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Post subject: Fender deville new tubes too hot
Posted: Sun Dec 29, 2013 5:20 am
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Hello all,

I just installed new JJ tubes in my 410 deville and when I turn the amp on the tubes glow red then blue really quickly and it make a noise like its gonna blow, also when I went to turn the bias pod a piece broke off and now I don't think it works now and maybe the bias is now set permanently high and causing the tubes to glow blue? Any help is appreciated.


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Post subject: Re: Fender deville new tubes too hot
Posted: Sun Dec 29, 2013 8:06 am
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Joined: Fri Feb 05, 2010 4:31 am
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Location: Province de Québec, Canada
tmontanez wrote:
Hello all,

I just installed new JJ tubes in my 410 deville and when I turn the amp on the tubes glow red then blue really quickly and it make a noise like its gonna blow, also when I went to turn the bias pod a piece broke off and now I don't think it works now and maybe the bias is now set permanently high and causing the tubes to glow blue? Any help is appreciated.


Bias pod ? you mean bias pot ?

Sure your tubes are biased too hot . New tubes need to check and adjust Bias if needed , but always check .
Don't adjust bias if yo do not know what you do . Now you may need to replace bias pot and to modify ( ? ) bias circuit if pot have not enough range to keep JJ's tube safe .

Schematic here for Hot Rod DeVille;

http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: Fender deville new tubes too hot
Posted: Sun Dec 29, 2013 1:53 pm
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Hi tmontanez,

Are you sure that you know what you're seeing? The tubes should have an orange glow from the heaters at the bottom of the tube and maybe even seen at the top, that's normal. Many tubes will have a blue glow when the amp is switched from standby to play, that's also normal.

If the tubes are biased too hot, they can get a red glow on the Plates (the big boxy part of the tube), this is not normal but won't happen instantly. Frankly, it would be difficult to get tubes to red-Plate in a stock Deville.

You should take a reading at the test point of the amp or use bias probes to check the actual current of the tubes, and sounds like you should not be fiddling with the adjustment pot if you don't have a clue as to what you are doing.

BTW, the "Piece that broke off" of the bias pot, could that simply be the reddish colored stuff on the edge of the pot? If so, don't worry, it is really only there as a tell if the pot has been moved.

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Post subject: Re: Fender deville new tubes too hot
Posted: Sun Dec 29, 2013 1:59 pm
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First order of business is to immediately replace the bias adjustment pot if necessary and properly bias the power tubes (preferably by someone who knows what they are doing) before any damage is done to the amp.

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Post subject: Re: Fender deville new tubes too hot
Posted: Sun Dec 29, 2013 2:04 pm
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+1 Shimmilou , Bill


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