I broke My ART MPA! Please Help me!

skiscem

New member
Well, i got a used MPA PRO from another user from this site, the preamp worked perfect, but i wanted to change the tubes...
so i got like 20 tubes and started trying, everything fine until... I gound this tube that looks just like a 12ax7 but a little bit longer, put it in, and sounded very nice for about 30 seconds..
After that, the sound is very distorted and everytime i turn it on, the VU meter goes all the way up, sound comes out but again, very distorted and weak, i tried changing the tubes but no luck, the channel two still works perfect.
The tube on the broken channel still lights up normally.

I have fixed a couple of amps before and i built a tube amp before, but i have no experience with preamps like the ART.

I would really Appreciate is someone can help me troubleshoot it or guide to what to look for or what to replace, I really like the preamp and i cant afford a tech to fix it right now

Thanks

Alex

To Make things Cool, im offering a nice, tube present to the person that direct me to the problem and helps me fix it!
 
I have to check whenever i get home, but i think there is no number, all the tubes i have a freaking old and most of them dont have anything :mad:
 
I think your unit has a 7812 regulator with a big ol' heatsink attached. That is likely the component that was destroyed. Check the voltages on that part.
 
I think your unit has a 7812 regulator with a big ol' heatsink attached. That is likely the component that was destroyed. Check the voltages on that part.


Well there is 2 black heatsinks inside, close to the transformer, what should be the voltage? if the regulator went bad, is it easy to find a replacement?
 
i know for a fact the tube was wrong, the pin patter is the same but the tube itself is like 3/4" taller, i feel really stupid, but it actually sounded very nice for the 30 second it survived
 
No, actually if the other channel is working that's not the problem, never mind. Instead I'd look to the ICs on either side of the tube. But I need to look at the comparative pinouts of these two tubes to see what is likely to have gone wrong.
 
No, actually if the other channel is working that's not the problem, never mind. Instead I'd look to the ICs on either side of the tube. But I need to look at the comparative pinouts of these two tubes to see what is likely to have gone wrong.


im going to get more info about the tube and pictures tonight wheneve ri get home
 
No, actually if the other channel is working that's not the problem, never mind. Instead I'd look to the ICs on either side of the tube. But I need to look at the comparative pinouts of these two tubes to see what is likely to have gone wrong.


But there is 2 heat-sinks, it should be one for each channel.. right? :confused:
 
OK, here we go: pin 7 is plate on EL84 but grid on 12AX7. That would apply plate voltage (+45V if this is the original Pro MPA) to something where it doesn't belong. That could be the output of the preceding opamp, depending on which side pin 7 is on--I think the ART uses a long-tailed pair configuration on the tube with one grid as input and the other for feedback, but I forget what feeds the feedback grid.

Wait, sorry that's backwards--the plate voltage is on pins 1 and 6 of 12AX7, that's the mysterious "internal connection" on EL84. I dunno where that ends up :confused:

Anyway, replace those IC opamps--was that NE5532 in the Pro MPA? I forget. Might have been TL072. Definitely I remember they were DIP-8 package though.
 
No, regulators for different voltage rails, I think there is +15V, -15V, and +12V, plus probably a TIP112 to regulate the +45V (phantom supply and tube plate).
 
OK, here we go: pin 7 is plate on EL84 but grid on 12AX7. That would apply plate voltage (+45V if this is the original Pro MPA) to something where it doesn't belong. That could be the output of the preceding opamp, depending on which side pin 7 is on--I think the ART uses a long-tailed pair configuration on the tube with one grid as input and the other for feedback, but I forget what feeds the feedback grid.

Wait, sorry that's backwards--the plate voltage is on pins 1 and 6 of 12AX7, that's the mysterious "internal connection" on EL84. I dunno where that ends up :confused:

Anyway, replace those IC opamps--was that NE5532 in the Pro MPA? I forget. Might have been TL072. Definitely I remember they were DIP-8 package though.


I will try that, are the IC Opamps easy to find? let say Radioshack?? or whats the best source for those..

Thanks!
 
I will try that, are the IC Opamps easy to find? let say Radioshack?? or whats the best source for those..

Thanks!

Military spec componants are the best...go to an electronics supply that can get those for you...and replace both channels in case it makes a difference...you might get lucky and they may have old tubes still sitting arround if the place is old enough.
 
Military spec componants are the best...go to an electronics supply that can get those for you...and replace both channels in case it makes a difference...you might get lucky and they may have old tubes still sitting arround if the place is old enough.

South Florida sucks for those kind of places, and the only decent electronics store, was closed a year ago :mad:
ont he other hand... i found on Craigslist a guy selling 3 boxes full of tubes (150 total) for $25
and i found like 10 matched pairs of 12ax7 and a few telefunken :)
 
The op amp on my mpa is the JRC 5532D and from what i have read, it is a cheap-not so good op amp, since im going to change one, why not both with something better.
What do you guys think about the LME49720/LM4562 http://www.national.com/pf/LM/LME49720.html
it seems to be highly regarded by Hi-Fi Enthusiasts and the requierements are almost the same, and some people replaced those with good results..
should i go for it, or is there any better option
 
Rumors of 5532's demise are greatly exaggerated. You can play chip swaps if you want, but I'd place highest priority on getting your unit to work again. To keep that simple, replace with the same part.
 
Well, i replaced the opamp like mshilarious said... and it works!!! thanks a lot, but still have a probme, the led meter and the vu meter are weird.
all the LEDs are lit up and the needle on the VU meter stays all the way in.... any idea of what could be causing this?

thanks
 
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