DC Huuuummmmmm

Apogee · 12th February 2007, 04:00 AM

Hi Everyone,

I've got a pair of Quicksilver KT-88 monoblocks. I just pulled them out of storage...

In one channel, I've got a quiet hum. If I didn't know better, it sounds like a little bit of dc leaking... I don't have my meter handy to measure offset (just moved).

I just lost the 5AR4 on that block. Sovtek, ~3 months of use since new. Amp was playing for about an hour, set the bias and noticed the hum. Turned the amp off to move the power cord to a different location, and saw it spark when I turned the amp back on. Immediately shut it down.

Figured that it needed one of the main filter caps. Was able to locate my capacitance meter so I disconnected all four electrolytics, 2 main filter caps (300uf 500v), 60uf 100v and 220uf 100v. All test good with very low leakage!

All other caps are film caps around the small signal tubes and I haven't checked them yet.

Just checked all tubes (except bad 5AR4) and they test very good on a calibrated Heath TT-1.

I put it back together, put in a new 5AR4 and am listening to it as I'm typing this. The hum is still there, but much, much softer.

The other channel is dead quiet with no hum detectable.

At this point I'm puzzled so I figured that I'd toss this out to the tube guys for help!

Any suggestions of where else to look?

I don't have the schematic or else I'd post it.

Thanks in advance!

arnoldc · 12th February 2007, 04:37 AM

Cold solder?

rdf · 12th February 2007, 05:06 AM

The Sovtek 5A4 has been uniformly unreliable for me in an EL84SE supply. A big shot in the dark, have you tried swapping rectifiers?

Apogee · 12th February 2007, 05:42 AM

Looked at solder joints and they all looked good.

Rectifier is brand new right out of the box. The last one hummed much louder than this one does and sparked, so I immediately shut it down.

The new one is working fine but the hum is still there, just much more softly.

I'm planning to troubleshoot this for real, just posted because I thought someone might say "I had one of those and this is what failed on mine..."

I was hoping...

rdf · 12th February 2007, 06:11 AM

Sorry, I meant swap rectifiers between amps. Was thinking the new Sovtek could be defective too. Like I said, a shot in the dark.

12th February 2007, 04:00 AM	#1
Apogee diyAudio Member Join Date: Feb 2002 Location: Tacoma, WA	DC Huuuummmmmm Hi Everyone, I've got a pair of Quicksilver KT-88 monoblocks. I just pulled them out of storage... In one channel, I've got a quiet hum. If I didn't know better, it sounds like a little bit of dc leaking... I don't have my meter handy to measure offset (just moved). I just lost the 5AR4 on that block. Sovtek, ~3 months of use since new. Amp was playing for about an hour, set the bias and noticed the hum. Turned the amp off to move the power cord to a different location, and saw it spark when I turned the amp back on. Immediately shut it down. Figured that it needed one of the main filter caps. Was able to locate my capacitance meter so I disconnected all four electrolytics, 2 main filter caps (300uf 500v), 60uf 100v and 220uf 100v. All test good with very low leakage! All other caps are film caps around the small signal tubes and I haven't checked them yet. Just checked all tubes (except bad 5AR4) and they test very good on a calibrated Heath TT-1. I put it back together, put in a new 5AR4 and am listening to it as I'm typing this. The hum is still there, but much, much softer. The other channel is dead quiet with no hum detectable. At this point I'm puzzled so I figured that I'd toss this out to the tube guys for help! Any suggestions of where else to look? I don't have the schematic or else I'd post it. Thanks in advance! __________________ "If I had my life to live over again, I would have made a rule to read some poetry and listen to some music at least once a week." - Charles Darwin

12th February 2007, 05:42 AM	#4
Apogee diyAudio Member Join Date: Feb 2002 Location: Tacoma, WA	Looked at solder joints and they all looked good. Rectifier is brand new right out of the box. The last one hummed much louder than this one does and sparked, so I immediately shut it down. The new one is working fine but the hum is still there, just much more softly. I'm planning to troubleshoot this for real, just posted because I thought someone might say "I had one of those and this is what failed on mine..." I was hoping... __________________ "If I had my life to live over again, I would have made a rule to read some poetry and listen to some music at least once a week." - Charles Darwin

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12th February 2007, 04:37 AM	#2
arnoldc diyAudio Member Join Date: Apr 2003 Location: Makati	Cold solder?

12th February 2007, 05:06 AM	#3
rdf diyAudio Member Join Date: Jun 2004 Location: big smoke	The Sovtek 5A4 has been uniformly unreliable for me in an EL84SE supply. A big shot in the dark, have you tried swapping rectifiers?

12th February 2007, 06:11 AM	#5
rdf diyAudio Member Join Date: Jun 2004 Location: big smoke	Sorry, I meant swap rectifiers between amps. Was thinking the new Sovtek could be defective too. Like I said, a shot in the dark.

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