Trace Elliot GP12 SMX 300

[terryc]

Hello everyone, my son is looking for some advice/help on the above. I am going to paste what he sent me.
Hope you seasoned people can help on this, many thanks.

I hoping I can get some advice on this as I cant seem to find anything on the internet about my intermittent problem with my Trace Elliot head
I have a Trace Elliot GP12 SMX 300, awesome amp but has been the bairn of my life for problems!. It has blown twice (transistors went!) and its been fixed and serviced to get it running again by a trusted amp electronics technician. The problem that I'm having now is the amp keeps cutting out while I'm in mid practise with the band. I play with an active bass so I do not push the amp very much (doesn't go past 4 on the volume)
The only way to describe this is that is just stops my bass from making sound and then it producing a very harsh earth buzz that comes from the speakers (the sound is similar to that off your thumb being put over a guitar jack while its plugged into a distorted guitar amp) This sound is horrific. But when I turned the amp off at the back and switch back on (in a matter of seconds) its fine again.
I'm completely baffled at this and so is the guy that has fixed my amp. The only thing I found similar to this problem is on a site (basschat) from a guy having similar issues with his GP12 SMX 250 combo where his cut out out at a gig but made a screeching noise instead of horrible hum like I get through mine. That guys problem people said its the amps valve but I'm very uncertain on this.
One thing I will mention, is that it seems that the amp only does this when i'm practicing and when its cranked up a bit, as when i'm at home practising at suitable levels (I played for two hours straight once to test this it didn't do it!
Any help will be most appreciated!
Kind Regards
Phil

[jacko]

Hi Terry. I had an SMX 250 a long time ago with similar issues. The fault started just after I bought my epic and I thought it was related to the bass battery as that was the newly introduced thing. - Needless to say, it wasn't the bass.   Couldn't find a tech anywhere near Worcester that could figure it out. However, when I moved up to Scotland I took the amp to a guy in Edinburgh who found a dry joint - it would only open up when the amp was warm so like your son, it only manifested at band rehearsals or on stage.

Graeme

[BeenDown139]

<my $0.02 USD>

back in the days of my youth i serviced many a power amp and SMPS power supply. Jacko kinda hit in on the head by mentioning heat as the culprit.

if i was still working in the amp shop what i would do in this case is connect the amp to a static load, hook a tone generator to the input, instrument the output with a scope and RMS voltmeter, drive the amp to 1/2 power and wait to see what happens next.  let it run a while, monitor the finals temperature to make sure nothing's getting outta hand.  gently tapping it when the temperature stabilizes will help eliminate something microphonic (like a cold joint).

sometimes ya gotta break out the big guns for a problem like this.  good luck and happy hunting.

[terryc]

Thanks everyone(up to now) The amp was serviced twice by a guy here in the UK(ex GP engineer) he soak tested it and it has been fine, he did find a small problem and it has been fine up to a couple of weeks ago. There is a valve in these amps and that has been suggested, a pretty cheap fix(12AX tube) so we may go down that line but any further input from you people is most welcome!
Terry C