|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
How hot is too hot for a transformer to run on a tube amp. I have an Allied Knight 935 (ecc82s with 3 12ax7s) and after about two hours the power transformer is hot enough, that you wouldn't want to hold your hand on it for 10-20 seconds. The output transformers are warm to the touch, but not nearly as hot. There is a hum adjuster on the back, would moving that help with the heat? I don't know how to bias this amp, or if it is self biasing, but the heat is starting to worry me. Any info would be appreciated.
BenAlled Knight 935, Sony CA70es CD player, Kenwood KT-6005 Tuner, KEF Reference 102s
Follow Ups:
That seems about normal. Having a shematic wouldn't hurt. About your tubes, An ecc82 is another way of saying 12au7, which is another small signal tube as is the 12ax7. Did you mean to say ecl-82 instead? This would then also be a 6bm8 which would be an output tube. (the 6xxx or 12xxx designations are American and the ECLxxx are the European equivalents). If so, you probably don't have bias adjustments. The Hum-null pot quiets AC on the tube's filaments and is adjusted by turning the volume and bass controls up and then adjusting the hum control for least hum heard thru the speakers. My Heathkit AA-32 uses 6eu7 tubes (a 6.3v version of 12ax7) and 6gw8 tubes (kin to the 6bm8, both in the same ecl family) and it's voltage-doubler, power supply isolation-transformer runs hot after a couple hours. I assembled in 1967 and am still using on a daily basis.
Yes, I ment the ECL-82. Sorry about that.
Remember that great flick with Marilyn Monroe, et at? Anyway, to answer your question, the design of the amplifier will largely determine how hot the transformers get. Some under-sized power supplies get quite hot during operation. I've experienced a few, both older and new designs that get a lot hotter than what you're experiencing. Not being familiar with your amplifier, I don't know what is "normal" for this amp, but it doesn't sound like you have a problem. That said, heat is the enemy of all the components in an amplifier, so less heat is better. Two things in particular to check to make sure your trannies are operating optimally...make sure you have at least 115V AC from the wall through a good power cord for your Knight--lower voltages mean the amp will pull more current for the same output, and that creates more heat. Also, get a set of schematics for your amp and learn how to bias the output tubes, and make sure the driver tubes you're using are the correct tube for the amp and are still strong. This will help somewhat with the transformer heat, and will help a lot as far as the longevity of the output tubes. A.G.Tannenbaum is a good place for schematics and manuals. Do a Google search for "Knight"--I think there are several sites devoted to these products, and you should be able to find one that details biasing procedures. Hopefully you'll get more specific info about biasing from the gurus here.
This post is made possible by the generous support of people like you and our sponsors: