Modding a Bassman 100

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by EightySix, Aug 14, 2008.

  1. EightySix

    EightySix Member

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    Hi everybody,

    Im looking for input on heavily modding this head

    I always liked the overall 'color' of the sound okay, just too clean for me because Im not using it for bass. I picked it up for $90 back in the day but I rarely used it because it was just overkill at the time. Now I can use the power, Im looking to mod the circuit and make it my workhorse.
    :drool

    Starting with the simple stuff first...


    -How much of a difference can being bias-able (and correctly biased) really make?

    -Any downside to running it on 2 6l6s for power breakup (Assuming the ohm load is correct)?

    Im thinking of running just the 2 inside 6l6s (now SEDs) and a higher ohm cab. I know its only 3db or so, I just want to hear the power section growl (After the other mods, below) If Im not mistaken these bassmen want to see a 4 ohm load normally, so I would need a regular 8 ohm cab, correct? ANY downside to pulling 2?



    Now, the not so simple stuff...


    -Is there a mod for the Master Volume with this or similar circuits besides just disconnecting it?

    I saw something about a mod to change the style of MV, something about pre and post? Is that worthwhile, or doable on these amps or would the guts have to be changed significantly? The reason Im tempted to keep the MV is for better sound at house levels, unless..

    -Would a Negative Feedback mod work well on this amp?

    Ive never heard an amp with this mod, that I know of. Can it make such a huge difference that it can be substituted as a gain control and just ditch the Master Volume? Or, again would the guts have to be changed significantly for this mod to work its mojo? Ive read about this mod being contolled by the old 1-n-2- Ground switch on the back, or by a pot to fine tune it.. Im guessing the pot-style has the edge?


    Beyond that Im still looking for ideas. Are there any simple mods that can be done on this circuit along with the mods Im considering to really wake it up? Or would the whole circuit have to be redone and just use the old transformers?

    Im not looking for a
    completely-over-the-top-distorted brown sound from the amp alone, especially since theres no channel switching anyway.. Im keeping my fuzz boxes. I thought about the Swartz mods but cant justify going all out, maybe if this head was in better shape, but its not. Im thinking of trying my hand at some mods, I have the patience, know how to discharge the caps and can solder. But first of all Im looking for SUGGESTIONS on how to help this particular amp out, whether I end up doing some myself just depends on how far I want to take it I guess.

    Info is hard to find on these amps.. Anybody that has done this or something similar, has some good links to share, knows somebody who knows somebody, somebody I should email, just want to shoot the sh*t and have knowledge, OTHER IDEAS I havent covered, etc everybody please chime in...

    Heres the original schem. for any wandering electricians, Im pretty sure mine is untouched because the sound was clean, clean cleeaaannn.

    http://www.drtube.com/schematics/fender/bassman100.gif

    :dude
     
    Last edited: Aug 15, 2008
  2. Blue Strat

    Blue Strat Member

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    UL amps are designed NOT to breakup (growl) so I don't think that pulling two of your power tubes will help much. This is really the wrong amp platform if you want growl.
     
  3. EightySix

    EightySix Member

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    BTW I should mention Im not talking to local amp techs because theres 0 known reputables ones in the Houston area, plus I dont trust anybody thats strictly after some scratch. I have more time than money.
     
  4. EightySix

    EightySix Member

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    Thats why Im going to have to mess around with the circuit. So far the people that Ive seen that mod these specific amps are going after even more gain than Im after, thats why Im positive just getting some decent breakup is doable.
     
    Last edited: Aug 14, 2008
  5. darkbluemurder

    darkbluemurder Member

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    Hi,

    Even though I have never owned such an amp I will give you some suggestions to try:

    1. Definitely remove those 2000pf caps that go from pin 5 of any power tube to ground. These caps kill highs. They are there to prevent oscillations which arose in some amps because Fender allegedly got sloppy on lead dress at that time. Most of the time they can be removed without any ill effect.

    2. I would remove that strange 0.0047uF cap between the treble wiper and the volume control. That kills bass big time - very strange feature for a bass amp.

    3. Since the amp has an UL power stage try to remove the negative feedback. Some of the Dr. Z amps (e.g. the Route 66) are UL and have no negative feedback. Since the stock feedback section is quite strong the amp should get much louder, brighter and grittier. See if you like it.

    4. If you like the tone without negative feedback then you could convert the master volume you have now to a cross-line master volume á lá Matchless/Bad Cat. They work a lot better in amps that do not have negative feedback.

    5. For more growl look to the phase inverter. Replace the 47k plate load resistors with 100k each, the 330k grid load resistors with 1 meg each, and the 33k tail resistor with a 47k. The junction of that 47k resistor and 0.1uF cap should go straight to ground (not through the 100 ohm resistor which was part of the negative feedback loop to be removed). After the parts change, try a 12AX7 for more gain.

    6. There is also a weird 220k in series with a 0.01uF cap going from the power tube grids (pin 5) to the negative feedback entry. I would remove that as well.

    7. Now, as a (preliminary) final step you could rearrange the bass channel (or the normal channel) to the following architecture: gain 1 - volume - gain 2 - tone controls. This would be more like Vox/Marshall only without the cathode follower and give more gain than the standard Fender architecture.

    8. Convert the deep switch to a midboost for more growl.

    That would certainly be a fun project.
    Good luck
    Stephan
     
  6. RedRock

    RedRock Member

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    I have converted a Bassman 100 to a Fender Custom Shop Tone Master.
    It sounds great. They both use only two pre-amp tubes and a phase
    inverter. A total of three 9 pin tubes. Well....you said you wanted
    to heavily mod it. The Tone Master schematic is not that complicated,
    and you could get distortion from the distortion channel in the pre-amp.
    You could also shunt some of the abundant top end in the distortion
    channel while you were at it. It also sounds nice with the standard
    Fender tone stack: 250pf, .047 and .1, or you could use a rotary
    control on the back of the chassis to choose between the stock
    Tone Master tone stack and the standard Fender, which sounds more
    full.
    See: http://www.schematicheaven.com/fenderamps/tonemaster.pdf

    You can skip the effects loops or install your idea of a good effects loop.
    The Tone Master effects loop transformers may be hard to find.
     
  7. darkbluemurder

    darkbluemurder Member

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    +1. After having posted above I also thought of converting this amp to Dual Professional specs without the 6V6-Reverb in front as another option.
     
  8. EightySix

    EightySix Member

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    Wow.. BIG ups to these guys

    Darkbluemurder, I appreciate your step-by-step, thats exactly what I need my first time around

    I wanted to get your take on this mod I found, in lieu of your #3 & 4-- http://www.regiscoyne.com/tech/resonance/

    Would this 'Resonance mod' kill off the possibility of it being a decent sounding house amp since I would lose the MV? If not, how would this mod affect your #5 and #6? Also on #6 did you mean remove both the 220k resistor and .01uf cap, or just the resistor?

    Im guessing the tonal differences between converting it to a 'dual professional/tone master' vs modding the original circuit is the dual professional would be better cleans/more headroom, sacrificing gain?
     
  9. jbltwin1

    jbltwin1 Member

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    Uh, 100's aren't UL as far as I remember. That started with the 135 watt models. If I'm wrong, I apologize but that's a standard 100 power section. Master volume yes, UL, no.
     
  10. EightySix

    EightySix Member

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    I think youre right, the 100 is just clean because its a weird circuit, not because of the transformers. I emailed a moderator to edit the topic title
     
    Last edited: Aug 15, 2008
  11. doctorx

    doctorx Member

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    Yes, the Bassman 100 is not UL.

    I rebuilt my B100 to resemble the AB763 Fender Blackface circuit. The amp now is basically a Super Reverb pre in one channel, and a Twin/Showman in the other, but both without any reverb or tremolo. Plus being a 100 watt amp it is a lot louder: You should hear this thing through a 4x12, the sky cracks and the heavens fall.

    The resonance control is similar to the "Soul Control"; It is a variable negative feedback control, but it only affects the low end. On one end of the spectrum you get crunch and OD, the other is clean spanky Fender tone. I like it.

    You could implement a better master volume but the front end of this amp is is not high gain and I don't think it would sound all that. I don't like master volume amps anyway so I'm biased. ;) I prefer using an attenuator or a smaller amp.

    Pulling two power tubes will give you a slight reduction but it will still be stupid loud, this amp has a great deal of power. Mine sounds great for playing Hendrix/Trower/SRV kind of stuff I think because this amp has a massive OT.

    My 100 is a 76, and by that time CBS/Fender QC had really deteriorated. Sloppy wiring, inferior parts and chassis, etc. I was going to clean up the wiring but when I did the rebuild and got it working it sounded fine so I left it as is.

    Good luck,

    regis
     
  12. RedRock

    RedRock Member

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    ______________________________________________________________

    In my opinion the Dual Professional is a clean amp, somewhat like
    a Twin Reverb with a Fender outboard Reverb unit built into the
    front end. Zinky achieved a beautiful tone with it.

    The Tone Master is a channel switcher. It also has a remarkably
    clear unique clean channel, and a separate distortion channel
    with a post phase inverter master volume that is relay switched into the
    circuit only when the distortion channel is on. The distortion channel
    is capable of delivering anything from light distortion to truly huge
    levels of distortion depending on the setting of its gain knob.
    This channel is somewhat bright, but the top end can be tweaked
    with caps (100 pf to 470pf) to ground at various locations, or by
    a cross-line high-cut circuit like the one found in the Vox AC30.
     
  13. darkbluemurder

    darkbluemurder Member

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    You are correct. No UL. The screens are tied through 470 ohm resistors to the transformer. I must have overlooked that. Sorry if this caused any confusion.

    Nevertheless my suggestions stay the same. The Dual Professional and Tone Master do not have negative feedback either even though they are not UL.

    @Eightysix: the "resonance" or "depth" control works like a presence control but for the low end. You could install that instead of the external speaker jack if you do not want to scrap the master volume. If you want the resonance control, however, you have to keep the negative feedback loop or it will not work. Whether you want to stick with the stock 820 series/100 shunt is up to you - you may want to increase the 820 to 1k5, 2k2 or up to 5k6. You could also temporarily wire a 25kB pot as a variable resistor in series with the 820R. Once you find out the setting you like most, take out the pot, measure the resistance and hardwire the next closest spec resistor in the circuit.

    If you choose to go without negative feedback you cannot have a regular presence or resonance control since they decouple the feedback in the high or low frequencies. That's why Vox/Matchless/Bad Cat use a Cut Control instead. This is a high frequency control in the power amp that works independent of the negative feedback loop.

    Both #5 and #6 can be done regardless of whether there is a resonance control or not. In #5 you could use 82k/100k or 91k/100k plate resistors if you keep the negative feedback. The 33k tail resistor could be changed to 22k (that would be Blackface spec) then also. With #6 I meant to remove both the 220k and the 0.01.
     
  14. Dezzy

    Dezzy Member

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    I know this is an old thread, but I HAVE A BASSMAN 100, AND WAS WONDERING IF u COULD TELL ME WHAT i WOULD HAVE TO DO TO GET IT A SUPER IN ONE CHANNEL, AND A TWIN/SHOWMAN IN THE OTHER ? i LIKE THE NORMAL CHANNELS CLEAN SOUND, THAT IS MORE MY THING, SO GETTING THE BASS CHANNEL TO SOUND LIKE A SUPER OR TWIN WOULD BE AWESOME, PLEASE CONTACT ME WITH DETAILS, I AM GRATFULL THANKS, EMAIL AS FOLLOWS IF THAT IS EASIER thementor@fsmail.net I am in the UK
     
  15. doctorx

    doctorx Member

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    The difference is, well, just a couple of caps in the tone stack. The Normal channel has a 500pf for the treble, .1 for the Bass, and .033 for the Midrange.

    The Bass channel has been completely rewired, it has a 250pf for the Treble, .1 for the Bass, and .047 for the Midrange. The Bass channel is now the typical AB763 preamp circuit, used in the Twin/Dual Showmans.

    The Normal channel is more or less like the early Super Reverbs, which had a .033 for the midrange cap although I went with the 500pf instead of a 250pf in the treble control along with the .033 mid cap to smooth things out and thicken it up. It sounds killer with a strat.

    The coupling cap between the preamp and power amp sections varied with all the fenders, I settled for a .001 to keep the sludge out of the bottom end.

    Also, I installed a resonance control, which is basically a presence control which affects the low end instead of the high end. It allows you to vary the amount of distortion of the amp from clean to crunch. I used the pot for the master volume for it.
     
  16. Moustache_Bash

    Moustache_Bash Member

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    Has anyone tried this mod on their Bassman 100? I'm curious to know if anyone digs it.
     
  17. kevmin

    kevmin Supporting Member

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    I know this is a lot of modding, but I would shoot for a high power tweed Twin with a built in "outboard" reverb in the front end. That would be sooo cool!
     
  18. Moustache_Bash

    Moustache_Bash Member

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    Another, probably dumb, question...

    For the resonance control, is the capacitor that one puts in measured in microfarads? Is it suppose to be a film or ceramic capacitor?

    Sorry, I'm a boob at such things.
     
  19. doctorx

    doctorx Member

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    Yes, it is microfarads.

    I used a 400 volt orange drop film cap.

    No, you are not a boob, it's a legitimate question.
     
  20. Alvis

    Alvis Member

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    Great sleeper amps. Fun to tweak. I re-wired mine into a tweed & put a 4-8-16 Tonemaster OP tranny in there.
    It's a beautiful sounding set-it & forget-it head. One of my favorite "Marshalls"
     

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