Amp loses power after about 20 minutes...what's up?

[CDaughtry]

Guys...I have an amp that sounds absolutely fabulous for about twenty minutes and then it starts, intermittently, losing power. The power never fully goes away, it just starts sounding weak, loses gain, and loses some volume.
I changed power tubes and that didn't help.

Any ideas?

 

[Blue Strat]

What amp? Did you check/adjust the bias on the power tubes? Check the bias and watch it as the amp starts to decline. This may give a clue as to what's going on.

 

[CDaughtry]

It's an Aiken El 34 Intruder. I did rebias it, with a couple of different sets of tubes, and it still does it.

 

[Blue Strat]

It's an Aiken El 34 Intruder. I did rebias it, with a couple of different sets of tubes, and it still does it.

What bias setting did you use? Watch the bias setting as it warms up and the volume goes down. See if it changes.

 

[SouthernShred]

Have you emailed Randall? He will get back to you ASAP normally...

 

[picnic]

I had something similiar with my Bogner. The amp worked fine in the Florida store, but when I got it back to NJ, it would conk out after 20-30 minutes. Tech said there were a couple of solder joints that had cracked. They weren't trouble until the amp heated up and the joints would open up.
He said they were small and very hard to find. He resoldered and I haven't had any problems with the amp in over a year.

 

[redtoploader]

Like picnic said, I think something is heating up. Either a solder joint or a resistor heating up and changing value.

 

[Swarty]

I'd swap in some different preamp tubes before doing anything.

 

[meburdick]

Twenty minutes seems like "a long time" for a tube to start demonstrating an issue. I'm more inclined to think that it's a solder joint or something similar.

If the amp has a PCB, there could be a hairline crack that doesn't open up until it really warms up, especially if it's in an area where there are components that don't generate much/any heat.

Look closely at things with a magnifying glass... You might find the crack.

 

[Scumback Speakers]

Like picnic said, I think something is heating up. Either a solder joint or a resistor heating up and changing value.

This was my experience as well after a bellhop dropped my demo Major Crunch amp at the LA Amp Show two years ago. Took it back to my amp designer, found the cracked solder joints, re-soldered, and the amp was as good as new.

Sounds like it might be what you have happening, too, CD.

 

[bbarnard]

Yep I'd say that it is something heating up and expanding. Had a similar problem with my Gries. Had missing solder on a joint and the wires would heat up and expand and not touch and the amp would sound really bad. Tmac here on TGP found it and Dave fixed it right up for me.

 

[Dave Orban]

I had an output tranny go on my Chieftain, and that was the symptom...

 

[G.Boccasete]

It could be any of a dozen things now sounds crazy but I have even heard of a bad speaker causing this problem-try a different cab for an hour and see if it still does it.
Gildo

 

[doc]

I have a similar problem in an old Traynor Studio-Mate, but in mine it happens after just a few minutes. Sounds great for 2 to 5 minutes then the power drops. I haven't tried changing out tubes yet or doing a cap job, but I'm watching this story with interest. I'm not so sure its just a heat related issue unless its just in the preamp because I can turn it to standby for 1 - 2 minutes and the power is back when I turn it back from standby. I'm planning to sell the amp anyway, so I don't want to invest a pile of money into fixing it, but don't want to sell it if its not right.

 

[ktone]

I think something is heating up. Either a solder joint or a resistor heating up and changing value.

90% chance this is it. After many years as a bench tech and hundreds of tube amps I have found that simply going through applying DeOxid (controls AND tube sockets) and resoldering every single solder connection fixed 90% of those "tough dog" problems - especially the intermittents such as this. I said every single solder joint! Sometimes you can see the bad solder joints but just as often you cant.

Don't forget, if you are working on a printed circuit board, to scrape off a little green and flow some solder on to the trace. Sometimes the trace breaks where it meets the circle and you can't see it under the solder.

 

[redtoploader]

90% chance this is it. After many years as a bench tech and hundreds of tube amps I have found that simply going through applying DeOxid (controls AND tube sockets) and resoldering every single solder connection fixed 90% of those "tough dog" problems - especially the intermittents such as this. I said every single solder joint! Sometimes you can see the bad solder joints but just as often you cant.

Don't forget, if you are working on a printed circuit board, to scrape off a little green and flow some solder on to the trace. Sometimes the trace breaks where it meets the circle and you can't see it under the solder.

Yep.

Especially with old amps. Old carbon resistors are especially prone to this...I realize this isnt an old amp we are talking about, but just a tip to anyone looking.

I had an old ac30 on the bench once that was getting barely any voltage on one of the plates of the PI tube(sounded bad of course). The resistors checked out within spec...when no voltage was applied. Turned on the HT and one of them went sky high in value. Another tricky thing to look out for.