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Restoring a vinatge Bassman-Amp

Discussion in 'Amps and Cabs [BG]' started by Jim Ingraham, Nov 14, 2004.


  1. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    Shillington
    This is my latest folly:
    [​IMG]

    I bought this on Ebay as a '73 or '4. Not much details listed on the auction. Turns out its '68 with the Blackface AB165 circuit. Sweet :bassist: So i open the shipping box and am knocked out by the smell... someone had re-covered the thing with a cream tolex, glued down with Elmers glue and covered with mustard and ketchup( or blood) stains and reeking of 20 year old tobacco.. yycchhh turned it around and look in the back and its filled with nests... mouse nests , birds nests, bees nests... its like been out in somebodies shed for the last 15 years no doubt.
    Any rate, as bad as it looks and smells, after i blew out the nests and plugged her in she fired right up. A little crackly and with a pretty good hum, but not too bad. Plugged in my Pbass and instant warm tuby goodness issued forth. Good enough for now... so back out to my shop where i ripped off the offending covering, pulled the chasis and hardware and sent the guts off to my amp guy
    So today i got the amp chasis back from repair but im not ready with case yet (still waiting on Tolex and a few hardware replacement pieces)... so i temp slapped it back together to play with it all day. What a Sweeet amp!! :D No it doesnt get very loud(only 50 watts but more on this later), but for the volume it has in it, the tone is the best ive found yet.
    The preamp turns out to be remarkably versatile... you look at the controls, see volume, bass and treble for each channel and go mmmm big deal.... until you start to play around with it a bit. You have 2 channels, a deep and a bright and you can patch them together and dial in a mix of the two channels with the indvidual volume knobs. The range of tones seems limitless. And then you have a great sounding, grinding tube distortion sound right before it decends into unusable mush. All in all a most excellent amp for a couple of hundred bucks.
    Now on to the volume issue. The guy working on the electronic end of this project( and a fellow bass player of course) has developed a gizmo. He calls it a Tadpole. Its sort of a DI box but it plugs in to the speaker output on an amp, splits the signal and sends one half srtaight thru to another speaker output and the other goes thru a bunch of little electronic parts that some how take current away but leave volts...i dont understand how it works really. bottom line is ...outa your speaker output you get another speaker output plus a line level signal output. This isnt really a preamp out... its more like taking the entire output of the amp including the power tubes and reducing it to a usable strength signal, which you can send to a PA or another power amp plugged into more speakers. The possibilities seem very intriguing to me.
    Unfortunately, i dont have this Tadpole thingy yet, so it remains to be seen how it will work, but stay tuned as i shoud have it by next weekend. :bassist:
    Ill post updates of the restoration as it goes along if anyone is interested.
     
  2. James Hart

    James Hart

    Feb 1, 2002
    toms_river.nj.us
    Endorsing Artist: see profile
    Very cool!!! It kinda looks cool in the buff :smug:

    FYI: most DI can take speaker level. Both the ones I'm looking at will (Avalon U5 and Radial JDI)
     
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    The Tadpole is probably a very simple design. Hook up a voltage divider (two resistors) across your speaker outputs, and voila, an instant "poor man's line out".

    Here's an example:

    http://www1.korksoft.com/~schem/gibsonamps/ga19rvt.pdf

    Check the far right of the schematic, where the speaker is connected to the output transformer. The two resistors are connected across the speaker ouput. Basically you just adjust the values of the resistors until you get the signal level you want. You can replace the bottom resistor with a pot to give you more flexibility. The Falcon uses a 100:1 resistor ratio, which is pretty typical and should work fine with a Bassman.

    Here's a couple of other thoughts. On my '65 AB165, I modded the bass channel to run "straight through" like an old Fender tweed amp (ie removed the tone stack, and added a voltage divider to reduce the gain to an acceptable level). This mod is totally awesome when you're bridging the channels. Give you lots of flexibility with both the crunchiness and the tone. Turns out the second tone stack doesn't do a whole lot for you when you're bridging (at least, a whole lot "more" than just one tone stack would do).

    The other thought is, if you're going to be flipping sounds a lot, take out all those extra inputs on the front panel and replace them with switches. I found some toggle switches on an old Neve distribution amp that fit perfectly into the 1/4" holes. Now I have one input jack and three switches. The first switch flips between channels A and B. The second switch does "one" or "both" channels. The third switch takes the bass-channel tone stack in or out of the circuit. Works great! :bassist:
     
  4. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    Shillington
    @Nonsqtr:
    great reply, thanks for a bunch of great ideas. :hyper:
    Unfortunately, my amp guy, while he's very good, will give me a very hard time and stern lecture about any mods that alter the appearance (ie. replacing jacks with switches) :ninja:
    Id like to pick your brain a bit if i could:
    How will the Tadpole idea work in real life? Lets say im using my bassman bassically as a preamp into my QSC bridged into one of my bigger sets of speakers. If i set the Bassman volume to just below distrtion threshold, will the quality of the tone remain intact but just louder? Have you ever tried this?
    Also, lets say i find several settings i really like by monkeying around with the 2 channels.... is there a way of inserting a footswitch into the loop somehow that i could switch back and forth?
    I'd like to try the the "straight thru" mod. Could you point me to some schematics of this ?
    thanks again for your interest and help :bassist: :bassist: :bassist:
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Don't sweat the resale value, as this is a CBS vintage unit that doesn't have much anyway; a Blackface from '65 would have five times the value even though the basic circuitry is identical. The main mod to consider is the B+ filter caps. The originals need replacing anyway being so old. Cap technology has advanced a lot since then and you should be able to find new ones with much higher capacity than the old that will still fit in the tank. Also check the chassis for an open space to fit another tube socket by the power transformer, you may be able to replace the SS diode bridge with a tube rectifier, which adds a lot to the tone at high volumes, giving a natural compression. You can also do a bi-amp thing with the pre, running the bright channel normally to drive a midbass box with the internal power amp while having the deep go to a switched jack to go out to a separate amp to drive a sub. You also can derive a balanced out by tapping each side of the inverter tube in the power amp section, isolating it with caps and dropping the voltage with a fixed divider network or a stereo volume pot.
     
  6. Jim Ingraham

    Jim Ingraham

    Nov 14, 2002
    Shillington
    thanks Bill
    I dont understand a lot of what your talking about but im trying to have my amp guy sign up and join in the discussion.
    My plan is to take one line out to a separate power amp nested into some vintage "Fender" looking new 15" boxes which ill build to match the amp. these will be small and light... modelled on Bagend s15d's... but ill put Fender fronts on em. these will give me (i hope) good stage volume.
    Then ill run another out to the board and from there itll come out thru the Tuba24.
    I dont know if any of this will work... but ill have a bunch of fun trying :bassist: :bassist:
     
  7. I'm assuming you're concerned with volume, but it may end up being a non-issue... have you tried the head in a band setting yet?

    I have a 50 watt Marshall from a few years earlier than that and it's the loudest amp I've ever played through. It doesn't seem loud by itself, but it buries everyone in a live band setting.
     
  8. j.s.basuki

    j.s.basuki Supporting Member

    May 14, 2000
    asia/australia
    I bought Bassman 70 of '70s, and in good condition last week. The tone is the best , it has 2 x 15 " speaker on sealed cabinet. I do not have to do anything since all in good condition. I paid for approx $ 700. I experience a little bit click clack few minutes after switching it on, do I need to change the tubes. Could you help to let me know where to buy?
     
  9. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    If done correctly (or "well"), the switch mod is completely reversible. If you ever want to sell your amp, just take the switches out and put the jacks back in. :)

    Yes, that's the idea. "Intact but louder". I've tried this, and it works well. (Sorry if I'm stating the obvious, but "never run the Bassman without a proper speaker load" - just the resistors by themselves don't constitute a proper load, so you'll still need to use a speaker with the Bassman if you're going this route).

    Yes, this is possible. But not easy. Basically it involves replacing the tone pots with multi-position switches, and then using a relay (or its solid state equivalent) that's driven by the footswitch. Another more elaborate approach would be to replace the pots with MIDI modules, which you could then control with a MIDI footswitch. These mods are pretty elaborate though.

    Yep. Drop me an e-mail and I'll send you some schematics. :)
     
  10. BigRed

    BigRed

    Apr 1, 2004
    Palestine, TX
    Stain it and keep it "woody" looking. That would be sweet!
     
  11. BassGreaser

    BassGreaser

    Aug 22, 2002
    Austin, TX
    I'm having my Bassman 135's power section retubed with S.E.D winged C's. I was actually going to do the woody look with mine.
     
  12. Passinwind

    Passinwind I Know Nothing Supporting Member

    Jim: The guy working on the electronic end of this project( and a fellow bass player of course) has developed a gizmo. He calls it a Tadpole. Its sort of a DI box but it plugs in to the speaker output on an amp, splits the signal and sends one half straight thru to another speaker output and the other goes thru a bunch of little electronic parts that some how take current away but leave volts...i dont understand how it works really. bottom line is ...outa your speaker output you get another speaker output plus a line level signal output. This isnt really a preamp out... its more like taking the entire output of the amp including the power tubes and reducing it to a usable strength signal, which you can send to a PA or another power amp plugged into more speakers. The possibilities seem very intriguing to me.

    What you're describing has been a standard feature on many Fender amps over the years. It takes about 50 cents in parts, not counting the output jack, to do this. There are plenty of other versions possible, so your friend could be on to something worthwhile and original.

    Bill: Also check the chassis for an open space to fit another tube socket by the power transformer, you may be able to replace the SS diode bridge with a tube rectifier, which adds a lot to the tone at high volumes, giving a natural compression.

    If you do that, you'll need to add a 5 volt filament transformer for the tube rectifier. The vintage deWds will vilify you for drilling holes, even though the amp's not worth all that much. Ted Weber makes solid state rectifiers with various sag characteristics, which would be an easier and cheaper way to get the same result, more or less. That would still require an extra socket though. If you like how it sounds now, your tech probably already re-capped it. I'd leave it alone, personally.

    nonsqtr: On my '65 AB165, I modded the bass channel to run "straight through" like an old Fender tweed amp (ie removed the tone stack, and added a voltage divider to reduce the gain to an acceptable level). This mod is totally awesome when you're bridging the channels. Give you lots of flexibility with both the crunchiness and the tone. Turns out the second tone stack doesn't do a whole lot for you when you're bridging (at least, a whole lot "more" than just one tone stack would do).

    Good idea, and you could add a simple treble rolloff control to that pretty easily if you wanted to. I like Baxandall tonestacks as an alternative too (think Ampeg B-15), and that could probably be done on the second channel with a little effort.

    Jim, I realize this is Greek to you, but perhaps your tech will at least consider some of these ideas. The Bassman is a great vehicle for mods. I might have to snag one myself, actually. :ninja:
     
  13. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The filament voltage would be tapped off the same rail that supplies all the rest of the tubes. Drilling for the tube is probably not necessary as many Fenders of the day used chassis that already had a (capped) hole for another socket, which was not used after the switch was made to SS diodes. Getting the SS diodes out and a tube rectifier in is usually one of the first mods done to CBS Fenders to restore the tone of the early '60s originals.