Problems with my 5150 combo

[trenolds39]

I have a 5150 combo that I picked up used last year from a music store, and I replaced the tubes with jjs immediately. I haven't had more than 5 hours playing time on it due to school, and I just recently started playing it. After I took it out a few weeks ago, it's been working fine until the other day. After taking it off standby, a couple of seconds later I heard a loud pop, followed by an unbearably loud hum/buzz. It's a very bassy hum, and it overpowers my guitar signal which I can still hear over the buzz. It does it even with nothing hooked up, and I took out the chassis to see if I could see any components that were fried, and everything seems to be in order. By the way, there was no smoke or burning smell of any sort when this happened.

Sorry if my problem seems vague - I'm not as technically verbose as everyone else here is, and I'm not too familiar with the more complicated amps like the 5150s.
Thank you in advance.

[Satamax]

Sounds like a blown diode in the bridge rectifier. Could be many many other options, but that could be one.

http://www.geofex.com/ampdbug/ampdebug.htm

[Enzo]

First thing I would try is new power tubes. Or at least remove your power tubes one at a time and see if the removal of one or the other makes the loud hum go away.

[trenolds39]

Originally Posted by Enzo

First thing I would try is new power tubes. Or at least remove your power tubes one at a time and see if the removal of one or the other makes the loud hum go away.

I have the original sovtek tubes, and I switched them out after trying the method you suggested, and it still does it. I should note that the hum is the same loudness, regardless of the post gain (volume) of the amp, and it does it even with everything set at zero.

[Enzo]

OK, that means the tubes are not the problem and that it seems to be in the power amp stage. I would then look at the power tube sockets and verufy B+ voltage at pins 3 and 4 both on all power tube sockets. I would verufy the bias voltage is close to the -50 or thereabouts. And that the bias voltage is free of ripple. And since this amp is constructed in similar fashion to the C30, I would be looking at each little wire jumper between the boards.

[Amp Kat]

There is also a ribbon cable that goes from the preamp to the poweramp. Make sure there are no burn marks on those cables right at the board. What about those flyback diodes Enzo. Do you think one could be blown ?

[Enzo]

If a diode is shorted, you blow HV fuses hard. If the diode is open, no problem, you just have no protection if the load disappears.

[trenolds39]

Originally Posted by Enzo

OK, that means the tubes are not the problem and that it seems to be in the power amp stage. I would then look at the power tube sockets and verufy B+ voltage at pins 3 and 4 both on all power tube sockets. I would verufy the bias voltage is close to the -50 or thereabouts. And that the bias voltage is free of ripple. And since this amp is constructed in similar fashion to the C30, I would be looking at each little wire jumper between the boards.

And here comes the noob comment: I actually don't have a decent multimeter to do this - could you recommend one that is good for working with amps, yet doesn't break the bank? We have some nice ones in our EE lab, but lugging this 85 lb amp around probably won't be helping the situation much.

Where are the HV fuse(s) located? The ones on the board separate from the main pcb weren't blown, and the ceramic one looks ok. And I don't believe there are any burn marks on the ribbon cable.

I'm also asking on the peavey forums, and after I tested all the tubes, someone mentioned it could be a grounding problem, such that there may be loose jacks, so I'm looking into that as well.

Finally, I don't understand what you mean by B+ voltage - could you send me a link to explain all that...I'm only a freshman in electrical engineering, so I suppose you would say I only understand basic electronics theory. Thanks for all the suggestions so far though - I appreciate it very much.

[Satamax]

http://www.geofex.com/tubeampfaq/taffram.htm

http://www.aikenamps.com/TechInfo_2.htm

[Enzo]

B+ just means the high voltage, there should be several hundred volts there. Typically it will be there or not be there. It is a lot less likey to be there but wrong.

Ceramic fuses ALWAYS "look" OK, you have to pull them and test with a meter. But if your heaters are coming on, then it is OK.

That the amp works and makes sound means the B+ is present in general, so the fuse is likely OK. But if B+ is not at the pins 3 and 4 of ALL the power tubers in an amp, then it will be unbalanced and will hum.

The heaters are at 6VAC, but they are also offset by +25VDC. SO check at pin 2 or 7 and see if it is +25VDC to ground more or less. If it lost that offset, that will increase hum.

Do a search here for selecting meters, I think we have discussed it a couple times. I like FLuke meters, and you can spend anywhere from $99 to the sky for one. You can get some reasonable meters for less from Radio Shack, even Sears who sells their own brand AND FLukes these days, and online from a million places.

If you are in an EE program, talk to the people there about meters. SOmeone light have an older one they would give you or sell you cheap.