The Boogie Board

Hi guys,

My Mesa Boogie Studio Preamp sounds superb when I first turn it on and it does sound good for at least 20 minutes or so. After that, the lead tone changes in that the low end gets really undefined, I loose attack and bite.
Clean tone remains ok. I changed a lot of tubes, pretty sure it is not ear fatigue, I also have in my rack my JMP1 which does not do that.
Tubes being out of discussion, I would guess that a particular electronic part gets too hot and changes its characteristics. Any idea what is going on here? The preamp does sound out of this world when in parameters so to speak.
Thanks, Adrian

In the mean time I installed a fan inside. Kept it on for 2 hours, no change in the sound, it is perfect.
I also talked to a friend of mine who knows electronics, he told me that the culprit may be a tantalum capacitor in the cathode bias that changes capacity because of the heat.
The fan is silent, no annoying noise. As I kept it on on the floor, I could smell the heat of the electronics inside. It's a pitty Mesa did not provide holes on the top of the unit to allow the heat to go up but I am happy with the present solution.
Not really roomy inside the preamp, I installed the fan to blow towards the back, the heat goes out through the holes in the back. But I had to take the reverb tank out, I don't use it anyway. However, it may be more efficient to blow the air through the lateral holes but I would need a very slim fan. I can find one but it will only work at 12 volts and I would need a separate source for it. The current fan works at 230 volts but it is kinda bulky.

No additional noise generated by the fan in the signal chain.
Cheers

Possibly a thermal conductive resistor or part of the PCB becoming comductive under thermal stress.