#1
Hey guys, I have a 2 year old 6505+ head that has developed a hum. I've read some things online that said filter capacitors reduce hum and when they go south you can hear a hum through the poweramp. I only have this hum with a noise gate on in the fx loop with the fx loop turned on as well. When the fx loop is switched off you hear normal high gain preamp hiss. Think my issue is filter capacitors or not? I've tried everything that I can think of to eliminate my hum problem and have come to the conclusion it must be an internal amp problem. Thanks
#2
u change the tube for the fx loop?
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#3
It usually takes 10 yrs for a electro cap to go bad. So 2 years its shouldnt be a cap.
#4
It sounds like it is one the pedals in the fx loop to me. Try removing the pedals and run a short patch cable from the "send" to the "return." If you still have the hum this way, it's most likely the tube.

If it were the filter caps, you'd here the hum even when nothing is plugged into the amp (no effects/pedals, and no guitar).

After looking at a schematic, though (http://h.imagehost.org/0114/pv5150c.jpg) It looks like the fx loop's tube is always in circuit. So it is one of the pedals in the loop.

If you use a power supply/daisy chain for your pedals in the loop, this can cause ground loop problems. I suggest removing pedals, one at a time, until you find the culprit.
#5
Hey guys thanks for the quick responses. I have tried all of these suggestions already with no luck. I have replaced the preamp tubes and power amp tubes. I run all my pedals through a voodoo lab power brick. I have 2 noise suppressors, a boss ns-2 and an ISP decimator. What I've done is removed the noise gates from my pedalboard to just run them individually through the loop to remove all other pedals and cables from the equation. I run a short patch cable from send to the ns-2 with a cable to return. It doesn't matter if its run off battery power or the power brick as you turn the threshold up or use the mute function you get an annoying hum. Same exact thing with the ISP pedal, and I've used numerous different patch cables with the same results. If I run a patch cable straight from send to return theres no hum, just normal preamp hiss, so for some reason my amp doesn't agree with what these noise gates are doing. It hasn't always been like this, Its only been doing this for about a month now.

edit - and when muted or the threshold maxed out on either gate, the guitar signal still makes it through quietly to the poweramp. While muted or maxed out, any effects that are on are not applied to that signal. The post volume doesn't have any effect on this signal making it through either. Its like somehow part of the signal is bypassing the effects loop.
Last edited by bluenova at Dec 5, 2009,
#6
The schematic I found does show a relay to switch the loop in and out. Looks like its a parallel loop, drawing is a bit screwy, which would explain why you still get signal thru to power amp even w gates all the way up.

Sounds like its your pedals as its fine without them. Do you really need 2 noise gates?
#7
Quote by Tackleberry
The schematic I found does show a relay to switch the loop in and out. Looks like its a parallel loop, drawing is a bit screwy, which would explain why you still get signal thru to power amp even w gates all the way up.

Sounds like its your pedals as its fine without them. Do you really need 2 noise gates?


I actually purchased the 2nd gate thinking that it had to be a problem that developed in the first noise gate I had. I kind of justified the expense thinking that I could put one before the amp and one in the loop when I eventually get this thing sorted out. But now I still cant run the one in the loop as the humming the amp makes is louder than the preamp hiss is to begin with. The amp used to be completely silent with a gate in the loop without any of that annoying preamp hiss or quiet guitar signal making its way through just like a gate in the loop is supposed to work.

What are some of the effects of a tube socket going bad? Maybe, even though I replaced preamp tubes it didn't matter because a socket is faulty. The amp sounds brutal like it always has and I would imagine a bad socket would have some serious tone suck.
#8
It's not a filter cap, and it's not a tube socket. If it's only happening with certain pedals in the loop, it's a problem with the pedals and/or the loop, not the power supply.

If sounds like you're asking questions based on guesswork, not understanding what the problem is. Filter caps and tube sockets are not good candidates for the issue you're having.
It is a strange problem to have, you'll probably need to have a tech look at it.
#9
Are you sure the hum hasn't always been there? Sometimes your mind ignores it, then after it latches on it's impossible to get out of your head.

If it's Just when using a noise gate then the solution is surely just not use a noisegate? Your tone will thank you for it as well, your high end will be nice and bouncy again.

Assuming it's actually the amp: your first port of call should be the ground scheme of the FX loop. As you said you've tried different cables it can't be that; have you tried shorting the loop with a cable? Do you still get the hum? If so you may need to use a ground-lifted cable.

I wouldn't recommend working on the grounding unless you really know what you're doing and since you had to ask the question I assume you do not... Take it to a tech.


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#10
Quote by power freak
Are you sure the hum hasn't always been there? Sometimes your mind ignores it, then after it latches on it's impossible to get out of your head.

If it's Just when using a noise gate then the solution is surely just not use a noisegate? Your tone will thank you for it as well, your high end will be nice and bouncy again.

Assuming it's actually the amp: your first port of call should be the ground scheme of the FX loop. As you said you've tried different cables it can't be that; have you tried shorting the loop with a cable? Do you still get the hum? If so you may need to use a ground-lifted cable.

I wouldn't recommend working on the grounding unless you really know what you're doing and since you had to ask the question I assume you do not... Take it to a tech.





Whelp, I have tried shorting the loop with a cable and just got the normal preamp hiss with no hum. I do also think the grounding is probably the issue since turning on the gate sounds like it introduces a ground loop hum. So, I think I better get me a tech to look at this thing because you are right, I don't really know what I'm doing haha. Thanks for the help guys.
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