Resurrecting a nOrh SE9 Integrated Amp

[wcakey]

Need some help here.

I am attempting to resurrect a nOrh SE9 integrated amp. The problem I am having with it is that it will run seemingly just fine for about 10 min, then develops a hum and dies -- blows the 500 mA slo blo fuse. The tubes (5AR4, 2 x EL34 and 2 x 12AX7) work perfectly in other amps, so they are unlikely to be the problem. Switching out the 5AR4 for a 5U4 allows the amp to run about twice as long before it dies in the same fashion.

What is the most likely culprit causing the problem? Power supply caps, or some other caps or some resistor somewhere?

Thanks for any suggestions.

 

[DF96]

Check C6 and C7 for leakage. Monitor EL34 grid voltage.

The EL34 may be running a bit too hot. Check current by measuring cathode voltage. Calculate power dissipation. Should be below about 30W. Try putting a 470R in place of the 390R cathode resistor.