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Hoffman Amps Forum image Author Topic: Princeton Reverb restoration 3-lead cap question  (Read 2567 times)

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Offline Swing_guitar

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Princeton Reverb restoration 3-lead cap question
« on: July 19, 2010, 04:09:23 pm »
I am restoring a 1964 Princeton Reverb by replacing the electrolytic caps. The board contains two Mallory caps labeled 25/25 uf 25 VDC. The caps have two leads coming from the (+) end of the cap body, and one lead from the (-) side. I cannot locate direct replacements for these parts. Can I simply use two separate 25 uf. 25 VDC caps instead of the original single-body units? I have seen some information describing these as cathode bypass caps. If I can't use two single units, what would be the appropriate substitute?

A second question relates to the lack of adjustable bias for the power tubes. Is there any real need to modify the circuit to make the bias adjustable? I have modified the bias circuit on other amps (Fender Twin Reverb) for use with matched power tubes, but am reluctant to mess with this one unnecessarily. Any thoughts? Thanks.

Offline LooseChange

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Re: Princeton Reverb restoration 3-lead cap question
« Reply #1 on: July 19, 2010, 06:45:57 pm »
Use two separate 25uf caps. And don't worry about the bias control.
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Offline Fresh_Start

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Re: Princeton Reverb restoration 3-lead cap question
« Reply #2 on: July 19, 2010, 09:29:00 pm »
Peronally, I'd use two separate 25uf caps just like LC said.  However, if you want to use a single cap, F&T makes a multi-section 25uf/25uf-25 volt electrolytic.  But it's $8 and 1" x 2".  Antique Electronics has them #C-EM25-25-25.

Do you have a way to measure the power tube plate dissipation?  If it's in the right ballpark, don't worry.  However, resistor values may have drifted and/or wall voltage gone up and it might be worth putting in adjustable bias - especially if the bias is off what you think it should be.  Just in case you hadn't found the right page in the Library:
http://www.el34world.com/charts/Biascircuits.htm

Cheers,

Chip
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