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Thread: Bugera 6260 issue

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    Bugera 6260 issue

    I would appreciate any help tracking down an issue on a Bugera 6260 amp. My son was playing through a borrowed amp and the sound went bye bye. The 5th preamp tube (middle of amp) has no heater glow along with the power tubes, the other 4 preamp tubes glow. When I pull the preamp tubes, unscrew the preamp board, flip it over and reinsert preamp tubes (trying to track voltages), all the sudden all tubes glow. Then when I put it all back together, the heaters go out in the 5th preamp tube along with all power tubes. Any advice on what I should check?
    Thanks in advance,
    Brian

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    Lifetime Member Enzo's Avatar
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    That extra small tube is the phase inverter - part of the power amp. The power amp has a plain old vanilla 6vAC heaters. SO sounds like the 6v is going out. SO a pair of heavy yellow wires from the transformer is the start, it flows through a 10A fuse F4. Then through a ribbon cable to the power tube board. My first suspect is the 10A fuse. Is the clip loose? Look at the ribbon cable, are any of the pins looking burnt or discolored?

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    Bent Member Chuck H's Avatar
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    I couldn't find a schematic so I didn't post. My first suspicion was that the preamp and power amp/phase inverter were separately powered at the filaments. Without a schematic I couldn't confirm this and I wasn't aware of the fuse. So...

    Thank you Enzo for having some familiarity with this amp AND...

    If it's the fuse, it likely blew for a reason. The most likely suspect would be a bad power tube. Or a power tube that suffered a filament short due to some other incorrect operating parameter. If it happens that the filament fuse is blown then the power tubes AND their operating conditions should be checked before powering up with tubes in place again.

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    Thanks for the responses gentlemen.

    Thanks guys. It is helpful to know that it is separate from the other 12ax7's. I was beating my head against the wall trying to trace the vo li tage from the other tubes ove tge this last one and not finding the path. So it will be heated with a/c? I will check for that. All the fuses are in good shape. Id i know that I am looking for a/c hopefully i will have better luck. There is no schematic that I can find. Ive been trying to use one from a 5150 II, but it is a little different and I am not all that good with them anyway.
    There is a little burn on the connector at the yellow wire. It reads about 5.6v.
    Any idea why having the board upside down might cause the tubes to glow?
    I replaced all the tubes with tubes pulled from a working amp (same guy the amp was borrowed from recently retubed his other amp.
    Thanks again

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    Lifetime Member Enzo's Avatar
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    Fuse could be blown, fuse could also just be losing contact in the clip.

    Remember, these amps are basically clones of recent Peavey amps.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Snibbornairb View Post
    There is a little burn on the connector at the yellow wire. It reads about 5.6v.
    Any idea why having the board upside down might cause the tubes to glow?
    Little burn, intermittent voltage,.. I think you're looking for a bad connection very near where the AC filament voltage for the power tubes enters the board. Possibly in the burned area. Maybe not though. 5.6V doesn't seem consistent with anything in a tube amp. It's unfortunate we don't have a schematic.

    Can you please confirm that you have located and identified the 10A filament fuse that was referenced.

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    Supporting Member Jazz P Bass's Avatar
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    Here is the schematic: 6260 RevG.pdf

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    Quote Originally Posted by Chuck H View Post
    Little burn, intermittent voltage,.. I think you're looking for a bad connection very near where the AC filament voltage for the power tubes enters the board. Possibly in the burned area. Maybe not though. 5.6V doesn't seem consistent with anything in a tube amp. It's unfortunate we don't have a schematic.

    You nail it. Disonnecting the connector and taking a measure right at the pins gives me 6.2vac. Connector in place has nothing. Continuity between pin on top and trace underneath is good. The burn doesn't seem that bad but maybe I will sand the contact a bit and try again.

    Can you please confirm that you have located and identified the 10A filament fuse that was referenced.
    The fuse connected to that winding is good.

    Thanks so much!

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    Bent Member Chuck H's Avatar
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    Ok. Looking at this schematic it's a little uncertain, but it looks like the AC filaments (the ones that are intermittent) and the DC filaments for the preamp tubes are using the same power transformer secondary winding. That same winding also looks to have some low voltage supply that is probably for the switching circuit. Since the DC filaments seem to be working it's probable there is an intermittent connection where the un-rectified AC straight from that winding meets the board. This may be jumpered from an initial circuit for the DC filaments and the low voltage supply through a connector cable (I've never seen inside one of these amps).

    Again, there's a reason for the failure. The burned area and the intermittent AC filaments may not be related, but it's a hell of a coincidence if they're not. I think you should trace the AC filament circuit. Wiggle all connections to try and detect an intermittent one. And test the DC filament circuit and low voltage supply from that same winding for relative goodness according to the schematic. It's possible a bad component in a peripheral circuit off that winding burned the board and caused the failure.

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    So it seams it may not just be the connection. When I pulled all the power tubes, I then had the 6v on the traces below.

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    Quote Originally Posted by Snibbornairb View Post
    So it seams it may not just be the connection. When I pulled all the power tubes, I then had the 6v on the traces below.
    It's normal to have filament voltage rise when tubes are pulled. Filaments draw a relatively large amount of current that loads the winding. It's the overall low voltage that might be a clue. Something may be over loading the winding. Since it still seems low with the tubes pulled the problem may be in a peripheral circuit off that winding (see my post above).

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    I really appreciate you time on this, Chuck. I did see your recommedation to look through the associated circuitry and components and it is pretty much straight traces that go through the fuse then to the connector to power amp board and right to the heater pins.
    I did take some sandpaper ro the contacts on the connector that brings the 6v from the tranformer to the board. After doing so I do have heater cotage on tge phase inverter and power amp tubes. I now have sound going to the speaker.
    Problems i still have: not knowong what caused the overheat in the 1st place, and also, in the process of disconnecting all the connectors from the control panel (which was apain with the glue and what seems like pretty cheap connectors) I broke a wire in a tine 3 pin connector. I have to obviously find a replacement connector and reconnect the wires.

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    Well, as I mentioned, the schematic seems to show three circuits operating off that winding. Now, it may be different in your amp. There are often differences and schematics can be out of date with changes. But if there are peripheral circuits then what I said applies.

    Can you report on the burned area of the board please? What components are on or connected to that area. Most boards have components numbers. If we're lucky they even match the schematic

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    Quote Originally Posted by Jazz P Bass View Post
    Here is the schematic: 6260 RevG.pdf
    Just saw that this was posted.
    Thanks so much!

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    Quote Originally Posted by Chuck H View Post
    Well, as I mentioned, the schematic seems to show three circuits operating off that winding. Now, it may be different in your amp. There are often differences and schematics can be out of date with changes. But if there are peripheral circuits then what I said applies.

    Can you report on the burned area of the board please? What components are on or connected to that area. Most boards have components numbers. If we're lucky they even match the schematic
    There are no burnt component and the board looks in good shape, although it is black so...
    The white plastic connector from transformer has some discoloration. The last pin in the connector is what was hot, after sanding the pin I got voltage back on the traces. I soldered a little bodge to reconnect the pin to the panel with all the pots. Now it is working fine.
    Any suggestions for follow up. My son been playing for 10 min or so and still going well.
    Thanks

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    Quote Originally Posted by Snibbornairb View Post
    Any suggestions for follow up.
    Well I just looked at the schematic more closely and realized I was in error regarding the peripheral circuits. The AC filament winding looks to be connected to the other circuits I mentioned through a single 4.7k resistor (R104). Why, I can't say.

    I don't suppose there'a any follow up except that it's obvious that connection became hot enough to discolor. That's a result of excess current. And the filament voltage you report seems low at 6V unloaded/5.7V loaded. That may be normal for that amp. The schematic doesn't say. But at face value you have symptoms of an over loaded filament winding. Follow up for me would be to confirm the voltages you have there, which I can't do and the schematic doesn't specify. And if it's indeed low I would be looking for why. If there is a problem the amp may work for awhile, but on borrowed time.

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    The AC filament winding looks to be connected to the other circuits I mentioned through a single 4.7k resistor (R104).
    That elevates the 6.3V heaters to +24VDC and also means that there can't be a grounded CT for the 6.3V winding (or other ground reference like balancing resistors).

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    Ah! Thank you. That's where I was confused. Because the DCV reference was only to the one side, rather than to the CT. I haven't seen that before. I guessed it was probably an elevation circuit but didn't want to speak out of my depth.

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    Because the DCV reference was only to the one side, rather than to the CT.
    The 6.3V side where the resistor connects to will be at constant +24VDC while the other side will be at +24VDC +/- 6.3VAC, i.e. varying between 15.1V and 32.9V instantaneous voltage.

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    I remember working on a 6262 where the yellow filament wiring had to be soldered directly to the board due to the fact that the molex connector was not holding up well. Not sure what the connector looks like in this 6260 amp but the OP keeps mentioning it looking burned and cleaning pins got it working again. But how long will those pins hold up if that is the type of connector on the amp. I think we need a picture to confirm what we are dealing with...



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    The discoloration was just to a light brown, not nearly as bad as that picture. Could the addition currwnt be tge result of turning on standby switch before tubes warmed up? Not too familiar with tubes and like I said, it was my son who was operating the equipment when we lost the power section.

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    It has that same style molex connector

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    Do you know what caused the failure in the amp in the picture?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Snibbornairb View Post
    Do you know what caused the failure in the amp in the picture?
    THAT is the perfect question Although it's possible to diagnose if there's an issue in your amp as well. And that's good because it's not necessarily the same problem.

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    I think in the case shown, the type of connector used is just not a great idea for carrying this type of current. A slight bit of resistance develops across the contacts (oxidization) and you have a lot of heat developing. From there it's just a downward spiral, more heat causes more resistance causes more heat, etc.
    Very common for these and similar connectors in heater wiring circuits to have been bypassed by techs, often with no other underlying high current fault.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    I think in the case shown, the type of connector used is just not a great idea for carrying this type of current. A slight bit of resistance develops across the contacts (oxidization) and you have a lot of heat developing. From there it's just a downward spiral, more heat causes more resistance causes more heat, etc.
    Very common for these and similar connectors in heater wiring circuits to have been bypassed by techs, often with no other underlying high current fault.
    Agreed, but,..

    6V on unloaded winding and 5.7V loaded. Normal? I don't know this amp. I think it should be confirmed that that is the normal voltage for this amp and that there isn't something loading the winding.

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    If it has that molex connector it needs to be removed and soldered straight to board. As I remember there was a service bulletin from Bugera about fixing that exact issue.

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    Quote Originally Posted by Chuck H View Post
    Agreed, but,..

    6V on unloaded winding and 5.7V loaded. Normal? I don't know this amp. I think it should be confirmed that that is the normal voltage for this amp and that there isn't something loading the winding.
    I got the impression that measurement was done at the socket, not at the winding. So I'd expect some drop at the connector when winding loaded.

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    My memory said 5.7V for some reason. It was actually 5.6V. This is what I read:

    Quote Originally Posted by Snibbornairb View Post
    There is a little burn on the connector at the yellow wire. It reads about 5.6v....I replaced all the tubes with tubes pulled from a working amp...
    Then

    Quote Originally Posted by Snibbornairb View Post
    So it seams it may not just be the connection. When I pulled all the power tubes, I then had the 6v on the traces below.
    I interpret this to be read from the board traces without power tubes installed.?. It could be that it was read at the socket and the socket was presenting some resistance before cleaning? Which, BTW, is not usually best done with sandpaper. But since it was done and the amp was working I didn't mention it before.

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    A reading taken right at the winding both loaded and unloaded would help.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by g1 View Post
    A reading taken right at the winding both loaded and unloaded would help.
    As well as confirmation of a good battery and/or meter calibration. And still... Maybe this is normal for this amp.?.

    This is really just something I'd want to be cozy on before buttoning it up. But it's not my matter.

    And probably best to just do the direct board solder as suggested by the manufacturer. But still, confirm voltages.

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    And we are getting "ther is 5v on the heater" or similar statement. What we are not hearing is 5v with respect to what?

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    If I ever see a brown spot on a molex socket used for a heater connection I hardwire the two wires. End of story. No need to measure anything.

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    Thanks for all the thoughts, gentlemen. To clarify a few pounts in the discussion, at the winding, unloaded, I was getting 6.3v. On the thick board traces on the power amp board I would get high 5v rage (can't remember exact, would rise as tubes heated). The sanding I did was on the Molex connector pin and the 6.3v winding. I put it back together before the direct solder recommendation was posted. I will communicated that to the amps owner, however.
    Since I have had the amp apart and fiddled around with it I might just buy it from him anyway. I feel bad about the whole thing. Big lesson is dont borrow anything that you are not willing to buy.
    My son is back to jamming on it now, so that is the good news.
    Thanks again

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    I feel bad about the whole thing.
    No reason to feel bad, Brian. No misuse involved.The heater problem would have shown anyway.

    at the winding, unloaded, I was getting 6.3v.
    As it is unloaded, the voltage seems somewhat low. But I am sure it was always like this. What is your measured mains voltage?

    Any contact resistance of the fuse holder and the connector will reduce heater voltage on its way to the tubes. You can measure the voltage drop across the fuse holder and connector. Tubes typically work fine if heater voltage is within 6.3V +/-10%. So something like 5.7V won't be a problem at all.

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    Last edited by Helmholtz; 02-02-2020 at 08:45 PM.
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