Trace Elliot Series 6 GP12 Amp Humming

[daynacherry]

My GP 12 is giving off a low level hum through my speakers as I turn the master gain up,the hum level increases. Normally it is not a problem, but it is present at the XLR out which is effecting a clean recording signal.
I have had the amp open and when I disconnect the multipin that goes from the Pre amp board to the Power amp Board the hum goes away, and listening to the power amp on its own it is totally Hum free. The multipin has 4 wires(signal/earth, and a blue/red pair, which I am assuming is the voltage supply to the pre amp board. I was thinking of connecting a Smoothing capacitor accross the blue/red supply cables to the pre amp board in the hope that this may filter any hum in the supply.
Any thoughts/suggestion on taming the Hum will be most welcome.
thanks in advance
Mike

[sharkwagon]

I would post something in the Trace Elliot owners thread, as there is a lot of experience with these units over there.

[daynacherry]

Hi Sharkwagon
Do you have a link for that page/site

[B-string]

You should address any bad filter in the power supply section instead of adding additional. A cap that is loosing capacity is on its way to shorting out.

[Bertr]

I just bought a 250w GP12 SMX from 04/1993, which had some hum issues

It hummed even at pre and master at 0, I did more or less what lenky76 said here:
Earth switch-Trace Elliot GP12 SMX
but instead of routing the new cable that way, I used a stereo shielded cable (shield connected to chassis) which I routed along the front of the amp (under the pots), the left side and finally to the input of the pwr amp.

Hum is 99% gone, not worse that a good GTR valve amp with pre and post at 0.

I discovered that the trick was to keep the preamp/pwr amp connecting wire as far away as possible from the 4700uF caps.

So my question: are those bad? I cannot think that the amp left the factory humming that way.

Second question: when I turn it off, after 15/20s it pops 4/6 times (each pop 1 s away from the previuos one). Anybody solved the problem?

[Bertr]

Update:
-for the pops, a 10k 2w resistor in parallel with the power amp caps solved the problem.
-Hum is now COMPLETELY gone: It was the toroidal tranny still radiating a bit to the preamp. I enclosed it in a Hammond type alu enclosure. Amp is now quiet.