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AMP BH 420 mods

Discussion in 'Amps and Cabs [BG]' started by phatbottomend, Oct 13, 2018.

  1. phatbottomend


    Jun 2, 2010
    I recently purchased an AMP BH420, second hand of course. The amp sounds great! nicest amp i have ever owned! and through 1 of my 3 way cabs it sounds MASSIVE! The more I used it the more I found little problems with it that I am very keen to rectify.

    Firstly, I am replacing all the pots as they are quite scratchy.
    Next, I found that my Hi/low speaker output and crossover isnt working, so i am looking into that, starting by replacing the op amp, i decided to replace all the op amps at the same time.
    I have also decided to upgrade the pre amp eq to give me more high frequencies (i'm a slap player) using the values used in the schematic for the Gibson GB440.
    Now this is the only tricky part. I want to add a cooling fan as after a 3 hour gig the amp was VERY HOT!!!! VERY!!!!! Does anyone have any information on the best way to do this? Any info would be very helpful.

  2. Nothing wrong with replacing pots on an old amp if you have the electronics chops to do so and sounds like you might. But have you tried cleaning them with Caig De-Oxit?
    I’ve got an old amp scratchy pots and I just put up with it for years (didn’t get much use regardless) I just finally decided to clean the pots and the De-Oxite worked wonders.

    As far as cooling how ‘bout a big *** fan... ? :smug:
  3. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    I mounted a 120 Volt fan in the back of a rack case back in the day to try to keep a SM400 from being incredibly hot.
    Not so sure it was needed through
    If I'm not mistakem, Steve Rabe worked on the AMP as well as designed the SWR.
  4. phatbottomend


    Jun 2, 2010
    Thanks so much for all the info.
    After alot more research it turns out that the pots are incredibly difficult to find, so I think cleaning is my only option. I will give both circuit boards a good clean too. I will still be replacing and upgrading the EQ section capacitors and all the 5 Watt ceramic resistors.

    I am still keen to put a fan in the amp though, I know the design is supposed to use the chassis casing as a heat sink but after hours of being on, and pushed, the amp was almost to hot to touch.
    I had an SWR 750x for MANY years, I loved that amp, it used to get hot, even with a fan but the heat off this BH420 is crazy!

    Jim, do you have a contact for Steve Rabe? I would love to pick his bain.

    Thanks again
  5. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Man, I would leave the amp alone.

    If you choose to clean the pots, use only Caig DeOxit D5 and only the tiniest amount possible. Don't spray anything on the circuit boards. I wouldn't be replacing parts, iirc, the PCBs on those amps are not very robust and you are likely to end up with more problems than you start with.
  6. hennessybass

    hennessybass Supporting Member

    Oct 11, 2008
    Bayou City
    The AMP BH420 is one of my all time favorites! When I was young, I saved a bunch of money to buy a new bass. Went in the store to buy it, did a final try out and used an amp I'd never seen or heard of. I walked out of the store with a AMP BH420 instead of the bass.

    The chassis on that amp is a huge heat sink - that's how it cools itself. No need for a fan. Just make sure it has plenty of space around it.

    It's been doing fine for 30 year without a fan. Just let it cool down for a few after the gig.
  7. derrico1

    derrico1 Supporting Member

    Apr 12, 2005
    Charlottesville, VA
    Great amps. Even though I hardly take it out of the studio these days, I've hung onto a Thunderfunk 420 (which borrows a lot of the BH420's design), just because I love the tone (and the DI).

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