No thats a Zobel network.  This has normal voltage feedback

Brilliant!  What chip does the MAH3 use?

If it is cardboard, then there's your problem.  It should be a special silicone rubber/fibreglass washer (Sil-pad) to transmit the heat.  Cardboard is a heat insulator!
LM3876 or 3886 there's not much difference between them.  They have similar pin-outs, but 3876 is 56W and 3886 is 68W outputs.  Both will shut down if overheated. 

I expect this amp uses the LM3886 IC which has a heat sensor/thermal cutout built in. The symptoms you are getting points to this shutting down.
In the picture raul.jpg what is the brown material under the IC under the metal clip? Looks like cardboard?

I see now that their domain name expired on 30/04/2014 and is pending renewal or deletion!!!

Thanks phatt, that article has really given me something to think about.

Tony.

Guitar preamplifiers often have a very high input impedance >500k but what is the optimum impedance to load a pickup.
Is it higher or lower than this assuming the pickups are not active types.

Edited to remove typo.

Tony

First of all get yourself a schematic.  What amplifier is it - someone here may be able to help you with it.  If you dont know what you are doing then you are just going to destroy more chips. 

Looking here http://talk.philmusic.com/index.php?topic=116207.0 it doesn't appear to be the handle.

1875s always run hot when idle.  Have you insulated it from the heatsink and is the heatsink screwed to the chassis?  If the heatsink is screwed to the chassis then the chip MUST be insulated from it. The negative supply is connected to the mounting tab internally and if not insulated will cause a short.

There's also this guy in UK.  http://www.ampmaker.com/store/home.php

That should be of course SILICONE, not silicon. 

The + input to the TL072 is going to need a reference to earth, you should connect the 1M resistor between this point and earth and scrap the 47uF Cap.

How have you connected your guitar to the amplifier - to which points?  Have you used a stereo jack plug or a mono one?

I have some questions for you.

1. Why did you choose a stereo Amplifier?
2. Have you speakers on both channels?
3. Is the guitar only connected to one channel or both?

Regards  Tony