Old Tube amp - to recap or not ?

Discussion in 'Amps [BG]' started by pfschim, Feb 21, 2014.


  1. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    I have a situation/question to pose to the collective wisdom of TB.

    Last year, I grabbed a very clean Fender Studio Bass amp (just the head). 200w of all tube goodness built by Fender in the late 70's. So, this amp is at least 35+ years old. I bought it used (of course) and it clearly had some work done on it at some point, but I wouldn't guess that it had a cap job done since its birth in Fullerton.

    Now, the amp works and sounds great, no buzzing or any real hum, and it seems to kick out the volume just fine. I did replace the 6 original RCA 6L6GC power tubes that came with it, and then had it rebiased with a sextet of Ruby 6L6GC STR's. When it was opened up for the retube work, the tech and I got a good look at the amp guts, and neither of us saw any swelling or sweating on the caps. There was one that looked somewhat scorched, but the scorching could have easily been a cosmetic result of the original power cord shorting out (I seem to recall that this particular cap was very near the hot/ground wires that looked as if there had been an issue of some sort in the past).

    My question is: should I just go ahead and have it checked out and recapped, or should I leave well enough alone and go with "it sounds good, it must be ok" ? I'd really hate to have it worked on and discover that I liked it with the original caps better, because it sounds really good now.

    and ... discuss!
     
  2. Basshappi

    Basshappi

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    If it works, don't fix it.
     
  3. nashvillebill

    nashvillebill

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    35 year old caps? They are living on borrowed time. Recap it!
     
  4. Itzayana

    Itzayana

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    Old caps are part of the MOJO.
    If you re-cap it you might be sorry.
    I agree with Basshappi, If it works, don't fix it.
     
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  6. nashvillebill

    nashvillebill

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    There's no mojo in dead amps.

    Electrolytic capacitors have a known failure mode: the mechanical seal fails after many years, and they dry out. I've repaired amps with rectifiers that failed because the caps were dried out.
     
  7. JGR

    JGR The "G" is for Gustav Supporting Member

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    There's no mojo in worn out components (or crappy ones - can you say carbon comp?) - probably time to replace them. You should notice an improvement in punch and clarity which is a good thing. If you do desire a different vibe after you recap, then it's better to play with filtering values to get you where you want to go.
     
  8. nashvillebill

    nashvillebill

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    Incidentally, there's a crucial cap as part of the bias circuit. Lose THAT cap, lose bias, and say goodbye to your power tubes. You did save those RCA 6L6's, I hope; they are quite valuable if they are still good.
     
  9. beans-on-toast

    beans-on-toast Supporting Member

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    Lets assume the caps test good. I assume that the amp has been sitting unused. Heat is the enemy of caps. When the old caps get into service and exposed to the heat they could go down hill fast. The rubber safety seals can have cracks or be leaking which would allow goop to ooze out. So you are taking a chance.

    On the other hand, if you want to keep it in original condition as a museum piece, best to leave it the way that it is.
     
  10. Stumbo

    Stumbo Wherever you go, there you are.

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    I vote recap. Old caps are....old!:)
     
  11. middy

    middy

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    Oops. Disregard.
     
  12. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    oh yes I did. They all tested good and I put them away for safe keeping. I was amazed what I saw for prices on them. All six are worth more than what I paid for the amp. amazing!
     
  13. nashvillebill

    nashvillebill

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    This alone would be a gentle hint...

    Incidentally, many folks focus on just the filter caps, but electrolytics are also used a cathode bypass capacitors, and these can dry out just like the others. This affects the frequency response of the amp.
     
  14. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    well, there's part of my dillema. I was wondering if the combination of spec variability in the original components, combined with the impact of age on those components had anything to do with the great sound of the amp as it is. It really does sound good.

    Feeding my question was an interesting conversation recently (more of a lecture really) with Randal Smith on this topic. He was talking about the wide % range variability in the original component values, as well as the impact of age on those same components, as it related to current "tone" of a given amp. He was specifically speaking about trying to capture the "mojo" of Carlos Santana's legacy Mark V in a new limited production amp, and the challenges that those things imposed on the process. But his comments and experience could apply to any amp really, including mine.

    ok. you all have given me something to consider for sure. Thats why I asked.

    Thanks
     
  15. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    spoke to a SF Bay Area tech this afternoon. Erik from Euthymia Amps (?). He strongly suggested the recap (filter as well as the power/cathodes).

    He also spoke about making the bias "universal or global"

    what does this mean, and what does it mean to the operation of the amp going forward ?
     
  16. pfschim

    pfschim Just a Skeleton with a Jazz bass

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    well, I think the scorching had more to do with the power cable than the cap. it just seemed to be localized near the cap.
     
  17. beate_r

    beate_r

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    Even 35 year old (electrolytic) caps can easiliy live another 15 years if the amp is played and checked regularly. But they also can fail tomorrow. I own a Bouyer ST20 which is now 50 and still plays fine. Everything is in its original state. On the other hand my slightly younger Echolette M40s needed a recap. And two of the electrolytics in my 1967 Dynacord Bassking failed. But this was forseeable - the caps already showed some electrolytic material on the surface.

    My conclusion:

    in 70s technology i would have a look at just the electrolytic caps - the other caps should be quite robust. If there is no hum and the caps are visually ok You can usually risk using them. But as always with old hardware there is of course a risk of failure. In anyway i would try to go with minimal changes but do everything that is necessary.

    If Your desire is, however, to have a player with best possible robustness, You should consider to replace the electrolytic caps and thoroughly check the biasing trim-pots (or maybe replace these as well)
     
  18. Basshappi

    Basshappi

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    I will bow to those with greater experience.
    I read in the OP that his tech went through inspected the amp and thought everything looked okay. Thus my comment.

    The caps are old and a cap job shouldn't cost too much, so it would be a worthwhile expense to keep a great amp ticking for years to come.
     
  19. beans-on-toast

    beans-on-toast Supporting Member

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    That's hard to know. Take several vintage amps, all the same model, all look the same inside. You can find one that is absolutely magical. What makes this amp special can be hard to pin down. Sometimes it is due to the combination of system component values and tolerances that come together that make the amp different. Any one of them on its own would be within tolerance.

    The power supply is one of the sub systems that factor into the sound of the amp. Is your particular power supply making the amp special? There are a number of things ahead in line such as the components in the signal chain, the transformers, the speakers. Sometimes the design is simply very good and all the amps sound great. Although you'll always come across examples that stand out for one reason or another.

    If you do change the electrolytic caps, I would start with all the power supply caps and the ones in the power tube bias circuit. I'd only change the cathode electrolytic caps on the pre-amp tubes if they really need to be changed. These caps could be contributing to the tone. These caps, having a smaller voltage rating, may not be exposed to as much heat as the power supply caps. This can help them have a longer service life.
     
  20. JGR

    JGR The "G" is for Gustav Supporting Member

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    The whole magic vintage amp thing largely comes from Marshall who was notorious for using a wide range of components based on whatever was available at the time. Transformers being one of the biggest areas - varying voltage levels, primary impedance, and bandwidth being major contributors. You don't really hear folks talking about magic Fenders or Hiwatts or Ampegs. If your amp sounds good now, it sounded even better new which recapping will get you closer to. Quality tubes would then be the only missing link. Much better to have an amp you love based on a stable platform rather than something that is slowly dying and on the edge of failure. Tube amps are pretty basic and the whole mojo thing is perpetuated from a combination of BS marketing and lack of real understanding on the technical side, especially in the guitar arena.
     
  21. Codger

    Codger Supporting Member

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    If you are of a mind to do something then replace the electrolytics. It probably won't sound different but you will have peace-of-mind that you have replaced the most failure-prone passive components.
     

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