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Post subject: Bias voltage on HRD
Posted: Mon Jul 28, 2008 2:57 pm
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I have a HRD that I just checked the bias voltage on and it came from the factory set at 83mv. Is that the norm to bias that hot. Anyone checked theirs and is it close to that reading!


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Posted: Tue Jul 29, 2008 11:26 am
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Many of us shoot for about 68mV. Here is a link that goes into detail that will be redundant for you, but it's good to have.

Good luck.


http://studentweb.eku.edu/justin_holton/bias.html


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Posted: Tue Jul 29, 2008 12:01 pm
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I know that the bias should be 68mv, was on that site very imformative. Just wondering why it was set so high at the factory!


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Posted: Tue Jul 29, 2008 2:39 pm
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Here is the real link. www.eurotubes.com
There are videos on the site that explains it all. The plate voltage on a Hot Rod Deluxe is around 450 volts. At 70% dissipation at 25 watts each comes out to around 85-90 mv depending on your amps plate voltage. 68mv are Justin's settings.
I've been running mine at 85mv using JJ 6l6's for the past year with no ill effects like red plating or burnt traces on the board. I ran mine at 68mv and thought it sounded ok. At 85mv it sings. It breaks up a little earlier, but that's the sound I want. YMMV


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Posted: Tue Jul 29, 2008 3:38 pm
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Well I set mine at 69mv using groove tubes 6l6ge, they came with the amp, doesn't sound to bad but I think it sound better at 83mv. Is that to high for those tubes! Going to order a set of jj's right now!


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Posted: Tue Jul 29, 2008 7:24 pm
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According to the average plate voltage, 68mv is way too cold. It really opens it up at higher settings. If it ain't broke, why fix it? If it came from the factory at 83mv why change it if you're not having problems with it?


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Posted: Thu Aug 07, 2008 6:09 am
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I agree with supro. The short answer is that 83mv/ma should actually be a good setting for many folks with typical tubes in the HRD.
Not only do Fender recommend an overly safe setting...but the test point also yields low figures according to many Techs.


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Posted: Thu Aug 07, 2008 7:02 am
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Yep, and when you bias too cold, you're tubes will last a really long time but the amp will sound a bit sterile. Too hot and you'll fry the tubes and possibly more.


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Posted: Thu Aug 07, 2008 2:00 pm
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I don't know if the circuit is similar enough, but my Blues Deluxe RI was biased to 73 mV from the factory. The bias of an amp does seem to change over time, but how much so I'm not sure. I biased a new set of tubes I installed five weeks ago to 44 mV and discovered last night that it was testing at 45 mV. (I'm not reporting the decimal point differences, but suffice to say there was a 1 mV difference.) Furthermore, I never gig my amp, it doesn't leave the house. The children don't jump on it, etc. So I don't know what might be considered a standard deviation over time nor am I sure what causes it to alter.


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Posted: Mon Aug 11, 2008 9:01 pm
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senggedorje wrote:
The bias of an amp does seem to change over time, but how much so I'm not sure. I biased a new set of tubes I installed five weeks ago to 44 mV and discovered last night that it was testing at 45 mV. (I'm not reporting the decimal point differences, but suffice to say there was a 1 mV difference.) So I don't know what might be considered a standard deviation over time nor am I sure what causes it to alter.


Just like your new strings will go out of tune after they have been on the guitar, output tubes will drift also.
New output tubes need to wear in & stabilize before a constant reading can be obtained.

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Posted: Tue Aug 12, 2008 5:07 pm
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#1StratMan wrote:
Just like your new strings will go out of tune after they have been on the guitar, output tubes will drift also.
New output tubes need to wear in & stabilize before a constant reading can be obtained.


A-ha, thanks for the info ##1stratman. That makes total sense. It's great having the Fender Forum as a resource. I really appreciate the time and experience of my more knowledgeable peers!


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Posted: Tue Aug 12, 2008 7:56 pm
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Yep, and don't worry about a couple MV it will drift over time.


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Posted: Wed Aug 13, 2008 3:23 pm
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63supro wrote:
Yep, and don't worry about a couple MV it will drift over time.


yeah, especially when the tubes are over worked &
underpaid, like mine- :D

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