Follow-up -- AMI "K" amp problem from last August
Last summer I was battling my AMI "K" amp, and several folks here
were kind enough to offer me some help. The problem was a crackling noise that appeared on power up, and I thought my output transformer might be bad. I ended up replacing the transformer, and the problem went away! However, after using the amp for a couple of weeks, the noise gradually returned -- except now it was a constant noise, whenever the amp was on. Frustrated, I decided to just forget about the whole thing for a while. So... Yesterday I decided to dive into it again. I replaced the few ceramic caps I hadn't replaced before, but that changed nothing. I poked around with a scope, and was able to determine that the noise started before the signal reached the second 12AU7 (V2). I already knew the problem was in the power amp, because the noise was present even with the pre-amp unplugged. All arrows seemed to point to the resistors that attach to the first 12AU7 (V1) -- which I *thought* I had checked back when I re-capped the amp. Nonetheless, I found that one of them was reading about 190K instead of 100K. Maybe I missed it, or maybe its value drifted after being used again for a while. I replaced it, and the other 100K resistors on that tube as well. And... the horrible noise is gone! I learned a lesson: When repairing a tube amp, double-check those resistors, and replace if there is any doubt about them. Thanks again to those who offered advice on this. -- Russel Willoughby |
Any 1950's and early 60's amp I work on now gets the resistors checked for
tolerance. I, too, have been finding some that are waayyy off value. |
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