Preamp tubes troubleshooting

  • Thread starter Thread starter KarolisZ
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KarolisZ

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Hi all,

couple of years ago I bought ADA MP-1 tube preamp and now decided to change the tubes. Manual says there should be a pair of 12AX7's, so I bought EHX 12AX7's. Inside my preamp I found GE 5751 and GE 6201 tubes and replaced them with the new ones. At first everything was cool, slightly more gain, brighter sound, etc. Unfortunately, after couple hours of playing my amp was becoming more and more quiet, and in the end the sound was gone completely and only with max gain on distortion channel I was able to hear some clean sound. When I put the old tubes back, everything was completely normal. Yesterday I burnt another pair of new EHX's after couple hours of playing. What could be the problem? What bothers me, that originally there should be 12AX7, but now it's working on 5751 and 6201 and not with a pair of 12AX7. Might there be done any modifications which I'm not familiar with? It was bought on Ebay, but I cannot contact the seller.

Thanks!

P.S. Sorry my English... :)
 
Sorry, that sucks. I'm not aware of any mods for the MP-1 that would change the tube type that it requires. Dunno? It might be that you got bad tubes. You may have just burnt through one in each pair. New tube QA is not great unless you bought them from somewhere that tests them/burns them in.

Try putting them back in and changing one tube out at a time and see what happens.
 
Might not be the tubes, but you say the old tubes work. Could be bad tubes. Also the MP-1 use a dual opamp (solid state) as the first input stage, that could be flaky as well. A common mod is to replace those with a higher spec one. Hard To say. The MP-1 is a pretty low plate voltage unit and not known for frying tubes.

The 5751 is a low gain 12AX7, the GE6201 is a military spec 12AT7WA which is even lower in gain.
 
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