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  • Bias cap question 5F2A

    I'm getting ready to build my first amp. It's based on a 5f2a with some modifications. One modification is a voicing switch like Dave Hunter's Two Stroke Amp. This will allow me to select the value of the bias capacitor on my 12AX7 preamp tube. The two cap values are 25uf and .47uf.

    When I picked up the .47uf cap I accidentally picked up a "non-polarized 0.47uf electrolytic" cap. Will this work or should I return it and buy a polarized electrolytic cap?

    Thanks,
    Pete

  • #2
    The non polarized part will work just fine.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      Thanks

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      • #4
        I have another question regarding this cap. The normal cathode bias cap on a 5f2a is 25uF at 25v. Many folks recommend running a 25uF 50v cap. The Two Stroke has the switch activated .47uF cap rated at over 400v. Why the big difference in voltage rating? Does it have to do with the change in capacitance or is this a case of what was available? Bottom line is will a .47uF 100v cap be sufficient?

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        • #5
          Originally posted by PeteG45 View Post
          will a .47uF 100v cap be sufficient?
          Yes. The dc voltage through the cathodes is around 2 volts for the preamp tube and 19-20 volts for the power tube, which is why the stock values are so low. (Not sure which part of the circuit you are referring to). Either way, your 100V cap will be just fine.

          Your .47uF cap was probably intended to be used as a coupling cap, hence the higher voltage rating - it was probably what he had on hand.

          Please post back and let us know how they sound.

          RWood
          Last edited by RWood; 10-25-2009, 02:40 PM.

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          • #6
            Originally posted by PeteG45 View Post
            The normal cathode bias cap on a 5f2a is 25uF at 25v. Many folks recommend running a 25uF 50v cap. The Two Stroke has the switch activated .47uF cap rated at over 400v. Why the big difference in voltage rating? Does it have to do with the change in capacitance or is this a case of what was available? Bottom line is will a .47uF 100v cap be sufficient?
            Electrolytic caps typically don't last as long, and don't sound as good, if they are run at voltages far below their maximum rating. Rule of thumb is to have the electrolytic cap running at about 50% to 80% of maximum voltage rating for optimum performance. But 25uF 6V caps ain't that common anymore, so lots of people use 25V-50V (or even 100V) caps for cathode bypass in pre-amp tubes and this is no big deal. Running a .47uF 400V film cap as cathode bypass is perfectly fine.
            Building a better world (one tube amp at a time)

            "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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            • #7
              Originally posted by tubeswell View Post
              Electrolytic caps typically don't last as long, and don't sound as good, if they are run at voltages far below their maximum rating. Rule of thumb is to have the electrolytic cap running at about 50% to 80% of maximum voltage rating for optimum performance. But 25uF 6V caps ain't that common anymore, so lots of people use 25V-50V (or even 100V) caps for cathode bypass in pre-amp tubes and this is no big deal. Running a .47uF 400V film cap as cathode bypass is perfectly fine.
              I've read this "E-caps degradation at lower then rated operating voltages" thing a number of times on a few forums ... is there some definitive word on this from manufacturers, engineers, smarter folk then me (etc.) and not just hearsay?
              The reason I bring this up is that I've seen a few amps from the 60's and 70's come into my shop where the cathode bypass E-cap is a 25v E-cap, running at less then 2vdc or 3 vdc and the stupid thing was installed backwards for 30-45 years! ... and it still seemed to function properly... forwards or backwards and when tested on my $3500 Sencore cap tester at 25vdc... it was within 5% to 10% of it's value with low ESR. !!??
              What's the deal?
              Bruce

              Mission Amps
              Denver, CO. 80022
              www.missionamps.com
              303-955-2412

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              • #8
                I'm a little confused (again). Aren't the caps used in a cathode bias portion of an amp like a 5F2A supposed to be electrolytic caps?

                I'm using two paralleled 6V6 tubes for output with separate cathode bias resistors and capacitors. Right now that's a 470 Ohm 5 watt cement power resistor and a 25uF 50v polarized electrolytic capacitor on each tube. On the preamp tube I have a 1.5K Ohm resistor and two switchable caps, one 25uF 50v polarized electrolytic capacitor and one 0.47uF 100v non-polarized electrolytic capacitor.

                Would another type of cap be a better choice?

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                • #9
                  Since he referred to
                  my 12AX7 preamp tube
                  I think everybody assumed you were refering to the cathode bypass cap on the preamp.
                  Jag
                  Senior Member
                  Last edited by Jag; 10-26-2009, 11:00 PM.

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                  • #10
                    the stupid thing was installed backwards for 30-45 years!
                    OMG Bruce! You might have just started another "Magic Mojo" myth!

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                    • #11
                      As long as the voltage rating is sufficient, non-polarized is fine. Even on the power tubes.

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                      • #12
                        Originally posted by Bruce / Mission Amps View Post
                        I've read this "E-caps degradation at lower then rated operating voltages" thing a number of times on a few forums ... is there some definitive word on this from manufacturers, engineers, smarter folk then me (etc.) and not just hearsay?
                        I confess I have to defer to the knowledgable Merlin for this wisdom: (see p 26)
                        Chapter 1: The Common Cathode, Triode Gain Stage

                        Originally posted by Bruce / Mission Amps View Post
                        The reason I bring this up is that I've seen a few amps from the 60's and 70's come into my shop where the cathode bypass E-cap is a 25v E-cap, running at less then 2vdc or 3 vdc and the stupid thing was installed backwards for 30-45 years! ... and it still seemed to function properly... forwards or backwards and when tested on my $3500 Sencore cap tester at 25vdc... it was within 5% to 10% of it's value with low ESR. !!??
                        What's the deal?
                        Dunno - I can only guess that maybe the low voltage wasn't enough to dissolve the insulating film that is there to block DC. Or maybe operating at low voltages that way actually acted to re-form the cap to be polarised in the other direction?? (I guess that's possible because, according to hoyle, Electrolytic capacitors electrolytic caps start out life by having to be formed by a slowly rising voltage)
                        Building a better world (one tube amp at a time)

                        "I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo

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