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Topic: Hot Rear Panel on Fender Twin |
Jim Cooley
From:
The 'Ville, Texas, USA
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Posted 10 Jun 2011 6:33 am |
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When I finished practicing PSG last night and turned off my Silverface Twin, my fingers touched the rear panel at the Power and Standby switches. The rear panel was really hot. I couldn't touch it for more than a second or two. The output tube end of the panel was noticeably warmer in general the the other end. I didn't notice anything unusual in the tubes. The amp had been on for a 1 1/2 to 2 hours. It was on Standby during part of that time while I played through another amp. The amp has been converted to adjustable bias. The 6L6 tubes are biased on the cool side of nominal, based on the amp's plate voltage.
I have owned the amp since the 1970s, and I have never noticed this. Did I leave it on Standby too long? Have I missed something?
_________________
Hey, mister, how do you pedal that thing anyway?
"The worst an honest man can do is make an honest mistake" - Augustus McCrae
"From the sweet grass to the packing house, birth 'til death, we travel between the eternities" - Prentiss Ritter
Too many steels, amps & other stuff, and an open mind. I have tube amp bias. |
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Ken Fox
From:
Nashville GA USA
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Posted 10 Jun 2011 8:05 am
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That is typical if the output tube bias is allowing the tubes to idle too hot. You will also notice, typically, that the output transformer is very hot as well due the the increased idle current on the primary windings of the transformer.
May not be the only issue that can cause heating, but it is a common one. |
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Jim Cooley
From:
The 'Ville, Texas, USA
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Posted 10 Jun 2011 8:48 am
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Hey thanks, Ken. That makes me feel better.
Would it make a difference if I biased the tubes a little cooler? Would doing that affect the tone? The amp has one JBL K120 and one Weber NeoMag 12. I get some noticeable highs. My plate voltage is @ 453V. The Weber bias calculator says that I should be biased a 46mV. The Jim Jones chart on the same page indicates 40mA as an average (60%) bias setting. I am at around 38mA. If I increased the bias, would it warm up the tone? Would the amp get too hot?
-Jim- |
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Ken Fox
From:
Nashville GA USA
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Posted 10 Jun 2011 10:32 am
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| Tone is pretty subjective . I set up amps at 60 percent. Silverface amps did not have a bias adjustemnt, only a bias balance. Has yours been modified? Aldo the higher the bias current, the hotter the tubes and transformers will be a t idle. |
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Jim Cooley
From:
The 'Ville, Texas, USA
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Posted 10 Jun 2011 3:29 pm
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Ken,
My amp has been modified from bias balance to bias adjustment. Thanks again. |
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Jim Cooley
From:
The 'Ville, Texas, USA
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Posted 12 Jun 2011 7:24 am
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Ken - You have email.
-Jim- |
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