#1
Hey all. Not sure how many users here are experienced with tube amp repairs, but I figured I'd go ahead and ask anyway.


I have a Peavey 6505 plus 112 combo that I bought used about 6 months ago. There were no issues with the amp according to the previous owner. It has been a great amp and I fell in love with the lead tone, until just recently.


I think I may have left the amplifier on standby for a few days last week. I'm not 100% sure about that. But there was a big storm the night before I found this problem. I don't think we lost power in the storm, but perhaps there was a quick power surge I didn't notice. The day after the storm, I went to turn the amp on and there was no power whatsoever. I checked the fuse that is accessible in the back of the amplifier and found it was blown. So I replaced it, and now it powers on. However, there is absolutely no sound coming from the amp. No hum, static, nothing. Not even noise from the reverb springs when I shake it lightly.


I have tried different speakers, plugging guitar into the effects return, placing a patch cable from the effects send to return, all with no results. All of the tubes are glowing when turned on.


What are some other things I should check for before taking it into a tech? I am pretty electronics savvy, I just have not really ever had to mess with amplifiers besides replacing tubes.
Last edited by MeGaDeth2314 at Jan 21, 2017,
#2
MeGaDeth2314 Check the HT fuse which will be an internal fuse (F2).

Obligatory warning: Tube amps can contain lethal voltages even when off and unplugged, so if you are not comfortable draining/working around lethal voltages, please take it to a tech.
Last edited by Will Lane at Jan 21, 2017,
#3
Thanks Will Lane . Checking the internal fuses was my next plan. How can I make sure that I don't kill myself? Do I just pull the fuses to check them, like the main fuse?
#4
MeGaDeth2314 You will need to drain the filter caps to be safest. Typically you connect pin 1 of V1 to chassis ground and turn the standby on while the AC switch is off. It may be different depending upon the amp so someone else may want to chime in to make sure you do not harm your amp/stop your heart.

Hopefully the internal fuse(s) will be in a convenient location, in a fuse holder- and not soldered to the PCB, underneath a ledge or something silly like that. Check it for continuity. Or if the filament is very visibly broken, replace. I sent you a PM with more details.

#5
Thanks so much Will Lane . I do not have a voltage meter, but I am confident I can discharge it the way he has shown in the video. If I discharge it and wait for a while, then use some kind of rubber grip tool to pull the fuse, would that be safe enough?


I have also heard about the rectifier diodes going bad in these amps. Do these need to be checked with a meter? How difficult are they to replace?
#6
MeGaDeth2314 Likely yes, but to be safe, no. I would want to be sure all high voltages are drained out the amp before I do anything with the internals. I would always use rubber-gripped tools to poke around the internals in the first place, but allowing your body to contact any part of the circuit (or allowing the metal of the tools to misdirect current between two points) is a dangerous game if the voltages somehow were not drained.

Checking the diodes is smart, but I think you will need a DMM to do it (one with a diode-check function particularly). They are as difficult to replace as anything else on a PCB, which is not very easy but doable.
Last edited by Will Lane at Jan 21, 2017,
#7
Will Lane

I think you may have been right about the F2 Fuse. I finally opened it up just to take a peek and the F2 fuse looks like it went out, the inside of the glass is all black like it was burned. The other fuses are crystal clear. I don't notice anything else that looks strange.


However, my preamp tubes don't appear to have pins like in the video. Do you know if there is something else I can connect to the chassis to drain the voltage?



F2 fuse is noticeably blackened



entire board


Also, the F2 fuse doesn't look like it's just plug-in and go. There are two thin metal wires connecting it to the board. Is this something I would have to solder or do you just push the metal pins into the holes?


Thanks for all your help.


EDIT: Nevermind, I found the pre-amp pins. They were just smaller than I expected.
Last edited by MeGaDeth2314 at Jan 21, 2017,
#8
MeGaDeth2314 Yup, that F2 is not happy. Replace it with an exact match. You can look at the schem and see what it says, or look on the metal barrel of the fuse and read the indentations. Make sure you get either slow-blow or fast-blow, whichever it is. It looks like it is soldered, the legs coming from it are attached to the fuse and the legs are in the PCB. I would carefully heat up just the leg of the fuse until the solder point flows, then pull out the leg then do the other side. Then reverse the process for the new fuse. That is presuming the new fuse does have solder legs, if not you may need to cut the legs off the old fuse and solder the new fuse to the legs.

And yup, on PCB amps typically the tube socket points will be soldered to the PCB rather than openly visible. You just poke the solder point and send the other end to chassis ground as demonstrated in the video. Attaching 10k resistance on your draining jumpers is smart so you do not get any sparking or a lot of heat when connecting the leads. However since you are having to poke the point, you cannot just connect the points and step away, you have to constantly hold the jumper on the solder point. Obviously use isolated jumpers, but another good rule of thumb is to put your other hand (non-probing hand) behind your back so it is not near the circuit.

EDIT: You may also be able to just solder the new fuse on top the old fuse as if you were doing a parallel connection but I am not sure.
Last edited by Will Lane at Jan 21, 2017,
#9
Will Lane

The soldering points for the fuse is on the back side of the board, which looked like a real bitch to take off, so I figured I'd try soldering the new fuse on top of the old one and still nothing. Not sure if it's just not enough contact or if something else is going on. Should I try cutting the legs off at the connection point with the fuse and soldering the new fuse to the legs?


EDIT: Another person from a different forum pointed this out. But in the close-up photo I posted, there is some blackening on the PCB right by the 2nd group of 4 diodes, between the diodes and the cap. Could this be a case of a bad rectifier diode/diodes? How can I test for that? Apparently that group of diodes is the most common to go bad.
Last edited by MeGaDeth2314 at Jan 22, 2017,
#10
That amp reeeeally didn't like that power surge.

It's quite common for the rectifier diodes on these amps to get too hot and burn out. It's happened to me before. If you see burn marks, replace them. Just be mindful that a lot of watts goes though that section of the amp, so discharge the caps first.

It's much, much easier to do this job if you just take the entire PCB out of the chassis. It isn't difficult, just tedious. When I was doing my Fender, Soldano, EQ and hot bias mods etc. to my amp, I took the board out of its chassis so many times I could do it in my sleep.

It was a lot of work but in the end I actually enjoyed it and it the results of all those mods absolutely transformed the amp. It sounds immense.

Unplug all the tubes. Unplug the transformer wires and make a note of where you unplugged them, I marked where they all went with a sharpie. You need remove all the screws that mount it to the board. look around the PCB and you'll see some screw heads. Take them out. You'll need to take out the screws that mount the power tube sockets to the board, you can find those on the underside of the amp's case where the springy clips that hold the tubes in are. Then you need to remove all the knobs on the amps' front panel (use a piece of string to help you pull them off) and undo all the 10mm nuts that hold the knobs on the front panel. Then undo all the 10mm nuts for each knob. A socket makes this process faster. After you've done all of that, the board should slide right out of the chassis.

Now you'll have access to the underside of the board and you can start desoldering and adding new components properly.
Quote by Axelfox
Reeeeeeeeeeeeeeee

Quote by H4T3BR33D3R
I also have to do that. Cottaging this weekend
#11
Quote by T00DEEPBLUE
That amp reeeeally didn't like that power surge.

It's quite common for the rectifier diodes on these amps to get too hot and burn out. It's happened to me before. If you see burn marks, replace them. Just be mindful that a lot of watts goes though that section of the amp, so discharge the caps first.

It's much, much easier to do this job if you just take the entire PCB out of the chassis. It isn't difficult, just tedious. When I was doing my Fender, Soldano, EQ and hot bias mods etc. to my amp, I took the board out of its chassis so many times I could do it in my sleep.

It was a lot of work but in the end I actually enjoyed it and it the results of all those mods absolutely transformed the amp. It sounds immense.

Unplug all the tubes. Unplug the transformer wires and make a note of where you unplugged them, I marked where they all went with a sharpie. You need remove all the screws that mount it to the board. look around the PCB and you'll see some screw heads. Take them out. You'll need to take out the screws that mount the power tube sockets to the board, you can find those on the underside of the amp's case where the springy clips that hold the tubes in are. Then you need to remove all the knobs on the amps' front panel (use a piece of string to help you pull them off) and undo all the 10mm nuts that hold the knobs on the front panel. Then undo all the 10mm nuts for each knob. A socket makes this process faster. After you've done all of that, the board should slide right out of the chassis.

Now you'll have access to the underside of the board and you can start desoldering and adding new components properly.


Thank you. That was very helpful. I am planning to do this once the new diodes arrive. Yeah I figured it was more tedious than difficult, but I really want to learn to do this on my own because the parts are so cheap.

I have a multimeter and can successfully drain the voltage from the caps now, so I shouldn't wind up shocking myself to death.
#12
Well, I replaced the F2 fuse and the 4 diodes. Still nothing.


I'm going to try new power tubes, assuming the fuse didn't blow again, and if that doesn't work then I'll take it to a tech.
Last edited by MeGaDeth2314 at Feb 8, 2017,
#13
Well, I'm throwing in the towel. I ended up replacing the fuse a couple times because it blew both without and with power tubes installed. I don't have the time (or knowledge) to go through and test every component, so I'm going to take it in.

Unfortunately there is only one place anywhere near me that fixes amps. Hopefully they don't charge me a ridiculous price.


Thanks to everyone in here for all the help. I actually had a good deal of fun learning about my amp and practicing some soldering.


Will Lane how did you get into amplifier repair? Do you repair guitar equipment for a living? I'd like to continue my research.
#15
Quote by MeGaDeth2314
Well, I replaced the F2 fuse and the 4 diodes. Still nothing.


I'm going to try new power tubes, assuming the fuse didn't blow again, and if that doesn't work then I'll take it to a tech.


Diodes are directional, are you certain you installed the new ones correctly?
“Ignorance more frequently begets confidence than does knowledge.”
Charles Darwin
#16
Quote by Arby911
Diodes are directional, are you certain you installed the new ones correctly?

+1

Not to mention as well that diodes are quite heat sensitive, so touching a soldering iron to the terminals for too long can damage them.
Quote by Axelfox
Reeeeeeeeeeeeeeee

Quote by H4T3BR33D3R
I also have to do that. Cottaging this weekend
#17
Arby911 T00DEEPBLUE I did place them in the correct orientation. I did have quite a bit of trouble getting the old solder off the joints, but the new ones went in fairly easily so I would be surprised if there was heat damage.


Can the PCB itself be damaged from too much heat? Like I said, getting the old solder off some of these joints was quite a pain in the ass and took a lot of heating, reapplying new solder and then wicking it up. I noticed what looked liked a bit of moisture around the soldered components on the PCB after soldering, but I assumed it was something that occurred naturally from the heat.
Last edited by MeGaDeth2314 at Feb 13, 2017,