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Fender Twin Reverb Problem

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Hey guys,


I've tried to register in specialised fender forums but it doesn't seem to work, so I'm hoping there's some Amp freaks here who can help me out anyways...



I brought my amp to the store to get the tubes exchanged and the amp rebiased.


I bought a matched Quartet of four JJ 6L6s and four Tung-Sol 12AX7s from the Tube Amp Doctor...



Anyways, I come to the store and try to run the amp, everything's fine until about 15 minutes after playing there's a sudden drop in Bass from the Reverb-Channel...


Does anyone of you had anything like that before? Could it have been caused from a faulty Output Tube?



Anyways, after it happened I go round the back of the amp to take a look at the Output Tubes, turns out one of the four tubes doesn't glow as much as the others... I tried switching it to another socket, same thing...


The problem is, that I've send one quartet back already for one of the tubes was broken and I can't imagine them sending out another batch of faulty tubes, I mean, it's the tube amp doktor, isn't it?!


please help, thanks



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tubes can be damaged in shipping but maybe they biased your amp wrong too


only way to tell is to get them to swap the tubes for you and try again.. if the same socket goes bad again; you know the place bias'd your amp wrong.




I've never had a problem with the Fender forum execpt when I blocking cookies from most web sites.




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You can't really judge a bad tube by its glow. Two perfectly matched tubes can have different glows due to the physical placement of the plates. Judge them by the sound.

But yes, try new tubes. You can always get a bum tube, so try a tube you KNOW is good. Pull them from another amp or something and test that way.

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Besides, "matched" tubes is a load of hogwash. The circuits' tolerances are much wider than the tubes, matched pairs don't really do much for you. If the tubes have a 2% tolerance, and the circuit you put them in a 12% tolerance, whats the point of matching the tubes? My numbers are made up but you get the idea I hope. Biasing does, but you can test it without biasing it.

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