Express 5:25 mystery

[Don]

I was wrong when I said that my 5:50 had that resistor from the factory!

I thought you meant the large resisitor with the heat sink on it under the board! It's the first thing that I noticed in the pic and what I focused on.

Mine looks like the GP pic.

 

[tmac]

Aha, I forgot that one was also in the picture

Anyway, I just installed a standard 120 Ohm 2W resistor, in addition to replacing the Zener-diode with a new one. It didn't make any noticable difference. It turned out that the Zener was not damaged (at least electrically), even though it looked like it.

Maybe it is just that the 5W mode is extremely sensible to the choice of tubes. The previous owner had installed JJ power tubes. The problem is independent of the positions of the two power tubes.

Apart from the weakness of the sound in the 5W mode, it is also very uneven, i.e. it has "swells" in the sound level. In addition, when play low notes, e.g. on the low e-string, I have very strong 15-20Hz oscillations in the sound.

I will get some Mesa EL84s...

 

[tmac]

No important difference after changing to new Mesa output tubes.

 

[tmac]

I reversed the order of the preamp tubes, and now the 5W mode is not weak at all... In addition, the sound is OK now. The only thing is that I no longer get much overdrive.

I guess this means that it is highly likely that a change of preamp tubes is in order, and everything will probably be alright after that!

If that's true, I'm a very happy man!

 

[soundchaser59]

Now that I think about it, I dont believe I have ever had good luck with any new amp....... until I change all the tubes.

Even my Epxress I had to change all the tubes to make it work correctly when I first brought it home.

Now days I just consider it part of the new amp deal...... like a new car with tax and license....