Bugera 6260 issue

[Snibbornairb]

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

[Enzo]

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?

[Chuck H]

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.

[Snibbornairb]

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

[Enzo]

Fuse could be blown, fuse could also just be losing contact in the clip.

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

[Chuck H]

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.

[Jazz P Bass]

Here is the schematic: 6260 RevG.pdf

[Snibbornairb]

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!

[Chuck H]

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.

[Snibbornairb]

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.

[Chuck H]

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).

[Snibbornairb]

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.

[Chuck H]

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

[Snibbornairb]

Here is the schematic: [ATTACH]56844[/ATTACH]

Just saw that this was posted.
Thanks so much!

[Snibbornairb]

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