5150 II cutting out, then comes back on

[cloy26]

I'll play my 5150 II at a fairly open level (4 or 5) at 4ohms. All is well until about 4 minutes. Suddenly the amps goes silent. So, I'll switch to standby, let it cool down, then shut it off. I'll wait 5 minutes, switch too 16 ohms (and switch the cab to 16 ohms) and to the right speaker output, and then turn it on and all is well...

wtf? Is this some safety feature or something?

 

[Sacha]

Most likely a dying preamp tube, or maybe a loose solder connection or something

 

[guitarguru777]

Sounds like you got a bad power tube to me.

 

[RichMinerva]

Sounds like you got a bad power tube to me.

+1

 

[53Crëw]

If a power tube goes bad it will usually reduce the volume slightly or cause noise, not cause an amp to go silent altogether. (Unless it takes out the fuse, too.)

My vote is for a preamp tube. First thing I do anytime an amp acts up, though, is to clean the tube sockets and jacks just to rule out bad connections. If it continues, then I'd start swapping out preamp tubes one at a time to see if I could isolate a bad tube. If that doesn't solve it, then it's probably a bad component on the PC board.

Just to rule out the easily overlooked... your speaker cable is in good shape, right?

 

[Ben Johnson]

Try re-seating the power tubes.

 

[lolzgreg]

Does it just get remarkably quiet, and then randomly fluctuate in volume? If so, it's DEFINITELY the connections inside the effects loop. If you get some DEOXIT and spray it inside the effects loops jacks (send and return) at around a 45 degree downward angle, and put more DEOXIT on a 1/4" cable and rapidly insert and remove the plug from the effects loop jacks (send and return), it should clear up the issue. You may have to do this a few times, but this happened to my 5150, and this is how it was fixed (I watched as the tech repaired it).

 

[::XeS::]

FX-Loop for me too.
It happened to me some month ago

 

[sodapop808]

Does it just get remarkably quiet, and then randomly fluctuate in volume? If so, it's DEFINITELY the connections inside the effects loop. If you get some DEOXIT and spray it inside the effects loops jacks (send and return) at around a 45 degree downward angle, and put more DEOXIT on a 1/4" cable and rapidly insert and remove the plug from the effects loop jacks (send and return), it should clear up the issue. You may have to do this a few times, but this happened to my 5150, and this is how it was fixed (I watched as the tech repaired it).

+1

I just leave a small 6" jumper in the effects loop at all times since I dont use the footswitch. If the footswitch is unplugged the fx loop is off.

 

[cloy26]

Does it just get remarkably quiet, and then randomly fluctuate in volume? If so, it's DEFINITELY the connections inside the effects loop. If you get some DEOXIT and spray it inside the effects loops jacks (send and return) at around a 45 degree downward angle, and put more DEOXIT on a 1/4" cable and rapidly insert and remove the plug from the effects loop jacks (send and return), it should clear up the issue. You may have to do this a few times, but this happened to my 5150, and this is how it was fixed (I watched as the tech repaired it).

It gets dead silent. Not even a hiss... Still the same diagnosis?

 

[53Crëw]

It wouldn't hurt to clean all the jacks and tube sockets with DeOxit. It's probably $5-$8 per can and good to have around. At least then you can rule out dirty/oxidized contacts.

Next thing I'd do is swap out preamp tubes one at a time and see if the problem goes away.

 

[Wolfeman]

If you are loosing the 'hiss' as well, you can rest assured it's some time of power delivery issue.

I would start with the phase inverter.

 

[cloy26]

I would start with the phase inverter.

I suppose I have to go find an amp tech in Orlando now. lol

 

[Wolfeman]

No, the phase inverter is just a 12AX7 that needs to be changed

I only recommend trying that first since if it is what's wrong it's the cheapest fix.

Then I would move on to power tubes.