Assumptions are the mother of all stuffups

Discussion in 'Amp Tech Center' started by Burning Fingers, Jun 3, 2020.

  1. Burning Fingers

    Burning Fingers TDPRI Member

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    I have an Australian "copy" of a Fender Showman Ab763..made in the 1970's by Eminar.
    The amp came to me after a local music shop had it overhauled and then sold it to a customer who returned it complaining that it was making horrible noises.

    After checking it out I agreed...horrible noises from bad and mismatched power tubes , dodgy caps between the PI and power tubes and dirty preamp sockets. I replaced all those and that solved the noise issue.

    But... the normal (non tremolo ) channel was insanely loud compared to the tremolo channel so I assumed there was a fault with the tremolo channel and checked every component and voltage in that channel only to be left scratching my head as everything checked out ok.

    I told the shop what I had found and they offered to sell me the amp for a song as they didn't want to pay for all the work that had been done (and was still to come) .

    So now I had yet another tube amp to add to the other 11 that I own...what to do with it ...ah ha ...I need a bass amp...so I will change the tone stack in the normal channel to make it more suitable for a bass guitar.

    This channel only has volume, bass and treble controls and I use the tone stack calculator to figure out the right values of the caps and mid resistor and attack the tone stack.

    Well ...start to attack the tone stack until I realize that it is wired in strange ( to me ) manner.
    The mid resistor is not there at all which means the tone stack has no path to ground and the bass pot is wired differently...see the pics. tone stack1.jpg tonestack2.jpg

    I put in a 6.8k mid resistor and rewired the bass pot in the usual fender way and bingo...the insanely loud channel is tamed and both channels now have the same volume. Changed 2 caps in the tone stack and it is better for bass.

    Some one had removed the mid resistor for some reason and rewired the bass pot and the last tech had not picked up on that ( unless he did it himself ??). If I had not wanted to alter the tone stack in that channel I would probably still be wondering what was wrong with the other channel.

    Never assume the previous tech did a good job...my assumption cost me about 4 hours time...but at least I have another tube amp at a bargain price...are 12 amps enough ?...probably not . Are assumptions ok...definitely not.
     
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  2. Fluddman

    Fluddman Tele-Meister Silver Supporter

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    Good one!

    Haven't heard 'Eminar' mentioned for a while. Brings back some memories.

    Re amp repairs, it's always good to be the first person to work on amp. Nothing worse than dealing with someone else's undocumented modifications.

    Glad you got it sorted - funny how things work out once you stop looking for the problem.

    Cheers
     
  3. Kevin Wolfe

    Kevin Wolfe Tele-Meister

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    That’s cool! Should make a decent bass amp. You could always tweak the value of that mids resistor to take out some of the inherent mid-scoop these circuits are known for. The old Bassman had a 6.8k in the normal channel and an 8.2k in the bass channel. Beefing up the filter cap values will also stiffen up that bottom end.
     
  4. Paul G.

    Paul G. Friend of Leo's

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    Lifting the ground there takes the tone stack out of the circuit, so it's not surprising the volume was ridiculously high.
     
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  5. Wally

    Wally Telefied Ad Free Member

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    +1 with PaulG’s analysis. That was a straight signal feed from the first stage to the second stage through the treble cap, it seems. Someone ‘lifted’ the tone stack.
     
  6. Dacious

    Dacious Poster Extraordinaire

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    Eminars are much underrated. Housebrick sized transformers, handwired with no tagboards. Had a PA head that like most tube amps kicks out way more volume than you'd think.
     
  7. Burning Fingers

    Burning Fingers TDPRI Member

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    Changed the mid resistor to 8.2k and the treble cap to 10nf and it is very suitable for bass...tone controls behave differently than one would expect but so much more interactive than the standard with 6.8k and 250pf. Have left the tremolo channel alone for use with a guitar.
     
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  8. Burning Fingers

    Burning Fingers TDPRI Member

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    Yes...huge transformers and quite heavy ...luckily it will live in my studio so I won't need a laborer to carry it for me.
     
  9. Burning Fingers

    Burning Fingers TDPRI Member

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    Yes it was brutally loud...100 watts of snarling, growling attack dog with huge fangs.
     
  10. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Fender Showman Ab763 makes a great bass amp. It has the tube amp warmth very much like the old Ampeg bass amps. I have mine set to work with guitar or bass speakers. I pull two tubes and run it like a Bassman with guitar.
    Heavy but the head it is not stupid heavy.

    Best of luck with #12.

    :cool::cool::cool:
     
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