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Post subject: '78 Princeton Reverb: Biasing?
Posted: Tue Nov 17, 2009 10:10 am
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I've got a 1978 Princeton Reverb, and from what I've been able to find out in various forums, as currently set-up with no mods, it can't be biased. However, I've also read that it's possible to re-fit the amp with a bias-capable resistor.

If this is possible, would it be a good idea to do so? Would it add any sound value to the amp or would it be more trouble than it's worth?

Any / all input would be greatly appreciated.

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Posted: Tue Nov 17, 2009 9:12 pm
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Hi Chris and welcome to the forum. The bias on the Princeton Reverb is set by the 22kohm resistor on the bias circuit. The bias current can be measured and adjusted if need be, by changing this resistor. A potentiometer could be inserted at this point to make adjustments easier. If you are happy with the sound of the amp and have no problems with the power tubes failing prematurely, then I'd say leave it alone. Keep playin' that gem! ART

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Posted: Tue Nov 17, 2009 9:42 pm
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Thanks, ART. I like your point of view - no sense messing with it if there are no problems with it - and I haven't any problems at all other than the normal wear and tear. Like you say: It's a gem. I appreciate your response.

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Posted: Thu Nov 19, 2009 8:25 am
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My Princeton Reverb is a '77 and when I put in Mazda NOS tubes, I had my guy check the bias to ensure it was optimized for those tubes. He changed the resistor values, but Art's suggestions are very good here.

If you haven't, you may want to think about having the filter cap can replaced since your amp is over 30 years old. Won't change your sound, but it's good preventative work. Mallory and others make this.

The other thing is the pull boost, which if yours is a 78 has it - no need to mod it out. I've found that as long as I don't actually pull on the volume knob, there's no difference in tone.

Great amp and hope you get another 31 years of playing out of it.

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Post subject: Thanks.
Posted: Tue Dec 15, 2009 1:22 pm
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Nedorama -

Thank you for your response and my apologies for not acknowledging your helpful input sooner.

I've never heard of Mazda Tubes before, but I'm intrigued enough to research them. Your advice to have the filter cap can replaced is something I'll look into as well.

Re the pull switch - I had it replaced very early on in the process with a switch in the back that gives me the boost without losing the reverb and haven't missed it at all.

I agree with your assessment of the amp's pedigree. I had heard that it had been used by Steve Cropper and Elvin Bishop and decided I had to have one. It's served me well in all kinds of circumstances.

Thanks again for your thoughts and all the best for The Holidays.

--Chris

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