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FDP Forum / Amp Mods, Repairs, and Projects / Loud hum on DRRI when turned on

Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Jul 16th, 2016 02:51 PM   Edit   Profile   Print Topic   Search Topic

I have a loud hum (much louder than the 60 cycles hum) on my DRRI. I have not used the amp in months and so this was totally unprovoked.

The hum is present as soon as the standby switch is flicked, regardless of whether anything is plugged into the inputs. Volume doesn't change it, but turning the reverb up and down slightly affects it, but not significantly.

I'm suspecting a power tube since it does not respond to volume, which means I'll have to change both 6V6 tubes at least to start the troubleshooting.

The original two power tubes are Fender Groove 6V6GT and the preamp tubes are also Groove, two 12AT7 and four 12AX7A. I'm assuming that the 5AR4 rectifier tube is not involved with the hum, but I could be wrong.

Any advice on whether any of these power tubes are really the culprit of the hum and if so, which tubes would you recommend? Same Groove tubes or are there any that might be better in terms of sound quality?

Any advice would be appreciated. Thanks

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 17th, 2016 10:06 AM   Edit   Profile   Print Topic   Search Topic

I would suspect the 5AR4 first, you could also have a bad power supply filter cap.

w

Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Jul 17th, 2016 12:42 PM   Edit   Profile   Print Topic   Search Topic

Thanks, Willie.

Would a defective rectifier account for what appears to be the 120 Hz hum? It would make sense that it is not converting the 120 V to full DC voltage.

I noticed that one of the 12AT7 preamp tubes isn't glowing as much as the other one. Any hint?


willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 17th, 2016 12:46 PM   Edit   Profile   Print Topic   Search Topic

If the tube is hot, the filaments are good.
Sometimes the filaments are not easy to see.

A bad rectifier will result in either no DC or very "ugly" DC. There are other causes for the symptoms as well but check the 5AR4.

w

(This message was last edited by willie at 02:48 PM, Jul 17th, 2016)

Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Jul 17th, 2016 01:18 PM   Edit   Profile   Print Topic   Search Topic

Thx. I'll have to replace the 5AR4 to find out.

The suspected 12AT7 gets a bit warm but definitely not as much as the other 12AT7 which is the phase inverter. It also has a white ring around the bottom, that I don't see on the inverter tube.

Could the 12AT7 that drives the reverb cause the hum? I tried to remove it but it sits quite tight in its socket and pretty difficult to dislodge.



Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Jul 17th, 2016 03:11 PM   Edit   Profile   Print Topic   Search Topic

I finally took out the suspected reverb 12AT7 tube. The hum is still there and of course the reverb now doesn't affect it.

I'll fall back on the 5AR4 hypothesis and hope it's not the filter cap in which case, it will have to go to a tech.

(This message was last edited by Iron Man at 07:10 PM, Jul 17th, 2016)

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 17th, 2016 06:25 PM   Edit   Profile   Print Topic   Search Topic

White ring usually indicates no vacuum. A problem like you have is usually very easy to diagnose when we have it on the bench...a matter of minutes really.

w

Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Jul 17th, 2016 08:17 PM   Edit   Profile   Print Topic   Search Topic

When I took that 12AX7 out the amp and turned the amp on without the tube, the hum was still the same.

It is possible that the ring that I saw was just the tube plate. I'll switch the rectifier to a JJ 5AR4/GZ34 and if the hum is still there, I'll take the amp to a tech. My gut feeling is that it is an onboard problem.

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 18th, 2016 08:36 AM   Edit   Profile   Print Topic   Search Topic

Highly recommend taking it to a qualified technician. Like I mentioned, diagnoses of such a problem is usually very simple once a tech gets it on the bench. There are several things that we look for that narrow it down very quickly. I doubt it is anything too serious...or costly.

w

Peegoo
Contributing Member
**********
**********
******

Eat. Sleep. Guitar.

Repeat
Jul 18th, 2016 09:17 PM   Edit   Profile   Print Topic   Search Topic

Willie, you're the tech version of the country doctor that makes house calls :0)

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 19th, 2016 05:57 PM   Edit   Profile   Print Topic   Search Topic

LOL...that's pretty close actually. I have about as much to show for 50 years of doing it as many country docs do as well... Lot's of memories and friends though. ;)

w

Peegoo
Contributing Member
**********
**********
******

Eat. Sleep. Guitar.

Repeat
Jul 20th, 2016 03:05 PM   Edit   Profile   Print Topic   Search Topic

And four worn-out horses in the pasture too, I'll bet :o)

Hammond101
Contributing Member
*********

So. Cal. USA

Jul 20th, 2016 03:25 PM   Edit   Profile   Print Topic   Search Topic

Just put me out to stud baby!

willie
Contributing Member
*******

Too Near Atlanta GA

Amp Tech Emeritus
Jul 21st, 2016 04:53 PM   Edit   Profile   Print Topic   Search Topic

amen....

w

Iron Man
Moderator

USA/Frisco Bay

My eyes are blind, but I can see
Oct 26th, 2016 11:51 PM   Edit   Profile   Print Topic   Search Topic

Finally got around taking the amp to a great tech in SF. Turns out the culprit was a 47uF cap. Replaced it, changed the Groove tubes to a pair of JJ 6V6 and biased the amp to 25mA.
Sounds great!!


FDP Forum / Amp Mods, Repairs, and Projects / Loud hum on DRRI when turned on




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