Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Swing_guitar on July 19, 2010, 04:09:23 pm
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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.
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Use two separate 25uf caps. And don't worry about the bias control.
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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