Tube bass amp!!!

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Jan 27, 2020.

  1. RollingBender

    RollingBender Tele-Afflicted

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    This followed me home yesterday!
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    Pretty interesting relic of (best guess) the late 60’s. 6CA4 rectifier, two 6BQ5 power tubes, and a pair of 12AX7’s. I stood on a rubber mat and played bass through it for a bit. Not horribly noisy, neither buzz or amplified bass volume but it would probably work with a light handed drummer. The last bit of the volume knob will bring about some distortion but it will hang on to the bitter end.

    Going to put in a grounded power cord and remove the death cap before plugging into it again.

    I’ve been wanting to build a tube bass amp for some time. Going to spend some time with this and get to know it for awhile. Based on the size of the iron, I’m guessing there’s some room for improvement here.
     
  2. BobbyZ

    BobbyZ Doctor of Teleocity

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    Pretty cool.
     
  3. mkdaws32

    mkdaws32 Tele-Holic Silver Supporter

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    Nice!
     
  4. Wally

    Wally Telefied Ad Free Member

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    Congrats! The date codes on the speaker and the output transformer indicate late 1965 production for those. I would think the amp is from 1966. A quick search did not get me to a schematic for that amp, but there is a schematic for the MKX amp at Schematic Heaven. That amp has tremolo, but one might find some similarities to your 500 there. that amp might make a good guitar amp. I would think as a bass amp it would be good for nothing more than low volume practice...solo or with a guitars. I doubt if it would keep a bass in the mix with a drummer.

    https://schematicheaven.net/bargainbin/gregory_mark_x.pdf

    looking at the lead dress, one notes a rather disheveled result. They did not subscribe to twisted pairs for the heater filament for one.as for building a bass amp, ime one would want to start looking at some higher power platforms. A Fender Bassman 100 or 135, maybe?

    I also would suggest that the electrolytics...especially in the bias circuit and probably the can...be replaced ASAP.
     
  5. Allan Allan

    Allan Allan Tele-Meister

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    Try playing guitar through it. Maybe you just found a budget BassMan.
     
  6. Wally

    Wally Telefied Ad Free Member

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    probably closer to a Fender Musicmaster Bass amp, which was a decent low volume home practice bass amp and a hot low wattage guitar amp, ime. There were two versions...one used 6V6 while another used either 6BQ5 or 6AQ5 power tubes.
     
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  7. Whatizitman

    Whatizitman Friend of Leo's

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    Somebody's been inside it recently. Looks like my work. The two series resistors on the cap can is my signature. OK no it's not my work. But certainly someone with about as much experience as I have. :oops:

    Definitely recap with new filters, resolder all the recent work, and get rid of that tape. I would just bypass the ugly weld job on the cap can, and use discrete caps. Yuck!

    Two 12AX7s. Probably has a lot of potential gain on tap. My MMB only has one, but it has a transformer PI. It starts to get pretty crunchy at 10oclock on the dial. I'd like to hear this one!

    EDIT: It's two channel. That would explain the extra tube. I assume one side of one them is the PI? Tone stack recovery? Am I anywhere near correct on this?
     
    Last edited: Jan 28, 2020
  8. Wally

    Wally Telefied Ad Free Member

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    I would use a multi section can, but there may be only two caps sections in that can. There is not a lot of room there for single caps....from what I can see. One could spend some time changing that wiring dress...separate the heater filament supply from the power supply wiring to the tubes, etc.
     
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