Results 1 to 11 of 11

Thread: Gulbransen APS-25 tube amplifier conversion

  1. #1
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Gulbransen APS-25 tube amplifier conversion

    I've recently acquired an APS-25 tube amplifier from a working Gulbransen organ. I am wanting to convert it to a guitar amplifier. I managed a partial schematic, and have lots of pictures but seem to be able to find very little information about it. It runs a 6CA4 rectifier, a 12AX7 preamplifier, and 2) EL84's for output. The amplifier is in really clean condition and was working (making sound), I made sure I kept all of the wiring connections. I know there are 2 power leads that run from the chassis the plug one is to power the motor for the Leslie speaker, and the other was for an ad-on rhythm generator. I'm sure both of these can be removed. I'm thinking speakers wired to speaker leads and input applied at input jack and I should have sound. Here are some pics of the amp I removed the tubes while I'm playing around with it. As soon as I can figure out how to attach photos I will, this is my first post and I am new to the site. I am also knowledgable but new to tube amps. Any warnings, tips, specs, or other information would be greatly appreciated.

  2. #2
    Forum Member CoyotesGator's Avatar
    Join Date
    Jul 2012
    Location
    The Institute for the Very, Very Nervous
    Posts
    847

    Re: Gulbransen APS-25 tube amplifier conversion



    once you host the pic at the site of your choice, click the picture framed tree tab above and link it directly.
    What happend?
    Who let the magic smoke out?

  3. #3
    Forum Member Don's Avatar
    Join Date
    Jul 2002
    Location
    Massachusetts
    Posts
    11,288

    Re: Gulbransen APS-25 tube amplifier conversion

    I'd love to see pics of this!

  4. #4
    Forum Member DanTheBluesMan's Avatar
    Join Date
    Nov 2002
    Location
    Southern NH
    Posts
    4,717

    Re: Gulbransen APS-25 tube amplifier conversion

    likewise calling for pics, too

    I hope you can make this work. Some cool stuff can come from prepurposed vintage tube gear. Is it always cost effective? Not necessarily but that doesn't eliminate the cool gear factor

  5. #5
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Re: Gulbransen APS-25 tube amplifier conversion

    I can't upload pictures from my computer files? I don't understand what a host site is. You mean like Facebook or do you mean I need my own webpage or something like that?

  6. #6
    Forum Member chuckocaster's Avatar
    Join Date
    Sep 2003
    Location
    spanish for lard.
    Posts
    8,605

    Re: Gulbransen APS-25 tube amplifier conversion

    Photobucket or something like that, TFF doesn't host pics.
    "don't worry, i'm a professional!"

  7. #7
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Re: Gulbransen APS-25 tube amplifier conversion

    Ok I will look into photobucket.

  8. #8
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Re: Gulbransen APS-25 tube amplifier conversion

    OK I hope I did this right. If I did here are some pictures of the amplifier. http://s1174.photobucket.com/user/mu...en%20amplifier this link should take you there.

  9. #9
    Forum Member CoyotesGator's Avatar
    Join Date
    Jul 2012
    Location
    The Institute for the Very, Very Nervous
    Posts
    847

    Re: Gulbransen APS-25 tube amplifier conversion

    Thats a cool start.

    Have you worked with kill you dead voltages before?

    What would you want it to sound like?
    What happend?
    Who let the magic smoke out?

  10. #10
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Re: Gulbransen APS-25 tube amplifier conversion

    Limited experience lots of schooling long ago. I know those caps can be bad news. I know nothing at all about the amp specs I just have a partial schematic. Enough to know the components but no voltage, or current values, etc. I was told Fender bought out Gulbransen, so I am hoping somewhere there is some kind of specs to be found. As for how I want it to sound from all that I read and remember from schooling that is the marvelous thing about tube amplifiers. Output characteristics can be altered with not only tubes of different manufacturers but alternate tubes as well (12ax7 vs 12at7 or a 5751). I am looking to be able to play around with the tubing until I feel happy with the sound.

  11. #11
    Forum Member
    Join Date
    Jun 2017
    Location
    Grants Pass Oregon
    Posts
    12

    Re: Gulbransen APS-25 tube amplifier conversion

    Ok I placed a 1/4 inch mono jack to the speaker leads and to the RCA input. I figured simple in and out should do for a start. Tubes lit up plugged in and I got nothing. I know for a fact it was working before I pulled it. So I am thinking one or more of the wires from the control plug is going to be the answer. I'm sure glad I kept the wiring harness attached. I have followed and id'd 6 of the 12 plug leads and wish there was some kind of literature on the pin out. The wiring goes as follows:
    1 2 3 4
    5 6 7 8 ^ TOP OF PLUG ^
    9 10 11 12

    1-Black wire ?
    2- Green/white stripe x2 one leads to one side of the I/o indicator light, the other to a trimmer in line w/volume expression pedal
    3-White/black stripe goes to one side of I/o switch
    4-White/black stripe goes to other side of I/o switch
    5-White/red stripe goes to Leslie speaker motor
    6-Tan wire ?
    7-White x2 one runs to the I/o indicator light ? the other
    8-White/red stripe goes to Leslie speaker motor
    9-Red + speaker lead
    10-Black - speaker lead
    11-White/red stripe ?
    12-Brown/white stripe ?

    So pin numbers 1,2,6,7,11 and12 are all the ones I really don't know where they went nor what they do and actually I'm guessing pretty assuredly that pin 2 green/white is going to be involved with volume control. Again any advice, tips, knowledge, or experience will be welcomed thanks.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •