Tubes overheating solder joints

[sshrugg]

This question is actually for my amp, a Marshall DSL401 - it's a 2002, so it's prior to the fixes in 2003 that solved these problems.  Anyway, it's applicable to other projects too, so I'm posting it here knowing full well lots of folks here have great answers.

The DSL401 has a problem with the power tubes getting too hot, causing the solder around the rectifier bridge to weaken.  I've re-soldered and added a heat sink, and I'm planning to add a 4 inch 120v fan.

My question is, should I use 60/40 solder, or is there another type that will make a good electrical connection, but won't melt at lower temperatures?

Oh yeah - also, do those shielding covers help with heat, or would they make things worse?  I know their mostly to protect from interference, but I thought they might be helpful for this, too.

Also, any other input about cooling is also welcome.  Thanks!

[Dave Z]

Well, just from a procedural perspective, an engineer would say fix the problem, not the symptom. THAT much heat is the problem, not the composition of the solder. And any higher temp solder might get the bridge too hot & fry it.

The 120V fan & a heatsink is a good idea, absolutely, as long as you have a channel for air in & out. I suppose you're going to put cool air across the circuit board where the recto is, but it also sounds important to get some air across those tubes. Frankly I'd put the biggest airflow across the tubes -- keep them cooler, they'll last longer & radiate less heat to other components (again, treat the problem not the symptom).

Channeling airflow will make a big difference. Make sure you pay for a decent fan so you don't get a horrid rattling after a short time.

Are you referring to the metal covers for the tubes? Not a good idea here, they'll trap hot air next to the tubes.

[sshrugg]

I suppose you're going to put cool air across the circuit board where the recto is, but it also sounds important to get some air across those tubes. Frankly I'd put the biggest airflow across the tubes -- keep them cooler, they'll last longer & radiate less heat to other components (again, treat the problem not the symptom).

Actually, the heat sink is on the bridge, but the fan will be behind the tubes, like you said.  Maybe that will be enough, without using different solder.  I did notice however, that the solder in the amp didn't melt as quickly as the 60/40 I used to repair.  That's why I'm asking.  I'm trying to figure out what they used.

Also, I've heard they use a different bridge on the newer DSL401s, but I'm not sure which.  If I could find that out, I'd swap that piece as well.

[ambulancevoice]

id use some of that adhesive thermal transfer stuff on the solder joints and on the other side of the adhesive thermal tranfer a heat sink

[Paul Perry (Frostwave)]

I would think that if the tubes were THAT hot, the heat would be damaging the tubes, as well!
Do the data sheets say anything about positioning?
And I don't think a metal shell around the tube would help with heat, unless the metal itself was cooled somehow, by fins or air stream.

[runmikeyrun]

i had that problem with my 70's V4.  It's the problem with amps that have the tubes hanging below the chassis.  The solder would melt and the components would actually drop off the board! 

I removed the front panel's grill cloth, cut a hole for a fan, and replaced the grill cloth so fan was invisible.  I then drilled holes in the chassis near where the components were overheating.  I riveted little pieces of aluminum behind them and bent them like scoops- the airflow would come in and the scoops would catch it directing it into the chassis and over the components that were overheating.  I think they stuck down into the airflow about 3/4".  The rest of the air (the majority) blew over and past the pre and power amp tubes.  There's a lot of preamp tubes in those things and it was making the whole chassis hot, the power tubes were only part of the problem.  Never had another problem.

[sshrugg]

*nods*  Thanks so much, people.  You're all tons of help!

[sshrugg]

It's the problem with amps that have the tubes hanging below the chassis.

Should I flip the amp upside down?   

[runmikeyrun]

hahaha well, if it's a means to an end...   my v-4 didn't have a typical metal grille w/ holes punched into it covering the back of it like Marshalls do, so i had to put the fan on the front.  I noticed when i got my peavey windsor that the bolt hole pattern on a fan i had matched exactly to the holes in the grille.  I almost did it then i remembered there is only 7 tubes mounted to the top of the chassis.  not exactly cause for concern. 

Provided there is no metal grille on the back of your amp, you could take a fan you would like to use and your amp to a metal shop and ask to see their metal with punched out holes, maybe take them a picture of this:



and say i want this cut to fit to the back of my amp and i want the holes in the grille to match.  They can probably find it for you.  There is a metal shop near me and they literally have 10 racks of all kinds of sheet metal products stacked up on it.  I bought a sheet of expanded metal to make a grille on my 8x10 and it was like $30 so a small piece shouldn't be much. 

I don't buy much anymore, just "liberate" cool little pieces from their scrap bins.  If you ask nice (and buy something) they might let you pick something out of them for a reduced price.  Not for free, cause they get money back recycling it.  But they have cool stuff in ours, all kinds of neat cutouts and patterns their plasma cutter makes.  They can also bend and weld you stompboxes out of neat stuff like diamond plate aluminum

[MKB]

I wonder how much of the problem is due to an underrated bridge rectifier and not the tubes getting hot.  The tube pins in an octal tube are soldered with regular solder, if the tube was getting too hot that solder would melt as well, causing all sorts of problems.  However, the ambient temperature around the bridge rectifier is being raised by the tubes near it, and the enclosed space in the chassis that the bridge is in doesn't help either.

It might be worth checking if the bridge rectifier is of the correct rating, and maybe upgrading it.  Using a low voltage drop rectifier bridge would help as well.  And you could always either mount it to a heat sink connected to the chassis, or directly to the chassis.  You have to get that heat out of the part.  Also if you replace the bridge, leave its leads as long as possible; smaller diodes actually use the leads to dissipate heat, so they can get very hot.

Another thing, when installing a fan in an amp, you often don't have to get too terribly specific about the mounting, it just has to move the air around a bit near the hot part.  Just a small amount of air movement goes a LONG way in heat dissipation.

[sshrugg]

...they have cool stuff in ours, all kinds of neat cutouts and patterns their plasma cutter makes.  They can also bend and weld you stompboxes out of neat stuff like diamond plate aluminum

Wow! Sweet!  *grabs phone book*

I wonder how much of the problem is due to an underrated bridge rectifier and not the tubes getting hot.
...
It might be worth checking if the bridge rectifier is of the correct rating, and maybe upgrading it.  Using a low voltage drop rectifier bridge would help as well.  And you could always either mount it to a heat sink connected to the chassis, or directly to the chassis.  You have to get that heat out of the part.  Also if you replace the bridge, leave its leads as long as possible; smaller diodes actually use the leads to dissipate heat, so they can get very hot.

Thanks so much!  I know that Marshall changed the bridge in 2003, but I didn't know much about them, so I didn't know what to swap that out with.  Also, I forgot that they had heat sinks that sit right under it like that, too.  I'm all over it.


Thanks again, everybody!

[grapefruit]

Are you sure it's not the bridge itself that is getting too hot?
Usually heat damaged joints on a particular component is because *that* component is dissipating too much power.

If it is I'd recommend possibly changing the bridge, make sure you use heatsink compound, and possibly increase the bridge heatsink.

Also, once PCB tracks and pads are heat damaged I generally run either tinned copper wire, or tin myself stripped multistrand wire, from the damaged joints to the next joint. If the next joint is far away use insulated wire, but don't rely on the dodgy track.

Cheers,
Stew.

[Paul Marossy]

Have you acutally figured out how many watts the tubes are dissipating? It sounds like they are dissipating too much heat to me. Do your power tubes not last very long?

[sshrugg]

Have you acutally figured out how many watts the tubes are dissipating? It sounds like they are dissipating too much heat to me. Do your power tubes not last very long?

I never took a measurement, but I know that the tubes are working just fine, and this is a pretty common problem with these amplifiers.  After some searching, I found a bunch of people who were able to just throw a heat sink on.  That, however, was for the DSL201.  My amp, the DSL401 has two more power tubes, so it runs hotter.  I don't know a lot about tubes though, so I should tell you I got the high gain replacement package at eurotubes.com recently (this was after I had problems.  I initially thought my tubes had just gone bad, and found the real problem after installing the new ones)  The tubes I had before lasted the entire time I've had the amp, and the amp sounded just fine with some biasing from time to time.  This has been a real long time, and as far as I know, they still work.

I'm going out of town for a little bit, but when I'm back I'm going to try throwing in the replacement bridge I got, as well as a fan.  I'll post results here.  Thanks everybody!