resistor blow up !

[Ahmad_tbp]

hi i have built this amp many ago :
http://www.ampslab.com/c200qc.htm
and use it in a guitar combo amp , it worked well till some days ago after a few hours of workin , its sound reduced and r22 burned up completely , but the whole amp work well ( not as before but not major change ) even after r22 burnin completly , i checked wires , cahnged the output transistors , changed the resistor w a higher wat , test it with diffrent speakers , but the problem still reamins , when i turn it on , after a few minutes the resistor start to burn , but not major change in quality of output sound , and all transistors and the psu working well and cold even while the resistor is burning ...

[edl]

Hi,

If the Zobel's resistor get's hot, or it burns, then your amp is oscillating.
Check the C7 Miller-cap, or even change it to 100pF.

Regards,

[Ahmad_tbp]

thnxk , in fact i completely removed C7 before !
i mean from the start ! , lol
i wanted to solder it again but it seems i blowed up all the circuit by a bad mistake , the input wire clashed w collector of mj15016 ( i used mj15015/16 ) and after a big spark ! there s no sound come out of it !
lol !!!
i m tryin to fix it and use c7 again , is it possible that burnin of r22 was becuz of lack of c7 ?

cheers

[edl]

in fact i completely removed C7 before !

That was a really irresponsible action!

i m tryin to fix it and use c7 again , is it possible that burnin of r22 was becuz of lack of c7 ?

Yes. R22's burning was caused by lack of C7.

So, let's solder a 47-100pF cap in place of C7! Your problem will be solved.

And always remember: in such amplifier topology Miller cap (here it's C7) is really necessary, never remove it!

Best regards,

[Ahmad_tbp]

wohoo !!! .
thank u man ... didn t fix it yet but at sure it will work fine so
Redards .
Ahmad