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Fixed my fried Ampeg V-4 today and it was quite simple

Discussion in 'Amps and Cabs' started by Natron, Jan 14, 2008.

  1. Natron

    Natron Member

    Messages:
    59
    Joined:
    Dec 11, 2007
    Location:
    Portland, OR
    Backstory: A few days ago, I saw an ad for a local guy selling an early-version Ampeg V-4 for $200. I emailed him right away asking for details. Turns out there were two catches. 1. It didn't come with power tubes. 2. He was fairly certain the output transformer was blown. I told him I'd take it anyway. I figured it'd be a fun project.

    What it did was, 1/2 second after you flipped the power switch on, the 6A power fuse would blow. I tore it down the day after he dropped it off and couldn't find any signs of arcing, shorts, blown components, overheating, etc. The filter caps had been replaced recently so I was doubting those went bad. There were no power tubes in it and I took the preamp tubes out as well so a shorted tube wasn't causing the issue. There were no other defects I could find, visually. I told a friend of mine about it and he agreed to come over today with all of his testing equipment since he builds musical equipment for a living.

    He came over and poked around for about a half hour, testing the transformers (which, thank goodness came up with no shorts he could find) and tracing the flow of power through the amp. He decided it could only really be bad filter caps, bad rectifier diodes, or some freakish unknown problem with the output transformer. We started simple and desoldered all the rectifier diodes to check them out.. Guess what. TWO of them were shorted. Walked down the road, picked up a $2.50 worth of replacements, walked back, soldered them in, replaced the fuse, flipped on the power, and it powers up juuuust fine now. So $202.50 total + buying dinner and a beer for my friend for helping + $76 matched quad of replacement 7027A tubes = great deal on a V-4, if you ask me. I'm excited! Can't wait to try this out on the 4x12 I have coming this week!

    [​IMG]
     
  2. BluesForDan

    BluesForDan Member

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    NH
    I had a v-4, back when I was wet behind the ears and didn't know **** all about stuff. I'd love to give one another a go, with what I know now.

    what are you using for speakers?
     
  3. Natron

    Natron Member

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    Dec 11, 2007
    Location:
    Portland, OR
    Currently just have that Genz Benz 2x12 and later this week, I'm getting a Marshall 1960BV (Celestion Vintage 30s) from a fellow local forum member (hey Todd!).
     
  4. dewman

    dewman Gold Supporting Member

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    thats a killer story...keep an eye on her for future problems, although its likely that when the power tubes blew they may have taken out the diodes - so maybe the future will be just great. Get higher rated voltage diodes. If I remember right about my VT-40, I replaced them with higher voltage ones after a tube blow out...wow- those amps have enormous volume and low end...very cool sounds!
     
  5. Natron

    Natron Member

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    Dec 11, 2007
    Location:
    Portland, OR
    I couldn't find higher voltage diodes than the ones that were in there.... 1000V ones were already there. But yeah I seriously have NO clue how I'm supposed to use this non-master-volume amp in my apartment with paper thin walls.
     
  6. Natron

    Natron Member

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    Dec 11, 2007
    Location:
    Portland, OR
    Oh and that's the reason I'm fairly sure I'm going to end up with a 2nd HotPlate for it. Pity there's no way to modify it for 16ohm operation.
     
  7. soulohio

    soulohio Member

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    vin drin dop
    I use an airbrake on 'the VT40'...also, a bad monkey will not only give you a nice wee push but you can also dial down the volume...gawd, i jammed with the amp two days ago and my left ear feels like there is a sponge in there...watch out...them ampegs are dangerous....but i am getting the nicest wah-like QOTSA tones outta that thing...thanks again Dewey!
     
  8. JLee

    JLee Supporting Member

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    Good luck with that! I run mine at a mismatch to reduce the headroom. Still loud as f**k! But it gets the most awesome grind imaginable!
     
  9. Natron

    Natron Member

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    Dec 11, 2007
    Location:
    Portland, OR
    Hmm... and I just checked the THD Hotplate manual online that states it's safe to mismatch the impedance of the Hotplate and amp as long as the Hotplate is equal to or greater than the set impedance of the amp... V-4 on 8ohm, Hotplate is 16ohm. According to them, I CAN use it...

    Makes me nervous though. I don't want to end up with that OT blown afterall..
     
  10. soulohio

    soulohio Member

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    could you elaborate on what you are doing? what is mismatched? why does this reduce the headroom? does this technique work on other amps?

    i need more grind out of an OR120 and I have yet another VT40 that needs a good going over. If I could get them to grind at a lower volume i would be a happy camper.
     
  11. Unabender

    Unabender Member

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    So you actually did not fix it yourself like the title suggested :) Nothing wrong with that, congrats on the amp.
     
  12. theinteriorleag

    theinteriorleag Member

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    Feb 19, 2007
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    tn
    very good score there. You saved enough on that amp that maybe you could afford to power-scale it...make it perfect for any setting.
     
  13. Scott Auld

    Scott Auld Staff Member

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    I believe you are correct, you can use a 16ohm Hotplate with an 8ohm amp. Make sure the speakers are 16ohm. :)
     
  14. Natron

    Natron Member

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    Location:
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    :messedup When did taking the amp apart myself and soldering the diodes back onto the board myself become "not myself"? Yes my friend helped diagnose the issue though.
     

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