Oahu Amp (by Valco)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ruraltom, Feb 14, 2012.

  1. Ruraltom

    Ruraltom Member

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    Hi all!

    Though I'd share something that fell into my lap and ask a question. Sorry about the crappy pics!

    A Valco Oahu amp, serial X57(xx), I believe that makes it a '57.

    [​IMG]
    [​IMG]

    This came to me from a relative today, really excited to get it home: very cool old school 'powered' magnet speaker, two old glass 6v6 tubes (RCA and Sylvania) and an old RCA 5Y3 rectifier (?) pretty psyched to give it a whirl, even without a three prong. Threw the power switch... nothing. Checked the fuse... none there!
    I can try the glass in my Microbaby in the interim, but anyone know what kind of fuse to toss in? Anyone have any info on these?

    Thanks!
     
    Last edited: Feb 14, 2012
  2. Krayon

    Krayon Senior Member

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    pull the old fuse and check its value ( its written on the end caps) always replace with correct type and amperage.

    also be aware that "field Coil" speakers bring the B+ ( high voltage) outside the chassis.. watch what you touch when the electricity is on.
     
  3. Ruraltom

    Ruraltom Member

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    If only.... no fuse, empty chamber... good for my round of Russian Roulette, bad for makin' this amp wail!
     
  4. Krayon

    Krayon Senior Member

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    start with a 2 amp based on how many power tubes.

    betting you have other problems than just fuse based on the look of that chassis and the water damage on the front of amp.
     
  5. Ruraltom

    Ruraltom Member

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    Thanks! One hopes not about other damage.

    I'll try the two amp, is that a slo-blo?
     
  6. Ruraltom

    Ruraltom Member

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    I'll also be looking to get someone the put a three prong on it in the near future... I assume that is possible.
     
  7. Krayon

    Krayon Senior Member

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    on an amp that is unknown when it was last operating.. its best to do full check of everything.

    start with tubes.. check each one with a tester to verify they are good.

    then check all the caps. replace any that seem suspect by test.

    then check resistors for value.

    while checking..also look for loose or broken connections. replace or fix as needed.

    look carefully at all wiring, damage or fraying needs to be dealt with asap.

    then test transformer for voltages without any tubes installed.

    once you have verified that everything seems to be in working order then its reasonable to reinstall the tubes and fire it up.

    I know not everyone can do all that.. but thats my method of dealing with an unknown amp.
     
  8. Ruraltom

    Ruraltom Member

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    Hmmmm... looks like I need to send it to a pro, I am essentially unable to do ANY of that. I take that it would be dangerous to just toss in a fuse and see if it sings?
     
  9. Krayon

    Krayon Senior Member

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    slo blo is fine to start .. but if that blows.. then take it to a tech before doing anything else.. unless you want to start working on amps..

    these are nice model amps with a killer tone when working properly.
    I've had several of them over the years..

    and if your interested , here is a website of a guy that clones these.


    http://www.vintage47amps.com/files/
     
  10. Ruraltom

    Ruraltom Member

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    Cool! Many thanks, Krayon.
     
  11. Timbre Wolf

    Timbre Wolf GoldMember Supporter Gold Supporting Member

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    6SQ7 preamp? 6SL7 phase inverter? What are those metal tubes?

    - Thom
     
  12. Ruraltom

    Ruraltom Member

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    Good Morning Thom,

    As far as I can tell -not being any kind of expert on the inner workings of amps- those three on the left are the only ones. Never having seen an amp like this I can only guess those other 'cylinders' poking out the top are old-school caps and/or resisters? Maybe an expert can chime in here. The metal tubes appear to be solid, not tube protectors.

    Tom

     
  13. '58Bassman

    '58Bassman Member

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    The large metal cylinders ARE caps and if this amp hasn't been powered up in many years, it would be best to either re=cap it before doing anything else or using a Variac to start at a lower voltage and working up to full line voltage.

    Go to the Valco page for more info. Yours is new enough that I would expect to see something- a description, schematic, etc. I have an Oahu that, as far as I can tell, was made between 1930 and 1935 and I found absolutely nothing online, but http://tdsl.duncanamps.com/ has data for just about every tube out there. This will show the voltages needed (as close as practical) for the tubes in your amp.

    These are very cool amps, although I haven't tried one like yours. FYI- teh speaker- known more commonly as a 'field coil speaker', uses a coil around the voice coil and polepiece to create a magnetic field so the voice coil will move and was the only way top make a dynamic speaker before permanent magnets were practical. The coil is also used as a choke in the power supply- if you look at the schematic for a 5F6A Fender Bassman, you'll see the power transformer and a choke, which drops voltage and adds some filtering.
     
  14. Tuberattler

    Tuberattler Member

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    I've worked on a lot of these type amps. This amp WILL NEED power caps re-newed if they've never been replaced. That should be 90% of your problems.

    Most of the time on pre '50 amps the switch/knob is frozen and you might want to make sure the speaker is working.

    If you're not sure, take it to some one, these are nice sounding amps with a nice round fat tone that has a sweet breakup just past 60% on the volume dial.

    Give us some detailed gut shots.
     
  15. Ruraltom

    Ruraltom Member

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    Thanks for all the info, guys/gals!

    Can one just pull the caps out? Minus the one that looks to be wired in. Not sure if I have the scratch to send it to someone right at the moment, if there's even anyone around here that can work on them.

    I'm not sure about gut shots, never been into opening amp chassis. Is it safe to do so?
     
  16. Tuberattler

    Tuberattler Member

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    No the caps cannot be just pulled out. They are soldered in and will have to be re-newed with like type spec caps...
     
  17. wyatt

    wyatt Member

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    That's not 1957. More like late '40's.

    The preamp tube with the plate cap would be a 6J7 pentode, Valco would switch to the 6SJ7 by the early '50's. The PI is probably a 6SC7, pretty equivalent to a 6SL7, but with a different pinout. Field coils speakers were abandoned by the end of the '40's. Other than the tube and speaker swap, the circuit (the "Supreme") stayed very much the same until the late '50's.

    Ultimately, I would expect many of the components to have drifted way out of tolerance. Caps from that time period just weren't anywhere near as well made and as long lasting as what was available by the early '60's. And Supro and Gibson really cut corners in part quality in their builds. While they can sound as good or better (at least different) from the Fenders based on the same Western Electric circuits, they just weren't build a solidly.
     
  18. Tuberattler

    Tuberattler Member

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    That amp is worth a re-build as I've owned several old 40's amps and they have a unique character that you will not find in modern amps.

    The big stand up cans/cylinders at either end of the chassis are the caps I'm referring to. You should be able to find markings on the sides of them which will indicate the cap capacity of the can. These most likely will have two, three or more capacitors incorporated inside each can. There are connections on the bottom of these cap cans that have icons that match the Icons on the outside so you know what value you're connecting to.

    The best two sources that I've found for these are Antique Electronic Supply http://www.tubesandmore.com/ and Weber speakers who can make custom ones for you http://tedweber.com/

    High electrical energy is stored in these cans. If you're not comfortable don't do it yourself. This is not an expensive job and will bring a smile on your face once running up to snuff.
     
  19. nmiller

    nmiller Drowning in lap steels Silver Supporting Member

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    Serial X57-- dates to 1951.

    That amp has a permanent-magnet Rola speaker. The OT is often mounted on the speaker frame, making it look like a field coil, but it's not - there are only two wires going into the speaker.
     
  20. guitarcapo

    guitarcapo Senior Member

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    Don't turn the amp on or think about playing it. If you have that urge just play guitar through one of your other amps until you calm down.

    The first thing you need to try to do is track down a SCHEMATIC. That's a blueprint for the amp that you will need (even if you aren't working on the amp yourself a tech will need it) It will have all the values needed to make the amp work as it was originally designed.

    That's not a daunting task these days. If you have information on the model number, schematics for a lot of these amps can be found online.
     

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