Help with Peavey Ultra Plus 120. Just makes a loud hum/buzz after 5 mins.

[mrmcsick]

https://dl.dropboxusercontent.com/u/...0schematic.pdf

Here is a copy of the schematics.

It first started as an intermittent problem but has progressed into a constant. It fires up and works great for like 3-5 mins but then suddenly all output stops except for a loud hum/buzz that isn't affected by the volume control. You can switch off the standby switch to stop it from coming out the speakers but if you turn it back on its still there. If I power it off completely for about a min its good to go again for the couple of mins. All tubes are good as I've swaped them all out individually and have extra's that are known good as they work fine in my Fender bandmaster.

I found a couple small things when I ripped it apart. There was one broken solder joint on 1 of the pins of one of the power tubes and also a broken solder joint on ceramic disk right after the power switch. I'll look at the schematics to refer to what they are exactly in a min.

Other than that I reflowed all the solder joints. Visually inspected the caps for leaking/bulging. Cleaned all the pots. Retensioned the power tube socket pins. Cleaned and looked for any visual damage of all the ribbon cables and other internal connections.

I'm not all that knowledgeable when it comes to electronic circuits and components but some basics and can't really follow a schematic without getting lost or understanding why each component is there and why. If you guys have anything for me to test I can try. Just have a basic multimeter.


Edit - ok it looks like the first component that had a bad solder joint is on the last page of the schematics on the power chord circuit. The black of the chord runs into the main fuse then to the power switch and from there to the component which seems to be labeled cl60 on there. From this component the wire runs to the black wire into the power transformer and 1 pin of the ground lift switch.

I'm not sure which pin it was on the power tube as I don't know which way to count the pins. If I go from the key hole in the socket do I go clockwise or counter clockwise from it. If I'm looking down into the socket where the tube goes in from the back panel of the amp it would be 1 pin counter clockwise from the key hole. The key hole is facing left towards the output tranny. It appears that that pin isn't even used on the actual tube.

[nashvillebill]

I'd look really carefully at C51 and C52, the two main power supply filter caps. Have the leads to one of the caps cracked or broken off between the board and top of the cap--the solder joint might look fine... Have you given the amp the patented "Enzo whack" (hit it hard with your hand) to see if the physical jarring affects anything?

Enzo will likely be along shortly with more advice, he is the real Peavey guru here. Well actually he's the all-around guru.

[mrmcsick]

I'll look at the caps in a min. It actually did used to jolt when I would smack it before I resoldered those 2 busted solder joints I mentioned. Doesn't do it anymore.

Those caps are top mounted caps so I would need to unsolder them to check the leads. Should I make sure they are drained and give it a shot?

[mrmcsick]

[mrmcsick]

This pin is in the same trace as 1 of the 2 white wires coming from the half power switch. (the single post sticking up just below the yellow circle)

[Enzo]

If the amp reacts to a whacking, then some connection is loose in it.

I am not inclined to think it is caps myself. I'd watch my power tube closely. It takes a few minutes. I'd expect a bad cap to show up immediately. I first suspect power tubes. Look at them closely as th problem surfaces. are any of the power tubes developing a red hot streak down the side of the inner structure? (Google "red plate tube" and see many pictures as examples) Three minutes sounds like a tube underbiased slowly failing.

If that isn;t happening, then I start taking voltage readings around the tube to see if any of the operating voltages are falling off or becoming highly rippled.

[mrmcsick]

The whacking noise was before I resoldered those 2 bad joints. It no longer makes any noise when hit. I'll look real close to the power tubes when it happens. Be back in a minute. Thanks guys.

[Enzo]

The black disc in the mains wiring is an inrush limiter. If the amp fires up at all, then it is working. A loose solder there could make the whole amp go off and on.

[mrmcsick]

No red plates. Took a vid, its uploading to youtube now. You can hear the amp randomly pop every once in awhile too.

https://www.youtube.com/watch?v=8ts6...ature=youtu.be

[mrmcsick]

Ok what pins should I be checking, 2,3,4,5,7 and 8? What kind of voltages am I looking for?

[Enzo]

Well, take some fresh readings when the amp is first on and working well. Specifically we want to see on each power tube:
Pin3 pin 4 - B+ voltage, which ought to be something like 485-500vDC.. Pin 4 will be a couple volts lower than pin 3. There should be maybe 2-3v ripple or less I suppose. You also ought to see something like -55v on the grids at pin 5 with zero ripple.

Now when the amp warms up and goes all hummy, retake the readings. Has the B+ fallen considerable? Has the ripple grown much larger? Then the bias on pin 5, has it lessened and has it gotten ripply?

[mrmcsick]

Pin 3 on all four tubes is reading 510vdc in standby then 490vdc when on regardless of the hum. Pin 4 I'm getting -8 in standby then 485vdc off of standby ( when the hum kicks in it goes down to 483 then climb back up to 485). pin 5 on all tubes I am getting -41vdc.

I'll recheck them later. Thanks

[Enzo]

OK< so the power tube voltages pretty much remain where they are during the problem, yes?


Um, when an amp makes a loud hum like that, best not to leave it on.


Pops and crackles are common signs of failing power tubes, as would be that loud hum. Do you have a different set of 6L6s you could try in the amp? Like from some other amp?

With just these four, you could try removing just one of them and see if it still does it. then put it back and remove the next one. After without each of the four tubes, we will know if one tube is associated with the failure.

It is also possible a flyback diode is failing,m in which case you could pull out all four power tubes, and leave the machine running. if it starts a loud hum even without power tubes, we know current is flowing through the output transformer. Possible transformer failure, but more likely a diode.

[mrmcsick]

Yes the voltage remains the same.

These are jj 6l6's. I have another quad of sovtek 6l6's and a pair of rca 6l6's and all 10 tubes work fine in my fender bandmaster. I pulled all the tubes and no hum comes out. Even with just one tube in, in any of the sockets it there.

[Jazz P Bass]

Have you checked the bias voltage on pin 4 of the 6L6 tubes.

Check the Vdc & Vac voltages.

If the Vdc is low the tubes will try to redplate.

If the Vac (ripple) is high enough, it will tell the tubes to conduct, which may indicate a failed bias capacitor.