How hot is it inside an amp's chassis? None of your beeswax?

Discussion in 'Amp Central Station' started by King Fan, Jul 23, 2019.

  1. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I'm thinking about re-stuffing new electrolytic bypass caps in the old 20uF paper tubes in a small 1955 chassis (GA9, 2x6V6 parallel SE, if it matters). In some experiments and reading it sounds like you can re-seal the ends of the stuffed tubes with melted beeswax. BTW, these antique radio pages contain an almost-overwhelming amount of info -- everything you might ever want to know -- about old caps.

    https://antiqueradio.org/recap.htm
    https://antiqueradio.org/col01.htm

    But beeswax has a nominal melting point around 144-147°F. So I got to wondering how hot it gets inside a tube amp chassis. Obviously hotter near the PT and power tubes, and obviously it varies by amp, chassis size, and chassis ventilation. 4x6L6? Overworked old PT? etc. I've seen thermal imaging data of PTs getting pretty hot, but luckily the re-stuffed caps are down the other end from the PT in this amp.

    Data wiz @peteb shared some measurements (from a Champ, Pete?) that in the actual circuit temps got up to 150° (for example near the cathode cap and rectifier) but the chassis near the PT only got up to 90°. If the latter temp governed the air temp, beeswax might be fine, but if convection or (more likely) radiation heating brought the re-stuffed caps up toward 140... well, ugly.

    So have any of you ever measured the temp inside a buttoned up tube amp chassis?
     
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  2. tubedood

    tubedood Tele-Meister

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    Hanging with you to see the replies on this also... on a few of my build projects I tried to keep things looking nostalgic and era correct.
    Re-Stuffing the old filter capacitors would look pretty neat at least for appearances.

    And thanks for the links!
     
  3. King Fan

    King Fan Friend of Leo's Ad Free Member

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    I was reassured at first that the advice to use beeswax came from an antique radio restorer. But although tube radios get warm (I think -- I'm not *that* old) maybe they don't get *hot.*

    FWIW my daughter the museum conservator tells me that carnauba wax is the highest-melting wax, at 180.
     
  4. dkmw

    dkmw Friend of Leo's Silver Supporter

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    I dunno the answer to your question, but I could cook food under the hood on my Peavey Classic 20 combo:eek:
     
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  5. King Fan

    King Fan Friend of Leo's Ad Free Member

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    If you had a simple glass thermometer that went up to say 150° or 200°F, I guess you could put the bulb inside through one of the input jack holes. Do I have such a handy item? Nope. :)
     
  6. LutherBurger

    LutherBurger Friend of Leo's

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    I don't know nothin' about what goes on inside an amp, but I applaud your thread title.
     
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  7. Nickfl

    Nickfl Tele-Afflicted

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    Does the chassis on that amp mount on the floor of the cab? If the tubes and PT are mounted above the chassis it shouldn't get all that hot inside. If it has the tubes mounted below the chassis like a tweed or both tubes and PT mounted below like a blackface, it may get noticably hotter.

    I'd offer to measure the temp in my gibsonette for you but we are in the middle of moving to a new house and that amp is in a closet at the new place right now.
     
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  8. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Thanks, Nick, it's a standard top-of-amp chassis.

    If you keep the amp in that closet in your new house long enough, yours could turn into a closet find too. :)
     
  9. RLee77

    RLee77 Friend of Leo's

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    Just use a hot glue gun. Or silicon glue. No messy wax to deal with.
     
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  10. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Good point, RLee. In fact, that antique radio restorer uses hot glue to fill the space between the new cap and the tube ends with hot glue. (Some folks wrap the cap in tape, or in stiff paper, to partially fill the tube.) Then he caps it with wax for vintage looks.

    We've had a fun discussion in another thread where we learned that hot glue is OK for some amp work but the low melt (?240°F) kind is *not* good right near the tubes or other super-hotties.

    I think the radio folks who re-stuff (I've read several) use the wax to make the caps look as original as possible. They suggest re-using the wax you melt out of the tube (200-250° oven) to re-cap the ends. But in my C-D caps, the 'wax' that came out seemed to have a super high melting point and turned very brittle as soon as you tried to mold it into place, or even pour it. I'd probably have needed a ceramic crucible and tongs and a Bunsen burner. It was that melting point that a) made me try their beeswax suggestion and b) later worry I'd created a scented candle that could melt all over the nightstand.

    As a final detail, I found getting the end caps to look good was tricky even with the beeswax. This whole re-stuffing thing is probably vanity, and IME vanity goeth before the fall. :D
     
  11. King Fan

    King Fan Friend of Leo's Ad Free Member

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    It's about time for true confessions. I already waxed the caps and soldered 'em into place, for several reasons, like I could re-use the original spaghetti and I needed to get on to other testing and soldering around their anchoring sockets. And *then* I got to worrying! Typical. :D

    IMG_0036.jpeg

    Also I'm thinking I should have polled folks who re-stuff caps to see if they ever use beeswax. I know @keithb7 said he does this a lot, and IIRC maybe @D'tar and @corliss1 had some experience. Guys? Anybody?
     
  12. kris ford

    kris ford TDPRI Member

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  13. D'tar

    D'tar Friend of Leo's

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    Awesome! FWIW... I have seen a ton of components covered in wax and none have flowed out except after a failure condition. Preamp bypass caps may have one of the easiest long lived life in a tube amp compared to other components. :)
     
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  14. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Excellent tool, and I can see it being handy to use for an open chassis's innards, and for remote sensing (tubes, PT, outsides of chassis). I'm not sure it'll let me see the temp *inside* the chassis closed up in the amp.
     
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  15. kris ford

    kris ford TDPRI Member

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    True! But could still be handy! MIGHT be able to aim the beam thru the input jack for a ballpark rough estimate?

    My lil Lady uses it to verify oven temp when she bakes. :)
     
  16. FenderLover

    FenderLover Friend of Leo's

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    I have a 1948 Gibson GA-25 needing a rebuild. When I pulled the chassis there was a huge lump of wax on the cab under the center of the chassis (It's a bottom mount, rear facing). The thing ran fine, but I didn't fire it up again after I saw that.

    So to answer your question, it could get hot enough to melt waxed caps.
     
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  17. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Thanks, gentlemen. These two answers kind of frame the problem. For sure many old caps leak when they fail. Much more dramatically, I've managed to blow up at least one modern cap and melted a modern resistor in more active 'failure' moments (yeah, I know, duh). I hope that pool of wax represents a prior 'failure condition.' Especially since a bottom mount chassis should run cooler with the tubes on top. But could it represent 'normal operating temps?' Hmmm.
     
  18. BobbyZ

    BobbyZ Doctor of Teleocity

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    Did an old Newcomb pa an auctioneer used to own. It had a pair of 6v6s and a 10" Jensen alnico speaker, I'm guessing early 1950s.
    All the coupling caps were the old paper wax types and looked like they'd been hot a few too many times. I just replaced them all along with the electrolytics, fixed the last "repairs" and put in a 1/4" input jack.
    I've been inside old radios with the same type of caps that still worked and looked really good. I don't get into radio restoration, if they work I'll use um awhile then maybe pull parts out for guitar stuff. Just haven't found a radio I'm interested in enough to spend much time on.
     
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  19. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Good points. I figure radios may not put up with either the electrical or travel abuse guitar amps do. I wonder how hot it gets in the trunk of a *car* parked in the hot sun. Not up there in Frostbite Falls, Bobby, I'm thinking in Bakersfield or Austin or Muscle Shoals...
     
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  20. King Fan

    King Fan Friend of Leo's Ad Free Member

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    Update: I've been playing the amp a fair amount -- couple of ~1hr sessions -- and the chassis, even at the input end, gets pretty hot to the touch. So far, knock on cardboard, the beeswax end plugs seem to be holding up just fine. Fingers crossed!
     
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