6505+ Head "motorboating"

[Leooooh]

Hi,


The head is ok when the volume (post gain) is under 1 or 1,5. As soon as I get it past 2 it begins to do the exact same sound of the video I linked in this thread. It happens in both channels (clean, crunch and lead) at nearly the same volume.

Here is the video:

https://www.youtube.com/watch?v=WYaZO7Rc0Lw


With some research, I discovered that this problem is called motorboating (at least, is what people call it).

After studying the head's schematic for a bit, I decided to replace the C33 and C37 (the 22uF@450V caps). In my understanding, if they are bad, there will be a bleed in all the channels that will cause a lot of feedback. Well, replaced both of them, and the problem persisted. Then I also replaced the C36 (47uF@500), since it will filter the phase inverter. Nothing changed.

I already plugged the guitar in the return of the effects loop, to see if the problem was in the power or pre-amp. After plugging the guitar in the return, no feedback was head from the amp, so I figured it is in the pre amp section.

I also tried to replace the V1, V3 and V4 tubes, but nothing changed. Did the same with the power tubes, and rebiased the amp, but the problem is still there.

Have you guys ever encountered this problem? I really dont know what do now.

Thank you a lot for the help.

[Ericur]

What I could hear from the clip, it could also be a bad solder joint.
But recapping all the small electrolytic caps would be the next cheapest option.

[Jazz P Bass]

I would go 'stage to stage' from V1 on down, and see if any of the coupling caps are leaking high voltage dc into the next stage.

And of coarse, if Any solder joints look iffy, touch thenm up.

[Ericur]

Absolutley, go nuts with the solder

[Mick Bailey]

Take care when replacing components that they're actually diagnosed as faulty. Evey time something is desoldered there's a risk of lifting a pad, installing a component reverse-polarity, or the new replacement being defective. It's better to isolate the fault progressively to narrow it down to a small area, then diagnose which component is defective.

You already established the fault is in the preamp section. Try to narrow it down within the preamp. Which controls affect the fault? Usually if a control makes the problem better or worse then the fault is in or before that section. Check for ramping voltages on the grids of the tubes or any DC. A useful test is to ground the grid of each tube in turn while the amp is oscillating to determine which tube is the source, starting at the input stage.

[Leooooh]

Discovered some issues in the power amp board. The conector that is responsable to deliver the filament tension for all the power tubes and to V4 and V5 is hot. Even the trail on the board in this region is hot.

The wire that is hot is the one responsable for the "ACFIL2". Wherever it goes, it is hot. I noticed this after making an inspecion on the conector that conects the main board to the power board. To be more accurate, the wire I am speaking of, is in the J59 (main board) and J11 (Power board).

I checked on the schematic, and the "ACFIL2" goes directly to the power amp board in all the the pin 7 of the power tubes, as well as the pins of the filament in V4 and V5. In the power amp board, it is also connected to the R200 that is connected to the C200 and R201. I already replaced the C200, but the wire and the way in the board keeps warming.

Could this have anything related with the issue I am experiencing, when turning up the volume?

[Leooooh]

So, the head is fixed. Found the problem.

The problem was the two wires that deliver the heater voltage to the power amps.

In the main board, you can find the J59 and J60 that connects to the J11 and J12 respectively. Those wires are responsible for delivering the heater voltage (ACFIL1 and ACFIL2) for all the power tubes and for V4 and V5 and both of them were carbonized and broke up as soon as I removed the whole connector to check for bad connectivity.

So I cut the bad wires and soldered new ones directly in the main board and in the power board. I also cut the connector that connects the J83 and J81 to the main board, since it was with some marks of carbonization, and soldered them directly in the mainboard, since I had an issue in the past that all the power tubes and V4 and V5 were not glowing. It was definitely because of this bad connector.

The last thing I noticed before closing the amp, is that the trails that lead to the J59 and J60 are still heating, but the wires the connect the main board to the power board are not. The same happens to the wires that come directly from the transformer as well as the trails that leads to V4 and V5. Could this be a heating induced from the power tubes?

Well, I took the head to rehearsal (3 hours), and it didn't show any more issues, just the great and crushing tone that I was missing of the 6505+!

Ahhh...And one last note. During my search for this problem, I sent an e-mail to Peavey to ask for help. Grant Brown was of a great help and told me that normally, this problem is caused by C33(22uF@450V) and C37 (22uF@450V) not filtering correct. So if anyone happen to find this problem, you should definitely take a look in those caps. I thought about them too, but both of them in my case were good.



Thank you a lot for the help.