Off Topic Amp diagnosis

[spychocyco]

I've got a Crate GT-80 that I've been playing for about 15 years or so now. It's a combo that's switchable between solid state and tube. I only think they made them for a couple of years. I've been pretty pleased with it over the years, particularly for the price. (Well, I got it free from my wife -- then girlfriend -- for Christmas, so the price for me was definitely right , but I think she only paid around $450 for it).

Recently it has started "fading out" for lack of a better term. After I've been playing for a few minutes, the volume drops and the amp sounds muffled. It lasts for about 20 or 30 seconds and comes back to full volume. Sometimes it does it repeatedly when I'm playing, sometimes it doesn't do it at all. I thought it might be a cable problem since I've been playing with old cables, and I knew at least one of them had a short in it, but I bought two new cables yesterday, and it happened again this afternoon.

I've had my eye on a Mesa Mark V combo, and this would be a good excuse if it was even remotely in the budget, but for the foreseeable future, I'm going to have to limp along with this one or play my 10-watter if it dies. So if any of you guys are good with amps, I'm looking for any suggestions as to where I might start looking to find the problem.

 

[INMT]

Try replacement tubes for the preamp section ( the power amp section is solid state). I'm betting that would take care it it.

 

[marduke]

yeah i would suggest replace the tubes too..

 

[spychocyco]

Thanks guys. I kind of had a suspicion that was the case. New tubes ordered.

They just don't make things like they used to, I mean, those tubes are only 15 years old.

 

[marduke]

haha 15 years!! wow thats a pretty darn good run lol. the tube in my valvetronix amp lasted 2 years before the volume died out... (similar to what you described) then put a mesa tube (was spare in my old mark4) and was great, that lasted 1 year and has died... ahwell, good thing i dont use that amp, not even for home practise. the mark 5 is perfect for that the 10w selection is a God send!

 

[spychocyco]

The amp spent a few years in storage not being used at all because I didn't have a place that was big enough for it (either soundwise or physically) and had a few more years of very light use, but they've still gone far longer than they should have. Guess I can't complain ... and $45 really isn't a lot to get it back in shape. Still, that Mark V would have been nice ...

 

[fbecir]

If it is not the tubes, it's certainly the capacitors. I had to change the capacitors of my Marshall DSL401. With the heat, the capacitors tend to go wrong.

Good luck with your amp

 

[jamminjim]

electrolytic capacitors go bad over time, the dielectric dries out and shrinks, and some can even ooze out from excessive heat....

so if the tubes don't do the trick, look at the "electrolytic" caps...

btw did you buy replacements for "all" the tubes, or just the finals (power tubes)? I'd probably replace all the power tubes, but I'd substitute a known good one in for the preamp tubes, one at a time as some preamp tubes can last for up to several decades without a problem..... no harm in replacing all of the tubes though, cause then you'll have some spares...

 

[spychocyco]

As INMT pointed out, the power amp on this one is solid state. There are three tubes in the preamp, a couple of 12au7s and a 12ax7. I ordered replacements for all three of those. Hopefully that will put me back in business. If it comes down to the capacitors, I'm going to have to call in the cavalry and take it to a shop.

 

[banjoplayer]

had a similar prob with a Marshall DSL401 - also a tube was bad

 

[fbecir]

had a similar prob with a Marshall DSL401 - also a tube was bad

The problem with the DSL401 is that this amp can become very hot. The original caps are made for a maximum temperature of 85°C. My amp tech changed them all for better ones (made for a maximum temperature of 105°C). I do not understand why Marshall made such a choice (the price difference between the two kind of caps is really small).
Now my DSL401 works flawlessly.

 

[banjoplayer]

The problem with the DSL401 is that this amp can become very hot. The original caps are made for a maximum temperature of 85°C. My amp tech changed them all for better ones (made for a maximum temperature of 105°C). I do not understand why Marshall made such a choice (the price difference between the two kind of caps is really small).
Now my DSL401 works flawlessly.

Yep, I know... the heat ....
First I had to do this repair (resolder the joints of the bridge rectifier - the heat soaked them - without installing a computer CPU heatsink). Soon after that the amp again had the problem of fading out and this time it was V1 that had to be replaced.
After all was fixed, I sold it. I had lost the fun with it...

But the described first issue is something special of the DSL401. in common I think I would try changing the tube first

 

[Tim O'Sullivan]

The problem with the DSL401 is that this amp can become very hot. The original caps are made for a maximum temperature of 85°C. My amp tech changed them all for better ones (made for a maximum temperature of 105°C). I do not understand why Marshall made such a choice (the price difference between the two kind of caps is really small).
Now my DSL401 works flawlessly.

One of the reasons I would not buy a modern Marshall.

 

[fbecir]

One of the reasons I would not buy a modern Marshall.

Don't get me wrong. The DSL401 is a great amp. I bought mine in 2003 and I have to change the caps this year. During 5 years, I used it a lot (almost every day at home and for all my rehearsals / gigs). It traveled a lot of miles and stayed hot a lot of hours ... Marshall is still the leader of the pack, especially if the amp is made in Milton Keynes (as a Frenchman I hate to admit it, but an amp "Made in UK" rules ).

 

[jamminjim]

wow fbecir - guess I should throw my handwired blackface deluxe reverb in the dumpster, along with my Champs and Princeton... and find British made amp.

sorry, I glossed over the solid state MOSFET finals. However there may be some caps in the circuit. Caps are cheap. I couldn't find a real schematic anywhere. Found lots of "functional diagrams" that people mistakenly called schematics.

Two 12AU7's and one 12AX7 not a huge tube complement in the amp.

 

[fbecir]

wow fbecir - guess I should throw my handwired blackface deluxe reverb in the dumpster, along with my Champs and Princeton... and find British made amp.

Well, we all know where Jim Marshall took inspiration for his first amps
I also own a Fender Blues DeVille just to be on the safe side ...