KMD XV100SD Head Produces "Static Crescendo"/Popping

[badgercore]

Hey all!

Just picked up a used KMD XV100SD this afternoon and having a bit of a problem with it.

Brought the head home and set the impedance in back to 8 ohms to match my 2x12 cab. I fired it up and fiddled around for maybe 5-10 minutes at very low volume and the background hum/static began to gradually get louder in a crescendo until it made a pop. Once that began it started doing it more and more frequently, so I quickly shut it off. Afterwards, I noticed the back was very hot, like the tubes were putting out some serious heat.

Is this possibly just due to the fact that I was playing it at such a low volume, or indicative of a more serious problem? Perhaps some tubes that need replacing?

I know very little about tube amps, as this is the first all-tube amp I've owned, so go slow if you can.

Thanks for your time!

[Steve Conner]

Hey,

It's most likely a dodgy tube.

You should find some tubes in the back that are larger than the others. These are the power tubes. Inside each tube is a large grey lump of metal called the plate.

Fire the amp up while watching the tubes and wait for it to do its static noises thing. If the surface of the plate starts to glow red hot and you feel the tube putting out huge amounts of heat, you have a problem. Most likely the tube is worn out and needs replaced. You shouldn't use the amp while it's "redplating" like this, the excessive current can burn out other parts of it.

If one of the smaller ("preamp") tubes is worn out, that might also explain the static sounds, but it's not a dangerous situation, the amp will continue to work and make annoying noises.

There could be other reasons for it, but a bad tube is the most likely one.

[Gtr_tech]

Could be a marginal plate load resistor in the front end/phase inverter. 'Specially if they used 1/4w parts. Stick a cable in the effect return jack and see if it still makes the noise (you won't be able to use it that way, the cable just seperates the front end from the output section). That'll tell you if the problem is in the front end or the output section.

[badgercore]

Steve,

I turned the amp on and started playing and it took about 10 minutes for the popping to come on. I checked out the tubes and all 4 power tubes seem to be ok, at least they're all glowing orange, not ridiculously red hot as far as my eye can tell.

Gtr_tech,

Popped a cable in the Effect In jack and turned it off Standby. No popping, but not sure what that tells me at this point. Even if there's no cable in the Effect In and I turn the amp on, it doesn't pop unless I'm actually playing something. The popping seems to kick in sooner if it's set to channel 2 (over drive) than it does on channel 1 (clean), though I'm not 100% certain that's not just my imagination at this point.

Thanks for the suggestions!

[Gtr_tech]

Check for cracked solder then. Especially on tube sockets. I've never been into one of those amps, but I'd bet the sockets are pcb mounted which usually causes cracked joints from vibration.

Look for circular patterns similar to this:
http://www.oz1db.dk/ft-736r/r0012823.jpg

Just resolder them adding fresh solder to the joint. Clean the shorting contacts of the effects return jack while yer in there.