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Thread: 6505+ blows output fuse on standby. Does not with no power tubes. (

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    Member Biffstar's Avatar
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    6505+ blows output fuse on standby. Does not with no power tubes. (

    G'morning folks and hello. I'm new to this forum, so bear with me. I'm looking for advice/info on what's going on with my Peavey 6505+. I've already scoured the forums (and all of the internet for that matter) for anyone who has come across the same issue, and I've found absolutely nothing. I've found similar issues that had a simple solution, like dead tubes, sure. But nothing like what I'm encountering.

    Just a quick disclaimer, I'm no stranger to fixing electronics, but this is the first time I've really dabbled in repairing a tube amp. I'm well aware of the high voltages and dangers present, blah blah blah. Don't worry; I'm aware of draining capacitors, etc.. not my first rodeo.) Also, I'm not interested in hearing any replies suggesting that I take the amp to a tech... I am a tech, so to speak... Just not for tube amps necessarily, but I'm more than capable of fixing it with a bit of further direction. I wouldn't be asking for help if I were looking to just send it to a shop.

    So let's go!

    I received this 6505+ on a trade, and the previous owner said the only issue was a blown output fuse, so I thought cool, not a big deal. Received the amp, and sure enough, the output fuse was indeed blown. I swapped in a new one and powered on the amp. Within 10 seconds, still on standby, a single power tube arced white light, big pop from the speaker cabinet, and the few blew again. Hmm... Bad power tube? Tested all the tubes, all checked out fine. Put in a new fuse again, and powered it up.. Within 10 seconds again, single random tube arced, loud pop, blown fuse. Tried again with an entirely new set of known good tubes; exact same condition.

    So here's the scenario in point form:

    -With power tubes in, single tube (seemingly random) arcs within 10 seconds, and the output fuse blows. This is while still in standby. I never get to the point of being able to switch it to play condition. See photo:



    -After fuse blows, all tubes heaters stay lit and look normal; amp will not come off standby (since fuse is blown). See photo:



    -If I replace the fuse and remove all power tubes and power it up, standby can be switched off and on absolutely normally. Note picture:



    -Tried running with 2 tubes, same issue... random tube arcs, blows fuse.
    -Built-in bias voltage test points read 55 volts (which is where it should be for these amps).

    Here's what I've deduced so far, with no luck:

    Because the amp powers on, doesn't blow fuses, and seems just fine with no power tubes in, I figured the issue would have to be at the power board. So I removed the power board and tested all the resistors, caps and diodes on it. I found one dead screen grid resistor on the board:



    I replaced this resistor thinking it was the cause of the short/arc... Nope. Still exact same condition. Recall, the arc still happens in a random tube, not necessarily the tube that this resistor was connected to. Same random arc before/after resistor swap.

    So the power board all seems fine. I tested the rectifier diodes on the main board (visible near the fuses in the photo above), and those check out ok. There are no visible burned/scorched spots or components anywhere on the board. Note that I'm only talking about the top/visible side of the main board. I tested the output transformer, and the windings all seem fine. Found no shorts on any tap.

    And again, this happens after around 10 seconds of power on. It's not an instant short/blow (the fuse is a fast-blow, btw).

    I haven't yet taken out the big main board, but before I do, I'm hoping somebody here might have some insight on what step(s) to take next. Ideally, I'd love if one of you experts can chime in and be like "oh! that's a dead ______" and have it be a simple fix, but I'm not holding my breath.

    Please help! Thanks!
    Last edited by Biffstar; 11-20-2017 at 04:18 PM.

  2. #2
    Senior Member Enzo's Avatar
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    I would not pull the big board unless I had a darn good reason. You think the problem on the power tube board? OK.

    A bad screen resistor is a pretty sure sign the tube for that socket was bad. Or the tube was replaced after a bad one blew the resistor.

    When you say you tested parts, may I assume you did it with your hand meter? A meter uses a volt, maybe two, for any tests, and does not put real world conditions on parts. A cap that doesn't leak at 2v might well leak like a screen door at 200v. The miniscule meter currents will not expose something that breaks down at 2 amps or even 200ma.

    Same with transformers. Your meter will tell you if two windings are shorted together or to frame, but how would you know if two turns shorted together? You won't. Nor will a meter expose failed insulation in a transformer that arcs. You might tell if the primary of the output transformer was shorted end to end or in some other gross fashion, but that is about it. Arcs can only be detected in operation.

    Go to RG Keens Geofex web site and there is a tech tip of a simple transformer tester you can make. It won't make it arc, but it will at least detect shorted turns. Really, justa battery and a neon bulb, it is very simple to make and use.

    When HV fuses blow, we often suspect the flyback diodes, but if they are failing, it blows fuses without tubes too. So I don't suspect them today.

    ANy tube that arcs inside - white flashes - is instantly on my bad tube list.

    Take your four tubes and one at a time, sub them into some other amp, preferably one with similar B+ voltage. I won't assume they are good at this point.
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    Member Biffstar's Avatar
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    Hi Enzo, thanks for the response.

    You are correct, I've been using just a multi-meter to test components, including the transformers. I agree, it's not the most reliable method, but I still don't suspect the transformers. They do not/have not ever smelled or emitted smoke. Sure, not a reliable method either, but I've yet to see any other evidence that they might be at fault. Again, I'm not a tube amp expert (so correct me if I'm wrong) but if a tranny were internally shorted, wouldn't fuses pop regardless of whether or not the tubes were present?

    As for the tubes, I originally started with a used set of four that came out of a working amplifier. Stuck them into the 6505+, and a random one arced. I tested them again in the other amp, all worked fine. Stuck them back in this amp, same order for consistency, and again, one arced in a different position. For good measure, I bought four new tubes, and again, a random one arced. So I really don't think that all 8 of them are bad when only one tube in a random position arcs. It's never the same tube, nor the same position that arcs before the fuse blows. There is no discernible pattern.

    My guess is just that one spot happens to overload and arc at random before the rest, blowing the fuse. It did the same thing when I ran it with only two tubes.

    The only thing I haven't done yet is try a single tube at a time in each slot. For argument's sake, let's say it does work/stays stable with a single tube... Where do I go from there? What do I check next?

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    i did some work recently on one of these,same problem,popped fuse at standby off.
    I found an open screen resistor,changed it,then proceeded to clean very well from dust,both compressed air and contact cleaner on all sockets.
    Then i started reforming capacitors because the amp was on a shelf for quite some time,after this procedure everything went fine.
    I've seen in other cases that molex connectors go bad,heater circuit draws lots of current and heats up the connections,potentially leading to conductivity or interruptions,if you unmount the main board i'd check out those lines and eventually make a mod to connect those lines to the sockets without passing on connectors.

    Where do you see the arcing,inside the bulb or in the socket?

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    Member Biffstar's Avatar
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    Yeah, I considered some of the ribbon/molex connectors too, and I even tightened a few down to give good connections. Continuity is present and strong in all of them, so I've ruled that out.

    I also don't believe it's the heater circuit because after the fuse blows, the tubes all sit there just fine with the heaters glowing at normal brightness (i.e. pic above).

    The arcing is within the glass bulb. I've checked the sockets themselves for carbon traces or any signs of shorting from pin to pin, and they're all fine. If any one socket had a problem, I imagine the same socket/tube would arc over and over... but that isn't the case.

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    are those tubes from Ruby? (chinese str)
    when a tube arcs inside its either too much voltage on the screen grid (and this amp has over 500V,easy) or it has metal flakes that fall down and short elements,which is easy in that position.

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    Member Biffstar's Avatar
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    The tubes I've been using are 4 Mesa and 4 Electro-Harmonix. Not sure who the original manufacturer was before they rebranded them though. If a tube was internally shorted, would I not see arcing immediately on power-on?

    No matter what I try, it always takes around 10 seconds from power on before a random tube arcs and blows the fuse. It's never instant.

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    Hello in the 'Peg and welcome aboard.
    Do you have a load connected when you are running the amp? This can be critical.
    Otherwise, alexradium brings up and interesting point about arcing inside the tube being most likely caused by excess screen voltage.
    If a power supply resistor supplying everything downstream of the screens opened, I wonder if the screen voltage would go high enough to cause your arcing.

    As far as I know, this should be the same schematic:
    Attached Files Attached Files
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    Member Biffstar's Avatar
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    Yes, of course I have a load connected. Haha I'm not that daft. It's a 16ohm Orange 2x12 visible in one of the photos above. Cabinet works fine, impedance reads correctly. No issues with the cab (before somebody asks!).

    If the screen voltage is too high, how would I even go about correcting it? By upping the screen grid resistor values? Right now they're the stock value at 100ohm.

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    tubes are not predictable,they can develop shorts after some minutes,anyway they don't need a real short,even a resistance of less than 300 kohms is enough for a tube to be defective.
    tube tester can detect that behaviour but not always because they operate at 30/40 Vac in that test,and when you heat up with 500V in the amp,boom!
    If the standby is really open,check that,i'd get a new set of Ruby's STR,or Sovtek WXT+ or JJ 6l6gc

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    Member Biffstar's Avatar
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    Having arcing at random tubes in random slots really doesn't point to bad tubes IMO... especially when I've tried two different sets. But I'm going to try one single tube at a time tonight after work to see if they are indeed the problem. I'll be extremely surprised if they are. 8 bad tubes that worked perfectly fine before sticking them in this amp is kind of hard to fathom.

  12. #12
    Supporting Member Jazz P Bass's Avatar
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    How about a full on picture of the tube sockets.

    It looks like there is something going on at the bottom of the holes.

  13. #13
    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Jazz P Bass View Post
    How about a full on picture of the tube sockets.

    It looks like there is something going on at the bottom of the holes.
    Agreed. It looks like some kind of corrosion or crud in the socket pins.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    If the schematic posted above is the correct one, the standy switch opens the screen supply, not the B+ as is common, so any tube issues will still be in play (though the screens are not powered, they are still connected together via their 100 Ohm resistors).

    Have you marked the tubes so you know that they are always going back into the same socket?

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    Member Biffstar's Avatar
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    Here are the four tube sockets. The pictures might be a bit deceiving, but they are clean.






    However, after doing a bit more searching on the internet, I came across this repair page:

    PEAVEY 6505+ | PCBA Removal

    Curious, I took a closer look at the pins under my connector, and noticed discoloration there similar to that repair site:





    Sure, the burning isn't as bad as what's on that page, but still skeptical, I ran a tester across the pins... 100% shorted out between the pins. I don't believe that's normal...

    Thing is though, that site says that these two wires/pins are for the filament, and would prevent the heaters from firing if they're shorted.. But my heaters seem to work fine... So something still seems off.

  16. #16
    Supporting Member The Dude's Avatar
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    You could just have cracked solder joints on those pins.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    g1
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    If they are the filament connectors, you will be reading the short across the filament winding of the transformer, which is normal.
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    Quote Originally Posted by Biffstar View Post
    If the screen voltage is too high, how would I even go about correcting it? By upping the screen grid resistor values? Right now they're the stock value at 100ohm.
    My point about the screen voltage was that if there were a fault further down the line (like open dropping resistor R59 for example, or some burnt trace), then the screen voltage would rise (due to reduced load on the supply).
    This is a total unknown amp to you, never worked (for you), and you can't trust what the seller said, so you have to start from scratch.
    Do you have a variac or a light bulb limiter? If not, I think building the lamp limiter might reduce voltages enough to where you could do some measurements without blowing fuses.
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  19. #19
    Supporting Member The Dude's Avatar
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    And, be sure to check the connector that goes on those pins. It could be charred also.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    I don't have a variac or a lightbulb tester, no, but I did some more narrowing down tonight, and I think I've got it.

    So I tested powering on with a single tube at a time. I hadn't done this yet; I had only tried pairs at a time (and kept blowing fuses).



    From the left:
    -Single tube in socket 1, powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news!
    -Single tube in socket 2, bam... arc after 10 seconds, blows fuse....
    -Single tube in socket 3 (after replacing fuse again), powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news again!
    -Single tube in socket 4 powers on, stays stable. Flip standby to play, stays stable and I get an idle hiss from the speaker cabinet. Good news x3!

    So in the image above, the socket 2nd from the left is toast. I had light smoke coming from the socket every time I had a tube plugged into any of the other sockets (where I promptly killed the power), but I get NO smoke from the socket if the amp is powered on with no tubes installed at all. I'm no tube expert, but I'm assuming that the tubes are all electrically connected in some way for there to be a short in that socket ONLY if other tubes are present? And would explain why a random tube would arc when multiple tubes are present?

    I removed the power board again to inspect that socket, and there are no visible signs of shorting or burning externally:









    And none of the pins or solder pads on the underside of the board display any shorts with a multimeter, aside from those that appear to be intentionally electrically connected, and match the other sockets' connections.

    So to reiterate, the socket does not smoke if no tubes are present at all in any sockets, but does smoke if there is one or more tubes in the other 3 sockets. Again, I'm not sure why that is.

    And for the record, all 8 of the tubes that I have, despite a few of them having arced several times during my testing, are still good and usable. Their life is probably shit now, but they're still usable in any case.


    Can these style of sockets be opened up? Do these split in some way to examine the pins & connections and clean/service them? Or is my only option to outright desolder this socket and replace it?

  21. #21
    Supporting Member The Dude's Avatar
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    With no tubes installed, check voltages on that socket and compare to the other sockets. See if something's missing- particularly bias voltage. Also, I'd resolder the socket pins anyway. You can't always see hairline fractures and it's obvious there's a problem there. After doing both of those and verifying voltages are correct, I'd try it again.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Supporting Member Jazz P Bass's Avatar
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    What's up with the NTE 110 ohm screen resistor?

    Shouldn't that be 100 ohms?

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    Bottom picture, that one trace goes through pin 1 and 8 (if I have my orientation right) and then goes over to something else, does that go to another tube? If you have a lot of current on that net it may cause that socket to heat up even though nothing is in it.

  24. #24
    Member Biffstar's Avatar
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    I've taken two full-sized shots of the power board, front and back if you guys want to follow the traces.

    That resistor is the one I replaced that had blown (as mentioned above). It's a 100ohm; note the second line of writing on the resistor in this picture. It measures 100ohm as well.

    The one thing I've noticed, which is clearly intentional, is that some of the sockets have different unused pins compared to others, and all of the 6L6's go in the same orientation. (again, I'm no tube expert on why that is...)





    So, does anybody know if I can just split open the dead socket to examine it? Or are these sealed and I have to outright replace it?
    Last edited by Biffstar; 11-21-2017 at 03:16 PM.

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    i wouldn't lose time on disassembling a socket,just replace it,its 3 bucks.
    The only concern for me is to understand if its just the socket or the board itself that has become conductive between the pins.
    Only way to figure out is use an insulation tester.
    Did you try with just the 2 outer tubes if its ok?

  26. #26
    Member Biffstar's Avatar
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    I didn't try with just the two outers, but I did try with 3 tubes, all in the good slots. The amp turned on and kept stable, but smoke appeared from the bad socket.

  27. #27
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    Quote Originally Posted by Biffstar View Post
    ISo, does anybody know if I can just split open the dead socket to examine it? Or are these sealed and I have to outright replace it?
    I don't think that the main body of the socket is made from multiple pieces, it's one molded piece and the pins are inserted into it. Remove it from the board and replace it with a new one. You can test the amp once the bad socket has been removed, as it should work without it.

    To satisfy your curiosity, you can cut open the dead socket to see what is going on inside. It probably has a carbon trace that is hidden from view.

    Also, what is the burned resistor on the left side of the pc board?

  28. #28
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    Quote Originally Posted by 52 Bill View Post
    Also, what is the burned resistor on the left side of the pc board?
    ^^^^^^THAT!^^^^^
    Wow, good catch! I thought that was a diode... is it THE problem? Maybe, maybe not. But whatever it is, it ain't right!

    Justin
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  29. #29
    Member Biffstar's Avatar
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    I've ordered a new socket.. should be here in a couple of days.

    Quote Originally Posted by 52 Bill View Post
    Also, what is the burned resistor on the left side of the pc board?
    What burned resistor? Which photo? I'm not sure I'm seeing what you're seeing.

  30. #30
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    Post #24, first picture, between the two leftmost tube sockets. There's a burned carbon film (light brown body) slightly to the left of the larger metal film (light blue body) resistor. There are several other CF resistors on the board to compare the burned one to for reference. Whether or not that one is your problem, it still needs to be replaced, once the reason for it charring is found.

    Justin
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    "When receiving a shock I emit a strange loud high pitched girlish squeak." - Alex R -
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  31. #31
    Senior Member Pedro Vecino's Avatar
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    It looks like an input grid resistor (2K2 1/4 watt). The other of the same side is also deteriorated by the heat. The third band (red) looks brown.

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    Follow the power traces measuring currents. If you don.t have resistors inserted you need to temporary mount some(big enough to dissipate the expected requested power). That.s a easy way to isolate and spot on where you problem commin from
    " I know I'm stupid but get courage when look around "

  33. #33
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    Hmm.. nice catch, guys! When I get home after work, I'll take a closer look at those two.

    So to confirm, the one at the far left and the one in the middle should be identical to the two red/red/red/gold resistors at the far right?

  34. #34
    Supporting Member Jazz P Bass's Avatar
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    The 'burnt' resistor is R204. It is the grid resistor (2.2K/ 1/4 watt) for the outboard tube V8.

    PV 5150-2_6505+ Output Board.pdf

    There are only 4 small resistors on that board.
    They are all the same: 2.2K/ 1/4 watt.
    One for each tube bias control grid. (Pin #5).
    Last edited by Jazz P Bass; 11-21-2017 at 08:31 PM.

  35. #35
    Supporting Member The Dude's Avatar
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    If a grid resistor is burned open, there will be no bias voltage on the associated tube, causing the symptoms you have. I still doubt the socket is bad. Going back to my post #21, check voltages on the sockets and see. I see no reason to replace a socket without first checking the voltages on socket pins. It's the best way to figure out what's going on. Or, we could just replace parts until it works.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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