sunn concert lead head. please help.

[briann]

i have been having problems with my 72(?) concert lead head.  after playing for a while, the amp cuts out, and temporarily disables the power supply...must be creating a surge and/or not recieving enough power. i have been running it from outlets seperate from all other equipment, which helps but still doesnt do the trick when playing live (i usually make it about 6 minutes into the first song, then it goes).  i will take any suggestions, please email me. thanks a lot, any help is appreciated.

adios,
brian

[Anonymous]

The symptoms sound like a thermal cutout... the output transistors are getting too hot and the thermal cutout activates removing power until it cools off enough.

This can happen if your total speaker impedance is less than 4 ohms, such as two 4 ohm cabs plugged into the speaker jacks.  If that's not true, you may need your output transistors replaced... there may be a lot of thermal insulation (sometimes caused by excess dirt that has built up over the years) preventing a good thermal contact with the heat sink that runs along the back.

[briann]

hey, thanks for the response... normally when i play, i run the head through one or two 8 ohm cabs.  i was told the problem is most likely a faulty heat protection device.. apparently sunn started putting a protection on solid state amps, and my amp-head-friend told me that is most likely the problem.. so i am thinking of bypassing that, rather than replacing it, and hopefully i wont mess anything up.  i am going to ask him about the possibility you presented, thanks a lot for your help.  fortunately i got my hands on another amp head thats louder and doesnt shut off... thanks again.

[JoeArthur]

You could bypass the thermal cutout... really pretty easy.  You might just want to check that there ain't no excess dust surrounding the cutout that could allow for it to accumulate heat and then cutout.

I wouldn't bypass it though... and if it is cutting the amp off, it does mean that something like the output transistors are suspect.