#1


Hey guys, bummed out here. On a whim a few days ago, I decided to try an older set of power tubes that I had replaced last year (they weren't completely worn out though, and functioning). Anyhow, they sounded better than my newer set (both sets are JJ 6L6s), so I left them in. They must have had less use on them than the newer set. Anyway, yesterday I noticed some excessive noise coming from the amp, so I replaced the power tubes with the other set -- and that solved the noise problem. But wanting that better tone, I decided yesterday to clean the pins of the other tubes with contact cleaner and try them again. Well, the noise got better, so I kept playing them.

But today, while I was in the middle of playing pretty loud, there was a loud POP and I immediately went over to turn the amp off. I put the other tubes in, powered it on -- and there was nothing -- no lights, no glow, not even a slight hum. So next, I checked the main fuse on the back of the amp and sure enough, it had blown. Easy enough, I said, and a couple hours later I had some new fuses. Well, as soon as I put one in and fired it up -- there was a very quick "huumm" and then lights-out. Tried another one -- same thing.

I did a google search and found a post that said to try removing the power tubes, but leave it on standby and turn the power switch on to see if it blows the fuse again. Well, it blew the fuse again, dammit. So next, I removed the chassis and that's where I'm at currently. I've looked all over and I can't see anything obviously wrong. The other fuses inside appear to be intact. The pins on top of the right power tube socket appear to have gotten hot, but not overly so. And I think that's the way it's always looked -- I've had this amp apart many times.

So now, I'd appreciate any advice on what to try next. Did it take out my power transformer? Thanks!
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#2
Not an amp tech here but what I would do is get some brand new power tubes and a brand new phase inverter 12AX7 but honestly it sounds like something is wrong with the power section and needs an amp tech. Tubes you can always need so not a waste of money.
#3
^^Yeah, I was thinking of maybe trying replacing the PI tube as well and trying again. I have a couple spares that I know work, maybe I'll give one a go. As for power tubes, the ones I have in currently are not the ones that were in it when it blew, and they were perfectly fine before the whole incident. I have another pair I could try though as well.

I'm not too excited about getting new power tubes just yet -- because I'm sort of thinking it might be time for a new amp. I have had quite a few issues with this one. First it was diodes failing then lots of tube failures. There are some build-quality/design issues too -- when I put the phase inverter tubes in, it flexes the whole PCB because it's not anchored there -- which seems kind of poor. Anyhow, I've had it about 6 years and if it's going to cost me more than $100 or so I might as well get an American-made 6505 120 watt head. There is nobody in my town that can work on amps -- and sending it to Peavey will cost an arm and a leg. If it isn't something simple, I may just give up on this one.
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#4
That's a shame. Great sounding amps w/ mods.

Replace the phase inverter valve.
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#5
Yes, it is a great sounding amp. Unbelievable, really, especially with fresh tubes and running through my big cab. No matter what, I'm going to have to have a 5150/6505 in my arsenal. So if I can't fix this, I'm getting a 120watt 6505 head (not the 6505+). After doing the mod to 5150 specs, that is my tone. Gotta have the original version of the circuit.

I just replaced PI tube -- no change. I only have one fuse left.
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#6
One thing to consider, you'll want to make sure that you have the right kind of fuse. Some amps require slow-blow fuses because of the current spike on power up. If you use a normal fast-blow, then if if the current rating is right they'll still blow. I had this issue with one of my Kranks, I popped a slow blow in and it took care of the problem.
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#7
^^Thanks for the tip. I think it requires a fast-blow, which is what (I think) I picked up. I gave the guy at Radio Shack the exact specifications printed on the back of the amp, but I suppose a mistake could have been made. I might just get a slow-blow fuse tomorrow just to see if it allows the amp to fire up -- but I won't run it for more than a few seconds until I'm sure it's okay for the amp.
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#9
^^On the back of the amp it reads "F3AL/250V" -- which from what I understand is a fast-blow fuse. I checked the bag of the fuses I bought and they are indeed the right ones.

I also tried a different tube (a NOS 12AT7 that I know works) in the PI and it was still a no-go.
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#10
There are definitely some amps that have fast acting fuses, if it matches up then that shouldn't be the problem...

You could pull the board and check for any visibly burned/melted components.
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#11
If you have a multimeter, I would check the power rectifier. Try not to electrocute yourself.
"Practice doesn't make perfect. Perfect practice makes perfect." -some dude
#12
Quote by Prime2515102
If you have a multimeter, I would check the power rectifier. Try not to electrocute yourself.


I checked that one time (two years ago or so) when my amp was powered up. It read "1100 V."

I'm going to take the whole PCB out tonight and have a look underneath where I can see the traces.
Atmospheric dark metal w/ black and death metal influences:
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#13
Quote by KailM
I checked that one time (two years ago or so) when my amp was powered up. It read "1100 V."

I'm going to take the whole PCB out tonight and have a look underneath where I can see the traces.


That's not entirely unreasonable I don't think. It's been a really, really long time since I studied up on this stuff (plus I'm half asleep), but I think a full-wave rectifier will double the output voltage. It does seem a bit high since usually you're looking at 450-500V going to the power tubes, but who knows (an electronics tech, that's who! lol).

I would just check for continuity (with the amp off lol).

I had this same problem with my Mesa Heartbreaker and it turned out the rectifier tube was bad, that's why I suspect the rectifier (I realize yours is solid state but the symptoms would be the same).
"Practice doesn't make perfect. Perfect practice makes perfect." -some dude
#14
Quote by Prime2515102


I would just check for continuity (with the amp off lol).

I had this same problem with my Mesa Heartbreaker and it turned out the rectifier tube was bad, that's why I suspect the rectifier (I realize yours is solid state but the symptoms would be the same).


+1 good advice on chasing around continuity. i have done that for longer than i would care to admit. :p

good thoughts on the rectifier too. they aren't too bad to replace if you know what you are doing (kalim i think you do from the degree you have modded it), i haven't done one but i did look into it.
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#15
Quote by KailM
^^On the back of the amp it reads "F3AL/250V" -- which from what I understand is a fast-blow fuse. I checked the bag of the fuses I bought and they are indeed the right ones.

I also tried a different tube (a NOS 12AT7 that I know works) in the PI and it was still a no-go.


My bad. I should have checked that first.
#16
Quote by 311ZOSOVHJH
My bad. I should have checked that first.


No worries.
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#17
Quote by trashedlostfdup
+1 good advice on chasing around continuity. i have done that for longer than i would care to admit. :p

good thoughts on the rectifier too. they aren't too bad to replace if you know what you are doing (kalim i think you do from the degree you have modded it), i haven't done one but i did look into it.


Yes, if it's a matter of soldering components to the circuit board, I can do that. I won't say I'm the best at it, but my mods and my last repair have worked for about two years. The rectifier is the next thing I'll check, as that seems to be what gets recommended on all of the Google searches I've done, regardless of the amp. However, there is no tube rectifier in this amp --- so am I looking at diodes? If so, I think I know where they are in the circuit, but would appreciate any help.

What are all of the components in a 6505+ that constitute "the rectifier?"
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#18
four larger diodes for full wave rectification, some times they come all in a little package.

the diodes can overheat and fuck things up. if i am on an amp where i know what i am doing (really simple amps) i will put a heat sink on there, as that can help.
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#19
Update: Well guys, last night I pulled the whole circuit board out to have a look at the bottom (and a better look at the top I suppose). I'm not really finding anything visually, especially on the bottom. I haven't been super thorough yet but after 10 minutes of examining the traces on the bottom as well as all of the solder points, I can't find anything that looks burnt or even discolored on the bottom of the PCB.

On the top of the PCB, I do see some discoloration around the power tube socket pins, but not an all-out fire or meltdown of any kind. I am positive this all has to do with the power tube failure I experienced. Dammit, I should have never put those tubes back in! When I get time, I'm going to try chasing continuity, but it would help to know kind of where to start. I'm also kind of wondering about the power transformer. Doesn't that sometimes get taken out with a power tube failure? If that thing got taken out, I might just scrap the amp and get a new American-made 6505. I'm not spending $200+ on this amp, as I've used it for about 6 years pretty heavily. Might have to just cut my losses eventually.
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#20
Here is a schematic if it helps you at all. There are 3 areas that look like rectification to me. Their location is on the first page (which is the actual schematic) towards the bottom left. 4 diodes in a squared shape. There are 3 other fuses there that are NOT the mains fuse. I would check those as well.

The "top" rectification circuit looks like it is the high-voltage section, for the B+. Also has the heaters for the power section. The rectifier section below that looks like the heaters for the preamp. Below that, looks like bias and the backlighting. Obviously the most suspect is the top rectification section for the high-voltages. If you do the math for what the voltages should be, you can check to see if those are the actual values coming off those rails. If you are able to find the locations of the diodes on your PCB (you should, page 2 shows the populated PCB) you can check them with the "diode check" function of a DMM. They are all separate from each other so it should be easy to just poke each side of each diode.

Of course you know there are lethal voltages inside the amp even with the amp off and unplugged. I would drain the caps before doing any work (continuity) that does not require the amp to be on. If the amp is on you need a proper speaker load of course. Also you guys know I have only recently started dabbling into electronics so I am probably missing or overlooking a lot.
Last edited by Will Lane at Dec 8, 2016,