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Post subject: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 10:58 am
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Hello all, I just got a new '68 Custom Princeton Reverb. I am assuming this is a fixed adjustable bias amp and that the gold pot located behind the PT is the adjustment. Am I correct?

Secondly, I pulled the chassis hoping to find a bias test point like in some other modern Fender amps, but couldn't. Am I correct that there aren't any test points or did I just not look hard enough?

Has anyone come across instructions on how to bias these amps? I'm embarrassed to say, I've always had test points and never bothered to learn where those test points actually traced to. If I can't find instructions, I can just get some bias test sockets. I was thinking about the Bias Scout from Tube Depot.

Thanks for your help.


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 11:19 am
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Can't find schematic but for all Fender I work on and see;

Yes for the pot
No test point.

Bias Scout may do the job,it can read current and plate voltage with the use of a voltmeter.

How to bias ;
Use the first line here;
http://www.tedweber.com/webervst/tubes1/calcbias.htm


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 11:33 am
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Very helpful! Thanks much.


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 12:29 pm
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You are welcome.
Warm up tubes, Stanby in play, at least 10 minutes before bias is done / job finish.
New tubes may need more warm up.


Many Fender amps play well with colder bias, tubes last longer
For these amp, Class AB, Weber bias calculator show a 70% power dissipation.
Try also at 60 % or 65% and listen sound with playing guitar.


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 12:37 pm
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Be sure the tremolo is switched off before measuring or adjusting the bias. The bias current is used to modulate the screen-grid voltage of the power tubes and your readings will be affected if the tremolo circuit is engaged.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 12:37 pm
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Very helpful, thanks. This amp doesn't have a stand-by mode, just an on/off. I'll just turn on the amp with the probe socket in place and let it warm up without playing.


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 12:42 pm
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Retroverbial wrote:
Be sure the tremolo is switched off before measuring or adjusting the bias. The bias current is used to modulate the screen-grid voltage of the power tubes and your readings will be affected if the tremolo circuit is engaged.

Arjay


Arjay, This is about the 10th time you've helped me out on this board! So, what about the volume? Should it be turned all the way down before measuring?


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 12:56 pm
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No, about the volume. With experience, no need.

It is just "safe" to be sure there is no sound in the amp


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Sun Jul 24, 2016 2:07 pm
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SkipBurz wrote:
Retroverbial wrote:
Be sure the tremolo is switched off before measuring or adjusting the bias. The bias current is used to modulate the screen-grid voltage of the power tubes and your readings will be affected if the tremolo circuit is engaged.

Arjay


Arjay, This is about the 10th time you've helped me out on this board! So, what about the volume? Should it be turned all the way down before measuring?


Set your amp's volume all the way down as a safety precaution, just as stratele has indicated.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Biasing a '68 Custom Princeton Reverb
Posted: Mon Jul 25, 2016 7:00 am
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Way I remember settings = measuring & adjusting idle bias. No instruments plugged in, controls set to zero. Speaker(s) plugged in.


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