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In the Forum: Audio Discussions
In the Thread: Initial thoughts about new/old Lamm ML2s

Post Subject: I don't know what happenedPosted by Lx_ on: 12/7/2009

Romy and Paul,

Thank you for your help and comments. I decided to move the amp on the floor and on its side so as to be able to monitor it during operation. As it had worked for 10 minutes after replacing the fuse, I was confident nothing should be damaged inside (well, to be honest, I didn't exclude a nasty fault, but it seemed less likely and I didn't want that to be). I was suspecting a problem with the regulation stage, because I could not raise plate voltage higher than 169V just before the second failure. I replaced the 3 corresponding tubes with the ones from the working amp, changed the fuse (which had indeed blown again). Then I switched the amp on, closely monitoring plate voltage and current. This time I was able to adjust to 175V as required. I also checked voltages as suggested by Romy: VR tube voltage (BTW: I have 84.5V or 84.7V for the two 5651s, though they are rated for 85.5V), which is indeed applied to the cathode of the 6AK5 (error pentode), and 6AK5 plate to V1 grid. Everything was in order.

After 40 minutes without a problem, I decided to put back the original 5651 and 6AK5. I thought the 5651 reference voltage might be too low or the error amplifier tube was unable to apply sufficient correction. But after another 40 minutes of flawless operation I knew I was on the wrong track.

At that stage I was suspecting a faulty 6C33C being unable to regulate correctly, which would make the error amplifier work beyond its operating range, thus leading to the fuse blowing. Note that those are just my guesses as I don't have the schematics and don't know where the fuse actually is in the circuit. Also, the V1 tube on the failing amp has a less shiny getter than all 3 other 6C33Cs.

So I put back the original V1 tube. Now all tubes were the original ones, so I was in the configuration that led to the two previous failures.

But the amp worked perfectly for the next 10 hours. That was yesterday. And it has worked just as well for the last 2.5 hours. I have no idea what can explain the difference in behavior.

One thing I have noticed is that the location of the fuse on the PCB is labeled 1.25A (see here). But the fuse that was there was a 1A, and I replaced it with similar ones. Is it possible that the missing .25A is just enough to sometimes make the fuse blow when it should not?

Anyway I will order two full sets of new tubes from Lamm, because mine are probably quite worn out by now. Especially the 6C33Cs. I have plenty of 6N6Ps, so those are not a problem (but I cannot tell Lamm not to give me those). As for other tubes I will put on the new ones when they arrive and keep the current ones as spares.

In the meantime I will be praying that the amp keeps working.

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