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Post subject: jazzmaster ultralight amp
Posted: Sat Jul 23, 2011 2:57 am
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I'm a pro player and I just picked up a Jazzmaster Ultralight transistor amp to backup my vintage tube Fenders (mostly tweed) figuring it would be good to have a reliable amp to cover for any tube problems. Trouble is the Jazzmaster craps out and shuts off all the time when I'm playing and nobody seems to know much about it. It's like it goes into overload and turns off. (Funny that my tube amps are much more reliable - maybe two tube problems in fifty years of playing.) If anybody knows about my little transistor Fender I'd love to hear about it. Are these amps unreliable and overly sensitive? I'd love to hear from a Fender tech.

UPDATE; The amp stopped working altogether. I took it to the Fender repair guy. No parts available at this time and after two months Fender says they are trying to locate a chip for it. Maybe next month?

Mike Chain
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http://www.michaelchain.com


Last edited by mike chain on Thu Dec 15, 2011 4:34 pm, edited 2 times in total.

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Post subject: Re: jazzmaster ultralight amp
Posted: Sat Jul 23, 2011 9:11 am
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Hi mike chain,

What impedance speaker cab are you using? Using the lower ohm speaker cab with a solid-state head will produce the most heat in the amp. If you are using a 2-ohm cab, try changing to a 4-ohm or higher to reduce heat in the amp. It won't produce as much power, but will run cooler.

Not many enjoy working on a solid-state amp, of any brand. Many, if not most, ss amps have some sort of thermal protection for the output transistors, and once that area reaches a certain temperature, the amp will mute or limit the output until the area cools down. It may be a problem of overheating, if the amps works/sounds fine for awhile and then loses power. Make sure that the output transistors and heat-sinks are all tightly together, make sure that you have no loose screws there. Maybe use a fan blowing onto the head.

Another possibility is a bad solder joint(s) somewhere on a circuit board inside the amp, it could work and then fail when up to operating temperature. To find a bad joint requires amp dis-assembly and careful inspection/testing, and maybe expensive if you can find anyone willing to do it.

Just a couple of guesses. :idea:

Here is another thread discussing the same amp recently:
http://www.fender.com/community/forums/viewtopic.php?f=13&t=49018

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Post subject: Re: jazzmaster ultralight amp
Posted: Sat Jul 23, 2011 11:19 am
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I'm using the 2 ohm cab Fender designed specially for the Jazzmaster head. I'll try another cab with 8 ohms and see if it fares better. Bad solder joints and overheating heads shouldn't be out there on the market. In the old days Leo would come down and talk to us players and now its hard to find anybody at Fender with answers or who cares. That's why so many of us went boutique in the first place. Thanks much for your input.

Michael Chain


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Post subject: Re: jazzmaster ultralight amp
Posted: Sat Jul 23, 2011 2:38 pm
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shimmilou wrote:
ss amps have some sort of thermal protection for the output transistors
--> There is a NTC that monitors the temperature in the schematic.
I think it is a class D amplifier (chopper) with passive filter if the current is too much important in the MosFets there are more losses (Rdson*I*I) than expected then possible shutdown to prevent irremediable damages. However this amp has been designed for 2 Ohms, I think you should visit a tech. :(

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Post subject: Re: jazzmaster ultralight amp
Posted: Sat Jul 23, 2011 3:44 pm
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mike chain wrote:
In the old days Leo would come down and talk to us players and now its hard to find anybody at Fender with answers or who cares. That's why so many of us went boutique in the first place.


+1000!

Or stuck with lovingly restored and maintained vintage Fender gear.

8)

Arjay

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