|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
70.48.135.241
I replaced the PS cap and the voltage dropping resistors (I used four 15W resistors in parallel instead of two 10W). All the voltages measure right (380V, 290V, 270V etc.) but the power transformer still gets very hot (too hot to touch) after several hours of operation. Is this normal for this amp?
Follow Ups:
Have you checked your line voltage coming out of the wall? When these Fishers were built most homes were in the 110-115 volt range and that is what they are designed for. Voltages have risen over the years and it's not uncommon to see 125 volts in some areas, occasionally even 130. This can cause the transformers to run hotter.
Line voltage is 120V - just measured it. I am also measuring 415VDC before and 384 after the voltage dropping resistors.
Have you adjusted the bias? You can bias the output tubes a bit cooler - see how it sounds. Service info is online below:
Thanks for your reply, Tom. Yes, I've adjusted the bias to 0.6V. Cooler meaning less than 0.6V?
Yes, try adjusting to .5V, see how it sounds. If it still sounds good, try even less... if distortion occurs, you've gone too far... you can't hurt anything (except your ears...) by lowering idle current.The current through the output tubes is about 75% of the total power supplied by the transformer. And it's the only part you can adjust...
Kurt
In the schematic it calls for -10v on the output tubes and thats with a 110vac on the line..THey do run hot but if its to where you cant touch it for a couple seconds you have a problem.Now it could be one or more of your tubes have heater to cathode leakage.I would test them on a good tester like a heath tt-1 or sencore or hickok.
Another thing you can do is install a separate filament tranny.The insulation does tend to break down a bit on power trannies after many years of use so it can run hotter than normal.
I am getting -9.6 to -10.1 on the output tubes. Line AC is 120V. I haven't tested the tubes yet, but that's next...
This post is made possible by the generous support of people like you and our sponsors: