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I have a Dynaco PAS 3 pre amplifier from the early 1960s that has been in storage for years.

I have been led to believe I should replace the capacitors in a couple of the boards. I have read your information on hype vs performance.

I have a list of what I need. They tend to be 400 V 0.002 to 1.0 mF capacitors but there are others too.

What type of modern capacitor would I use if I wanted the correct capacitance in the circuits?

My major concern is with value and durability. I have very little interest in esoterics, I just want the thing to work.

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  • \$\begingroup\$ mfd as in microfarad(uF) or millifarad(mF)? sometimes old caps have mfd markings when they actually mean uF. Can you post some photos of the caps? I believe they would be film caps (which have a way longer life than electrolytic caps) \$\endgroup\$ – Wesley Lee May 29 '16 at 14:09
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Shotgun replacement of capacitors is an unwarranted fetish. To be sure SOME capacitors degrade with age. But others are as good or even better than modern replacements. Wet electrolytic capacitors are subject to drying out and/or leaking over time. They loose their capacitance and cause audible symptoms.

Capacitors used for power filtering will allow mains-frequency ripple to enter the circuit which will be heard as hum. This is a prime symptom that filter or bypass capacitors need to be replaced.

The smaller value (0.002 to 1.0 uF) capacitors will be found in DC-blocking, AC-coupling capacitors in-line with the audio signal. If they lose value, it will be audibly apparent as loss of low-frequency response.

Other uses for smaller value capacitors will be in places like tone controls. And a change in value will result in the tone controls acting differently or even failing completely.

Note that those old fire-bottle circuits need high voltage capacitors which are rather uncommon in modern times of solid-state circuits. There are vendors online who specialize in components used in repair and construction of tube circuits.

Suggest actually evaluating the gadget before making any component replacement decisions. As the old saying goes: Don't try to "fix" something that ain't broke.

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  • \$\begingroup\$ So, replace the electrolytic capacitors and wax capacitors as a matter of course, and leave the ceramic and mica capacitors alone unless testing (up to the required operating voltage) proves they are faulty? (My source of information: Mr Carlson's lab: Tech Tips Tuesday- Radio Repair Tips) \$\endgroup\$ – Andrew Morton May 29 '16 at 17:37
  • \$\begingroup\$ Check the film capacitors (usually 400V somewhere around 0.1uF) for leakage - especially where they are used as coupling capacitors. The easiest way is to look for a red glow on the following valve's anode (or tube's plate) caused by positive grid bias through a leaky capacitor! I didn't say it was the best way... \$\endgroup\$ – Brian Drummond May 29 '16 at 21:38
  • \$\begingroup\$ I have found your information most useful. I took physics several times 40 years ago and although we worked with schematics I never learned the difference between a film or ceramic capacitor. I believe there was leaking from my pair of aluminum tube capacitors 2000 UF 30V in the Selenium Rectifier circuit. I have gotten a pair of 2200 muF 50V +-20%. Do you think these are what is needed? The remaining "Black Cat" seem to be .002 to 1 MFD referred to in the wiring instructions as either capacitor or tube capacitor. Thanks John \$\endgroup\$ – John Jun 1 '16 at 22:49

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