Guitar Amp Noise

[benni]

Hey everyone, I hope this is the right place for this.
Anyway, I'm getting some fairly gnarly noise coming from my tube amp, in a bad way. It's more of a hiss than a hum, it isn't consisent (has some pops and clicks), and isn't audible when in standby mode.

Any ideas what could be causing it - tubes or even speaker perhaps? I had a power tube die on me before, but that just killed any output altogether, so I'm fairly sure it's not that. Otherwise, I'm pretty stumped!

[Khron]

Re: Guitar Amp Noise

One of the preamp tubes going dud, perhaps?

[keeney123]

Re: Guitar Amp Noise

Hey everyone, I hope this is the right place for this.
Anyway, I'm getting some fairly gnarly noise coming from my tube amp, in a bad way. It's more of a hiss than a hum, it isn't consisent (has some pops and clicks), and isn't audible when in standby mode.

Any ideas what could be causing it - tubes or even speaker perhaps? I had a power tube die on me before, but that just killed any output altogether, so I'm fairly sure it's not that. Otherwise, I'm pretty stumped!

It would help to know what type a Amp it is. Model # Brand name? Also, if you have a service manual that would help? Do you have any electronic test equipment?

[Longbow]

Re: Guitar Amp Noise

Any ideas what could be causing it?

Yes, lots of ideas. This is where your career as an electronics engineer begins. How about starting with the schematic? Keep in mind that even when new, most guitar amps have quite a bit of noise, not including the noise picked up when you plug in an instrument.

[peter.uttley]

Re: Guitar Amp Noise

I'm pretty sure it will be a power tube problem ..what's the amp ?

[petehall347]

Re: Guitar Amp Noise

clean the valves pins and sockets ..
you could first try gently rocking them to see what happens .

[Concordian]

Re: Guitar Amp Noise

Benni, I'm assuming you're aware of the "normal" noises that can occur even with good amps (e.g. from guitar cables that aren't connected to a guitar, and from single coil pick-ups that pick up 60 cycle magnetic fields from nearby electrical appliances) and that you've eliminated those as potential causes.

That said, I've done so (eliminated those) with a Fender Hot Rod Deluxe III. Even without a cable plugged into the unit, it has white noise (hiss) and crackles and pops, at times. As it turns out, this is a common problem with these units, and they are supposed to be fixable by installing higher quality replacement components from one or another of the following kits:

https://www.fromelelectronics.com/co...ucts/hrdx-mods

Even Fender puts cheap caps in their tube amps, appareltly! And if you look at his other kits, I believe every one of them is for a Fender amp (i.e. several of Fender's other models appear to also suffer from component quality issues).

Since discovering those kits and the info that accompanied them, I got busy studying a schematic for an old 80's 15 watt solid state Peavey amp, then spent some time looking over its circuit board. Even without a cable plugged in, it was putting out whitenoise and (60 cycle) hum, especially on its distortion/gain channel, when gain was turned up. It was a horridly loud, nasty sound!

After measuring voltage drops and taking a look at the caps (capacitors) on the board, it became pretty clear that the caps had never had high enough voltage ratings to be safe in the circuit, given the ranges that can result from the tolerances of other components (i.e. how close they're guaranteed to be to their stated values) in the system. One capacitor was being overtaken by a voltage that exceeded its rating by 20% (30 volts across a 25 volt cap). Another was very close to its 35 volt limit.

Given the age of the caps, I figured it best to simply replace them all. I have some Nichicon caps on order that are all rated at 63 volts, each. Hopefully I'll be able to get those installed, soon, and let you know how it turns out. If that doesn't completely clear things up, I'll go in and begin to test diodes and resistors. But whatever the results, I'll try to let you guys know how it all turns out.

[Concordian]

Re: Guitar Amp Noise

Actually, I'm only replacing the electrolytic caps. I wasn't concerned about the ceramics or whatever other types might be in there.

[Longbow]

Re: Guitar Amp Noise

Even without a cable plugged in, it was putting out whitenoise and (60 cycle) hum, especially on its distortion/gain channel, when gain was turned up. It was a horridly loud, nasty sound!

After measuring voltage drops and taking a look at the caps (capacitors) on the board, it became pretty clear that the caps had never had high enough voltage ratings to be safe in the circuit, given the ranges that can result from the tolerances of other components (i.e. how close they're guaranteed to be to their stated values) in the system. One capacitor was being overtaken by a voltage that exceeded its rating by 20% (30 volts across a 25 volt cap). Another was very close to its 35 volt limit.

Well it is all very nice to blame everything on incorrect part values, but I'd first like to see a schematic showing some of those incorrect voltages. When a number of capacitors start leaking or failing, incorrect d.c. voltages can appear anywhere and everywhere.