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View Full Version : 60 cycle hum in a Fender amp


TimH
10-05-2006, 08:46 AM
Hey Gang,

I posted about this hum in my DRRI a while ago and Mike @ KCA suggested trying some tricks with the reverb tank which unfortunately did not work. As a refresher the amp makes a noticable albeit not loud 60 cycle hum with no guitar plugged in and no gain running through the amp. The only control which changes the level of the hum is the reverb knob. Perhaps on a club stage this would not even be an issue but at a church when the pastor preaches I can see it being a problem.

So I sent the amp off to a local tech who proceeded to bias the amp for me as I asked him to (unrelate work that I wanted done) and then tried to fix this hum. In the process he replaced some capacitors which he said we bad and says the amp is quieter now than when he got it. The problem is the amp is still humming which was the only noise I noticed when I sent it in. I called him back after discovering this and he was like "yeah that's a common thing with Fender amps...there is a mod you can do (something to do with mixing back in some DC current...I'm not technical enough to relate the mod to you all) that will fix this problem. So here I am now trying to figure out if I should send the amp back to this guy or is he just trying to get more money out of me?

So my questions are...has anyone ever encountered this issue with their Fender amp? Second, has anyone heard of this mod or anything like it?

Just to answer the question I know will come up...yes I play single coils and no this has nothing to do with the pickups. This happens when the amp is fully on with nothing plugged into it.

mbratch
10-05-2006, 09:09 AM
As a refresher the amp makes a noticable albeit not loud 60 cycle hum with no guitar plugged in and no gain running through the amp.Does it work fine when the guitar is plugged in? What do you mean by "no gain running through the amp"? Do you mean "no signal"?

Blue Strat
10-05-2006, 09:14 AM
Hey Gang,

I posted about this hum in my DRRI a while ago and Mike @ KCA suggested trying some tricks with the reverb tank which unfortunately did not work. As a refresher the amp makes a noticable albeit not loud 60 cycle hum with no guitar plugged in and no gain running through the amp. The only control which changes the level of the hum is the reverb knob. Perhaps on a club stage this would not even be an issue but at a church when the pastor preaches I can see it being a problem.

So I sent the amp off to a local tech who proceeded to bias the amp for me as I asked him to (unrelate work that I wanted done) and then tried to fix this hum. In the process he replaced some capacitors which he said we bad and says the amp is quieter now than when he got it. The problem is the amp is still humming which was the only noise I noticed when I sent it in. I called him back after discovering this and he was like "yeah that's a common thing with Fender amps...there is a mod you can do (something to do with mixing back in some DC current...I'm not technical enough to relate the mod to you all) that will fix this problem. So here I am now trying to figure out if I should send the amp back to this guy or is he just trying to get more money out of me?

So my questions are...has anyone ever encountered this issue with their Fender amp? Second, has anyone heard of this mod or anything like it?

Just to answer the question I know will come up...yes I play single coils and no this has nothing to do with the pickups. This happens when the amp is fully on with nothing plugged into it.

Did unplugging the reverb cables help?

TimH
10-05-2006, 09:44 AM
by no gain yes I mean no signal and all the knobs on zero...literally as little gain flowing through the circuit as possible. And no, plugging the guitar in does not help.

Mike, no unplugging the reverb cables did not help at all.

Blue Strat
10-05-2006, 09:50 AM
by no gain yes I mean no signal and all the knobs on zero...literally as little gain flowing through the circuit as possible. And no, plugging the guitar in does not help.

Mike, no unplugging the reverb cables did not help at all.

Try replacing the V4 tube (12AX7). If you don't have any spares, pull V5 and put it in the V4 socket.

Swarty
10-05-2006, 10:18 AM
Is it possible that the reverb may just be adding the typical hiss to an existing hum coming from elsewhere? I'm guessing this may be the case. A mismatched set of output tubes will cause hum as will a bum preamp tube. I would take a known good 12AX7 and swap it into positions 1, 2 and 4 and listen after each swap to see if the problem goes away. If there is no change I'd swap positions of the 12AT7s in 3 and 5 and see if anything changes. If not, I'd go to the output tubes and make sure they are reasonably matched as hum will typically commence if the tubes are 10mA apart. This would be the low hanging fruit/DIY approach. If this doesn't cure it, it is probably time for a tech/scope. The amp should be very quiet when idleing with only a tiny amount of speaker hiss present.

donnyjaguar
10-05-2006, 10:24 AM
It could be a bad tube, but I've found some Fender amps have hum on the reverb circuit regardless. However, I've been able to remove pretty much all of the residual hum by re-routing the grounds. Sometimes you have to be rather creative here. A knowlege of RF notch filters helps here. :)

Blue Strat
10-05-2006, 10:27 AM
It could be a bad tube, but I've found some Fender amps have hum on the reverb circuit regardless. However, I've been able to remove pretty much all of the residual hum by re-routing the grounds. Sometimes you have to be rather creative here. A knowlege of RF notch filters helps here. :)

Reroute grounds in a DRRI?

HammyD
10-05-2006, 10:30 AM
I have a 67 ProReverb that was recently restored. I went to a gig and afterwards I got a 60 cycle hum as you described. Wound up to be the bright stwitch on the reverb channel. I must have bumped it and due to the age and general abuse from the previous 30 years had the part ready to fall apart.

Had a tech replace it with NOS as the new ones just didn't look the same.

Try tapping on the switches and knobs to see if you can hear more or less hum.

TimH
10-05-2006, 11:01 AM
problem solved. Thanks guys!

BudLite
10-05-2006, 11:17 AM
How did you solve the problem? Thanks

Blue Strat
10-05-2006, 11:36 AM
How did you solve the problem? Thanks

Uh yeah dude! We spent time with you, so how about telling us what fixed the problem? Jeez!:jo