical installations inspected and certified as compliant to regulation by a trained electrician every 5 years, and the tenant must be given a copy of the certificate.
So your landlord may well have an electrical report that he’d let you see.

The amp has also lately become noisier. The 60hz humm is much louder than before (actually can't really recall hearing it before the issue started). There has always been the noise from not grounding the guitar by touching bridge/strings, but now that noise has become even louder and more resonating. When I touch the bridge, a lot of the noise goes away, but the 60hz humm is still there and considerably loud. Volume of the humm can be controlled by amps volume knobs

If you remove the signal lead from the front panel input, does the hum stop, or carry on unaffected?

 

[FleshOnGear]

In the UK, all rental properties must have their electrical installations inspected and certified as compliant to regulation by a trained electrician every 5 years, and the tenant must be given a copy of the certificate.
So your landlord may well have an electrical report that he’d let you see.

Wow! Sounds like it’s a little tougher to be a slumlord in the UK, compared to the US.

 

[Dawis67]

Didn't get time to test the voltages today. Though I have another safety question. Are the 3 bias pin voltages safe to touch? I've seen people in videos aligator-clip leads to the 3 pins and just switch em out as they measure one tube or the other. Now I assume they aren't dangerous but can't rule out internet jackasses.

 

[TheKman76]

Now I assume they aren't dangerous but can't rule out internet jackasses.

The safe assumption is that they *are* all dangerous and Internet users *are* all jackasses.

Now, what three pins exactly?

 

[Dawis67]

The safe assumption is that they *are* all dangerous and Internet users *are* all jackasses.

Now, what three pins exactly?

The 3 pins circled in blue, aka bias pins.

 

[TheKman76]

These are low voltage so, yes, there is little risk with these directly.
Of course, you must apply the approriate care when working inside a high voltage device.

 

[Dawis67]

Update:
Finally tested power amp voltages and bias. Briefly tested colder and hotter bias to see if and how it affects the issue.

First, the voltages.
- My initial measurement yielded [plate](bias pin): V2: 462V(29.7mV), V3:463V(33.8mV). Not sure if this matters but I initially measured plate voltage with the amp on but on standby, this yielded about 480V. The voltage dropped to 462V in play mode.
- Final voltage and bias: V2/V3: 447V(35.7mV). Aimed for 65%(16.25W) which ideally should have been 36.3mV at 447V. Essentially ended up a bit colder.

Bias effect on the amp issue:
The issue was very identifiable especially when contrasting a cold and hot bias. This glassy, thin and squeely top distortion remained the same on both cold/hot bias. It was like hearing a stronger or weaker distortion (depending on bias) with the issue layered on top. When I say glassy, thin and squeely distortion, its exactly what I've concluded that the issue sounds like. As mentioned previously, it almost exclusively effects higher notes (like around 12th fret and up).

 

[mab1376]

Update:
Finally tested power amp voltages and bias. Briefly tested colder and hotter bias to see if and how it affects the issue.

First, the voltages.
- My initial measurement yielded [plate](bias pin): V2: 462V(29.7mV), V3:463V(33.8mV). Not sure if this matters but I initially measured plate voltage with the amp on but on standby, this yielded about 480V. The voltage dropped to 462V in play mode.
- Final voltage and bias: V2/V3: 447V(35.7mV). Aimed for 65%(16.25W) which ideally should have been 36.3mV at 447V. Essentially ended up a bit colder.

Bias effect on the amp issue:
The issue was very identifiable especially when contrasting a cold and hot bias. This glassy, thin and squeely top distortion remained the same on both cold/hot bias. It was like hearing a stronger or weaker distortion (depending on bias) with the issue layered on top. When I say glassy, thin and squeely distortion, its exactly what I've concluded that the issue sounds like. As mentioned previously, it almost exclusively effects higher notes (like around 12th fret and up).

Did you ever figure this out? i have a similar issue but a bit more subtle. I get breakup and crackling on the green channel above the 12th fret. Gain is at 9 o'clock, and master is at noon (with an attenuator). It is more noticeable recording through a captor X.