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What might cause LOUD hum in a Conrad Johnson pv6

The PV6 today blared out a loud hum after it had been running fine for a couple of hours. I shut off all power to the amp, pre-amp, tuner etc. I noticed when I moved the preamp that one of the output cables was disconnected.
I don't know if it somehow disconnected and it caused a hum or became disconnected when I moved the pre-amp. In anycase, I later plugged the cables back in securely and used only a tuner STILL A HUM then a turntable and STILL A HUM.

I am looking for someone who might have an idea what is causing the hum. I opened the preamp No smell all tubes appear to be working. So I am puzzled.
randall
randallvar
phd
1,316 posts
 
This is one explanation: Microphonics. Try tube dampers.
elevick
2,636 posts
 
Is the hum out of both channels? Tighten transformer screws
Is the hum on one side? Try swapping the final gain tubes only, probably 5751's. Just change the right and left. See if the hum moves too. To find these, I think they were the very back pair on the left but can't quite remember?
phd
1,316 posts
 
Randallvar, is the hum coming from your speakers or is coming directly from inside the preamp?
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almarg
9,670 posts
 
Assuming you mean the hum is coming through the speakers, rather than directly from inside the preamp, and that you've somehow determined that the power amp is not responsible, I'd guess a filament-to-cathode short in a tube. Another possibility, more difficult to isolate, would be a failed capacitor, such as an electrolytic in the power supply section of the preamp.

Regards,
-- Al
sonofnorway
62 posts
 
Another possibility -

The original hum may have been from a loose RCA cable to the amp. If the center pin was making contact, but the outer outer ring was not, that would cause a (very) loud hum. Are you sure all cables are firmly reconnected? Could be a bad connection inside a cable, also. Try swapping in some other cables.
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