My everything breaks curse continues

Teetonetal

Middle of practice, my fairly new Pleximan, suddenly produces a really thick distorted sound whilst on clean, slowly fades out and that's it. no more sound. It powers on, valves light up. But no sound.

Amp dead or could this be a valve? It felt pretty hot but not stupidly so. 

ICBM

Probably a blown HT fuse caused by a faulty power valve - they will still light up, the filaments are on a separate circuit - although there are other possibilities too.

Check the HT fuse first if there's an external holder for it.

Teetonetal

Ok. Got a spare so will put it in.

ICBM

Make sure you have a clear view of the power valves when you switch from standby to on the first time after you change it. If one is faulty you will often see arcing or an unusual glow - although not always. If there's anything like that, try to turn off before the new fuse blows as well .

Teetonetal

Took out fuse, tested with a mm, continuity test and impedance seemed correct.

Replaced with new fuse

Amp worked.

But old fuse in old amp

Amp worked.

Test last nights speaker lead... all seems good.

So, is this now a ticking time bomb?

timmysoft

Have you taken it back up to the same sort of temperature?

Teetonetal

No, bit hard as I can only test in a flat...

Will push again next rehearsal I suppose and take a spare amp... you think it could be a temperature cut out?

It seemed hot, but not crazy.

ICBM

How quickly did the sound fade out - 1-2 seconds, or more like 10?

1-2 seconds sounds like some sort of HT voltage failure (which is why I suspected the HT fuse), but 10 seconds would be a valve filament failure. If it's the latter it will be a preamp valve not power, because one power valve going out would not stop the amp working, just make it quieter and badly distorted. In a preamp valve there are two filaments and it's possible for one to fail and the valve still appear to be lit up, but it won't work.

If it is a filament, and it works now it will be a bad contact between the socket and the valve rather than a failed valve, almost certainly.

Teetonetal

It faded out pretty quickly, 2-4 seconds I suppose. 

I've had it running all evening and it's still working fine and sounding as before...

ecc83

"No, bit hard as I can only test in a flat..."

Cannot offer specific help but a plea to anyone who might attempt such a "first line" repair? Knock up a basic load box. If you don't need pretty you could build a 100W load for about £20 thus allowing full power testing of most amps without waking the chavvy.

You might think you need a monitor speaker and the associated circuitry and that indeed would be nice but, driving 30-50 watts into a load will cause the OPT to "sing" so you will hear if it stops! You should in any case have a test meter so that on 100V AC range will show operation.

Dave.

ICBM

Teetonetal said:

It faded out pretty quickly, 2-4 seconds I suppose. 

I've had it running all evening and it's still working fine and sounding as before…

Hmmm….

If it didn't actually blow the HT fuse it's not a power valve fault, I think that can be ruled out - since a faulty one would either blow the fuse or just cut half the waveform - so testing it at high power probably isn't necessary.

2-4 seconds sounds a bit too quick for a preamp valve filament fade-out, although it is possible.

So that probably leaves an interruption in the HT supply to the preamp - that would also cause the sound to become thicker and dirtier before it died completely, as the voltage dropped. That could be caused either by a bad solder joint or a faulty resistor in the HT supply chain.

There are probably still other possibilities too… that's just my best guess at the moment.

TheMarlin

Rectifier tube.

DJH83004

I am pretty sure it has a SS rectifier, also switchable 5/50 watt output, which may well be contributing depending how they achieve it. I have seen one in the flesh but never to take apart, also don't know if there are schematics available, suspect dealer only. I would go with ICBM's thoughts on bad solder joint, around something that gets hot - they are the usual suspects :-)