mosfet 450w irf240 / irfp9240 amp help

[aletheian]

If anyone can help me out there, it would be greatly appreciated. I attatched a schematic for an amp that I have here. It was working fine, but then if burnt up R33 and R27. I replaced them, checked my voltages and powered it up again...everything AOK, or so I thought. This time when I powered it up, it promptly burnt up R25.
Any thoughts? Could it be as simple as my bias seting for Q2? or do I have some bad mosfets on my hands?

THANX!!

[jacco vermeulen]

checked the voltage on the source of the Q11 irfp9240 ?
If that one is blown you'd have a short, and those 3 R's are bound to burn.

Tube front stage ?

[aletheian]

Yep, it has a tube front stage...a grounded grid with a cathode follower into a funky mu stage.
I'll check that source out...not sure what the voltage should be there though.

[aletheian]

Holy crap were the mosfets blown! I took them out and tested them. They had only a few ohms of resistance between the gate and both the drain and source. I guess I'll be replacing them. I thought they might be OK since the thing made SOME noise. It was also nice enough to blow out my test bench speaker...grrr.

[CBS240]

Sometimes when I test output biasing, I put fuses in line with the colectors or drains. This may not be sufficent to protect the outputs from all sources of destruction but it can limit the current through the devices in the event of saturation or a biasing 'short'. Fuses are cheaper than new devices.

[Dragon2]

Hi

Seems to oscillate: layout for that kind of amp is CRITICAL!
Low values 47 ohm resistors on gates.
Must use star ground.
To check oscillation, install a zobel network at the speaker output.
4.7 ohm 2w in serie with 0.1 uf cap.
If the resistor is heating...
You can remove it when to amp will be ok.
Good idea to install low value fast fuses on rails after filter caps.

Good luck

Have a good day.