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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
12 responses Add your response
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.
randallvar OP
1 posts
 
Thanks for your help and the quick responses. I sent the preamp to conrad johnson. The tech thought it might be a capacitor. The hum was in both channels coming thru the speakers. When I get the report I will post it for it might be a help to others.
mrmitch
359 posts
 
Randallvar, I had a CJ PV-5 which had a similar hum. Sent it to Conrad Johnson where they replaced some capacitors a nd the power supply. Sounds better now than when it was new.--Mrmitch
rogerbent
2 posts
 
Loud him in mine was a bad 40 MF section of main power supply dual filter capacitor. Antique Electronics has a CE Manufacturing 40/20/20 MF, 500v can that will work fine. You will need to drill additional hole in circuit board, but it fits fine. It is 2 1/2" tall. Original is 2" tall. New one fits fine.
johnsonwu
469 posts
 
One RCA socket lost ground from the cable yanking. Love the tube dampers. Add heat and cause users to buy lots of replacement tubes:)
rogerbent
2 posts
 
Even after replacing filter caps, I had hum even with the volume turned down. I swapped tubes. No change. I checked voltages on the zener diode string and each was reading 36 volts instead of 33 volts. The regulator transistors Q2 & Q3 were bad. I used a MJE15032G for Q2 and MPSA42G for Q3. I replaced Q1 also with MPSA93G though it checked ok. Quiet as a mouse now! Ground rings on many jacks are bad. Next job is replacing them with jacks with nuts on inside of jack panel.
hifigeek1
770 posts
 
Given it's age it does sound like a power supply issue.
jtimothya
612 posts
 
One RCA socket lost ground from the cable yanking.

This was my first thought as well. I lived with CJ preamps in the PV-5 through PV-8 series and the internal RCA wire connections tend to weaken over time with just normal use, partlyh due to the less than robust pop rivets holding the phenolic that is the RCA plug support attached to the back panel. Plus, modern RCA cables tend to have heavier plugs and cables which may put strain on the ports. Beautiful vintage CJ sound from these units.
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