Bypass capacitor substitutions....

Discussion in 'Tube Audio' started by Oerets, Jan 3, 2018.

  1. Oerets

    Oerets AK Subscriber Subscriber

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    When recapping a pair of Eico mono amplifiers did not have the .025uf capacitors. Everything else could be found. Had enough .033uf capacitors and installed them for testing and figured would keep up the search for the correct value.
    Well the amplifiers sound fantastic, plenty of tight bass and clear highs. Been running for well over a week now paired to L-100's with increasing satisfaction.

    Now the to question.
    I have 0.22uf that could be installed now, should I?
    Or leave it alone?

    So far I have chosen to do the latter.

    My understanding of bypass cap is a little increase is better then a decrease in value.

    Hopefully someone with more experience will answer.


    Barney
     

     

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  2. nerdorama

    nerdorama AK member Subscriber

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    The 0.033uF should be fine and maybe give a little more bass. Any issue that might arise would be from changing the RC time constants through the amp if they were tightly coordinated in the design to prevent low frequency oscillation. I couldn't say offhand but you are likely just fine.
    John
     
  3. thorpej

    thorpej AK Subscriber Subscriber

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    Which Eico model?
     
  4. Oerets

    Oerets AK Subscriber Subscriber

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  5. Oerets

    Oerets AK Subscriber Subscriber

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    Thanks
    That was my assumption but wanted to hear from others.


    Barney
     
  6. thorpej

    thorpej AK Subscriber Subscriber

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    If you're referring to C2 and C5 (the only .025uF caps I see on the schematic), those are the coupling caps from the phase inverter to the output stage. .033uF is quite near the typical 20% tolerance common of caps in the day, so it's probably just fine.
     

     

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  7. Oerets

    Oerets AK Subscriber Subscriber

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    They be the ones!


    Thanks for confirming my thoughts.

    So will give it some of the old fashion leave it the FK alone. May if needing to open them up in the future replace with the .022uf at that time. Until then keep enjoying them.

    Wanting to try different tubes, have JJ EL84's for outputs Mullard's for the rest. The original Mullard's were pulled when doing the rebuild and never reinstalled.


    Barney
     
  8. mcgjohn

    mcgjohn Member

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    Keep in mind those 0.025uf coupling caps were likely chosen by the engineer to filter out possible rumble or other low freq crap that was much more common back in the day. Today, not as much a problem, so should be able to use .047uf or even 0.1uf without much of an issue. I have a pair of HF-14s that have been running 0.1uf for coupling caps for a few years now. I thought about installing copper foil caps which are much better than any of the metalized or aluminum foil caps, but also worried about the poor ventilation in that small chassis, so far have not done it.

    The original EICO caps were ceramic. Not the best for audio. changing these out for a film cap of your choice will really help.

    the power supply cap uses a half watt 10k ohm Carbon comp resistor to drop the voltage. may want to replace that with a 1/2 watt metal film, or better yet, a 2-3 watt wire wound 10K resistor, which will lower the noise floor.

    best
     
  9. Oerets

    Oerets AK Subscriber Subscriber

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    I went thru and replaced all of the resistors capacitors and matched the values for both amplifiers part by part. Every one! Used 1/2 watt carbon and the 5 watt with two 330ohm paralleled. Replaced both power cords with new even. Bring up the amplifiers to be as close to clones as was possible. The only difference were the transformers. One was cloth lead the other not.


    Barney
     
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