#1
Hey all. Turned on my 6505+ 112 the other day after a few months of disuse to find that I got no sound out of it. The amp powers on fine. Both power tubes and the PI light up but all the preamp tubes v1-v4 are dark. I swapped a few preamp tubes just for shits but that made no difference. The external power supply fuse F1 is ok so now I'm checking on the guts. No obvious defects looking over the board. Big capacitors look ok though I'm sure I would have heard it if one of those went boom. Fuses F2, F3, and F4 all check out. I did a little research and it seems that this problem isn't unheard of for this amp, and diodes D18,19,23,24 and the R82 resistor are common culprits. In checking those diodes all 4 of them appear to be shorted (same voltage drop in both directions). All the other diodes on the board test normally.

My question is whether shorts in those diodes could be causing the preamp to not get power or are more likely a symptom of some other problem. The R82 resistor looks ok but would be easy enough to just change as it's in a holder. I am leaning towards just swapping those diodes and seeing if it fixes things. Apparently they are slightly underrated in this amp which causes them to fail frequently. If I change them I might throw in 1N4004s instead. Was hoping to get input from a few of our resident amp gurus first.

Thank you

P.S. Was hoping to attach the schematic but it's not showing up well. Here's where I found it http://www.prowessamplifiers.com/schematics/misc/Peavey_6505_Plus_112_Combo-Peavey_6505_Plus_112_Combo_Schematics_and_BOM.html
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
Last edited by GeetarHeero at Nov 5, 2015,
#2
As you've already noted, the diode bridge is the most common culprit. Any of the 1N400X diodes will work, and it's a cheap and easy fix.

R82 in a holder? Are you sure?
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
Last edited by Arby911 at Nov 5, 2015,
#3
You're right on track. Those diodes are very likely the culprit. I'll have to get back to you on what I used to replace mine, but if memory serves I used ones that had a higher voltage rating than stock, and I've never had an issue since. It's a very cheap fix, but time-consuming because you have to disassemble the whole circuit board to swap them properly.
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#4
No you're correct, R82 is not in a holder. I was picturing something else. In any case I'll swap the diodes out and let you know if I have any success. Wanted to lay out my thought process and make sure it made sense to someone with more experience. Thanks!
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
#5
KailM, what trouble could you get into by desoldering them from the board and soldering the new ones on?
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
#6
^^The sort of trouble that could lead you to the hospital (or the morgue)

I'm actually serious though. But you probably already knew the dangers. Just make sure there is no voltage left in the capacitors before touching anything. With mine, the caps have always drained simply by turning the amp on and letting the power tubes warm up for a few minutes, and then simply unplugging the amp. But I ALWAYS check it with my multimeter anyway before fiddling around in there.

Remember that diodes are directional; they are marked with a stripe. I would just draw or take a picture of the way they are laid out stock and then put the new ones in exactly the same way. I had a heck of a time getting mine to de-solder until I just took the whole circuit board out and got to them from the other side. When you're putting the new ones in, it helps greatly to tin the leads first -- then it's quite easy and quick to solder them in. I always leave my component legs long and trim them afterward. As long as you don't leave the soldering iron anywhere for too long -- it's a pretty foolproof process. Good luck -- I hope it gets you sorted out!

As for the diodes, I'm pretty sure I used 600 volt rated diodes -- which is overkill, but I haven't had any issues since. I think 400 volt diodes are still better than stock, and would likely never fail either.
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#8
Quote by KailM


As for the diodes, I'm pretty sure I used 600 volt rated diodes -- which is overkill, but I haven't had any issues since. I think 400 volt diodes are still better than stock, and would likely never fail either.


Factory are 4003's, 200v rating, which is WELL above what that circuit ever sees. You can put 4007's in (1kv) and it won't change anything. The problem isn't the voltage, it's the current, all of those are 1a diodes, which is right at the edge of what's needed IIRC. (I can't remember exactly how to calculate peak and average rectifier current, only that it has to do with the input power, capacitor and load...if someone else can do it that would be peachy!)

You could replace them with 5402's (200v, 3a) and probably never have to worry again.
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
#9
Quote by KailM
^^The sort of trouble that could lead you to the hospital (or the morgue)


I know I always double check. I have a little doohickey I use to drain the circuit- one alligator clip to the chassis and the other with a 100 ohm resistor in the jaws. Then double check that everything is cold before I stick my hands in there. Thanks though, my wife would be pissed if I killed myself f*cking around with my amp

Quote by KailM
I had a heck of a time getting mine to de-solder until I just took the whole circuit board out and got to them from the other side. When you're putting the new ones in, it helps greatly to tin the leads first -- then it's quite easy and quick to solder them in. I always leave my component legs long and trim them afterward. As long as you don't leave the soldering iron anywhere for too long -- it's a pretty foolproof process.


This is exactly what I was hoping for, some pointers on difficulties you found having done it yourself. I got all the pull off wires disconnected, nuts off the pots, and screws out of the board but am still getting some resistance trying to lift it out of the chassis. Could it be the pins in the tube receptacles? Do I just need to keep pulling up evenly until the board comes free?

Quote by Arby911
Factory are 4003's, 200v rating, which is WELL above what that circuit ever sees. You can put 4007's in (1kv) and it won't change anything. The problem isn't the voltage, it's the current, all of those are 1a diodes, which is right at the edge of what's needed IIRC. (I can't remember exactly how to calculate peak and average rectifier current, only that it has to do with the input power, capacitor and load...if someone else can do it that would be peachy!)

You could replace them with 5402's (200v, 3a) and probably never have to worry again.


That's really helpful to know as well. I will do just that end let you know how I end up. Hoping I just throw these things and the amp comes to life. Will be a few days until I can get my hands on the parts I need but I'll definitely let you guys know how it turns out. Thanks for the pointers
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
#10
Quote by GeetarHeero


This is exactly what I was hoping for, some pointers on difficulties you found having done it yourself. I got all the pull off wires disconnected, nuts off the pots, and screws out of the board but am still getting some resistance trying to lift it out of the chassis. Could it be the pins in the tube receptacles? Do I just need to keep pulling up evenly until the board comes free?
:


It's easy to miss those screws that fasten the PCB to the chassis -- maybe you missed one.

If memory serves, I had to remove the power tube clips as well -- I think their screws actually fasten to the PCB.

Then, even after everything is unfastened, it's still a bit of a chore to finagle the board loose -- mainly because of the EQ pots. I would recommend removing all the tubes as well just to prevent damage -- but you probably already knew that.
Atmospheric dark metal w/ black and death metal influences:
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Pestilential Flood
#11
It was the screws for the power tube clips, now that those are out the board is loose. Thanks for your help. Like I said, it will be a bit until I can get those diodes to throw them on but I'll let you guys know how it turns out
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
#12
While you're in there, solder a .022uF capacitor in parallel with R86. That's the resistor directly behind the effects loop "send" jack.

That will almost completely neutralize the crappy buffer in the effects loop and make it 95% transparent. You may have noticed that if you use the effects loop at all, the overdrive tones take a huge nosedive; gobs of fizz and a lack of low-end. The capacitor mod will almost completely restore the low end and cut all that over-the-top fizz (there is still a very slight difference in tone between using the loop or not, but it's not an issue). Also, the mod is actually recommended by Peavey. If you use the loop at all, you will like what this does to your tone.

I can list a bunch of other mods as well if you're interested.
Atmospheric dark metal w/ black and death metal influences:
(My Soundcloud page):

Pestilential Flood
#13
Well, 2 month bump to report that I finally got my hands on soldering supplies and new diodes. I put four 5402s in D18, 19, 23, and 24 and the amp came to life. All tubes now have power and it is functioning normally. I don't have a lot of experience with soldering and desoldering, so it was a surprise to me but the most difficult part was getting the old diodes off of the board. After that putting the new ones in was relatively easy. I appreciate the recommendation for modding the amp but didn't want to get too ambitious on my first try working with the guts.

Thanks to all for your help
Die troll

Dean VMNTX (EMG set)
Peavey 6505+ 112
+ a buncha teh pedlulz
#14
Good job man, nothing quite like fixing/building your own stuff.

And yeah, desoldering on wave soldered PCBs (and PCBs in general) is a total bitch. Desoldering at all is a pain in the ass now that I think about it...
I don't give a shit if you listen to me or not
#15
Quote by GeetarHeero
Well, 2 month bump to report that I finally got my hands on soldering supplies and new diodes. I put four 5402s in D18, 19, 23, and 24 and the amp came to life. All tubes now have power and it is functioning normally. I don't have a lot of experience with soldering and desoldering, so it was a surprise to me but the most difficult part was getting the old diodes off of the board. After that putting the new ones in was relatively easy. I appreciate the recommendation for modding the amp but didn't want to get too ambitious on my first try working with the guts.

Thanks to all for your help


It's awesome hearing the amp roar back to life, ain't it? Especially after such a long absence. Good job tackling it yourself!

But seriously -- do that effects loop mod. It'll be like making a peanut butter and jelly sandwich compared to the repair you just did.
Atmospheric dark metal w/ black and death metal influences:
(My Soundcloud page):

Pestilential Flood