Bassman 70 School

Discussion in 'Amps and Cabs' started by abovethewave, Feb 13, 2008.


  1. abovethewave

    abovethewave Member

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    Apr 23, 2007
    Location:
    Washington DC
    Hey folks,

    A few years ago, I picked up a '78 Bassman 70, SF obviously. I was looking for alot of headroom and a good base tube tone to build from with pedals. For this, the amp has been great. I got it for under $300 and it's in mint shape.

    Lately, both my skills and my ear are surpassing what I am getting back from this amp. Naturally, I am considering an overall upgrade. However, I will keep it in the Bassman family and get an AB165 BF next. Before that though, I wanted to do some real homework on the 70 to see what I had to work with. I will probably never sell it and will just experiment on it. I am a Mechanical Engineer with some fundamental electronics wherewithal. Before I go tearing apart an AB165, I would like to have some experience here.

    So, as I begin my research I have some questions. There have been a few good threads on the Bassman and lots of web resources; however, i still have questions:

    • Circuit Identification: From my research at FFG that the amp is one of these - AB165, AA568, AA270, AA371. I am assuming that it's not a 165. I am guessing likely a 371. I cannot find any info on the amp or the tube chart to suggest what it is. Is the best way to open and compare circuits to schematics? Are there any other indicators I should be looking for?
    • Discharge: Is there a "best practice" for draining off caps so I don't blow myself up? I can tell that this amp has been serviced and I want to see to what levels by opening it up.
    • Tubes: The tube chart reads (2) 6L6 GC Power Tubes, a (1) 12 AT7 phase inverter and (3) 7025 pre-amp tubes. However, it has (2) Svetlana 6L6 GC Power Tubes, a 12 AX7A (Made in China) phase inverter, a GT-12AX7 next to the phase inverter and then the last 2 are GT-7025. Can you tell me what this combination was intended for by the previous owner?
    • Channels 1&2: On this amp, I believe I read that the 2 channels for both the Bass and Normal side have different gain structures. I am assuming this to be true and that #2 is less gain.
    • Jumpers: Before I fry anything, I understand to jumper the # 2 Channels and that provides an ability to bleed in the Bass side using the Channel Volumes and at a lower gain and EQ point. Should I try this?
    • Mods: Without completely Blackfacing the amp, there are some suggested mods, such as: http://www.tone-lizard.com/Bassman_Modifications.htm. I am curious if these are even worth applying. I suspect that an adjustable bias would be the place to start.
    Thanks for entertaining questions from a newbie. Again if the amp simply isn't worth it, I won't even bother. However, I can find more flexibility and opportunities to improve performance, I wll. I also hope to find some BF Bassmans this weekend to try out.

    Thanks in advance for any comments!
     
  2. SatelliteAmps

    SatelliteAmps Member

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    Normal Heights, Calif
    • If you take the chassis out of the amplifier, look at the power tubes. If there are tiny little 2000pf caps on pin 5 going to ground it is the AA371 schematic. That is one of the easy visual things to look for to tell the difference between the schematics you have listed. If they aren't there you probably have the AB165 schematic.

    • If you are just looking (and not touching) you don't need to discharge the caps. If you want to poke around, I highly recommend using a small wooden dowel (sharpened with a pencil sharpener if you want). Do not use a pencil as the graphite can conduct electricity.

    • The 12AX7 in the phase inverter spot is to drive the tubes a bit harder to get more gain out of them. You can swap it back if you want, with no harm or modification needed. The GT-12AX7 is essentially a 7025. (A 7025 is a low noise version of a 12AX7 essentially)

    • Channels one and two do not have different gain structures. Some amps have high and low inputs, but they aren't different gain structures.

    • Yes you should try jumping the channels and seeing if you like it.

    • The Tone-Lizard guy is pretty on top of what he is saying. His mods are all pretty good, and they are all reversible. Might make your amp into something you would enjoy more.
     
  3. Screamin Willie

    Screamin Willie Member

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    Jul 1, 2007
    abovethewave,
    On your circuit identification question, comparing schematics, the Bassman 70 is not the same as any of the crkt id's you've listed. Have you found any other info regarding this?
    How's your project going?

    Inquiring Bassman 70 owners want to know :)
     
  4. drbob1

    drbob1 Silver Supporting Member

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    Aug 9, 2004
    It looks to me like the Bassman 70 and the Bassman 10 are very similar preamps. I've owned 10s, and they're pretty lackluster in the tone/gain department. I suspect that blackfacing them might be a bit more involved than the earlier circuit designs, although I have no personal experience with this. If you're not emotionally attached to the Bassman 70 circuit, why not take it a bit farther and build a white Bassman or Tweed Bassman circuit into that puppy?
     
  5. abovethewave

    abovethewave Member

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    Apr 23, 2007
    Location:
    Washington DC
    Screamin' Willie,

    I have not opened the amp as of yet. Knowing my tendency to get very involved in my projects, I am sure that there might be some considerable downtime once I start. I am current lylooking for a low watt (5-10w) amp. This is a whole other subject, but hopefully will have it soon... When that is here, I will dive in.


    I am glad that there are others interested in this amp. One of the reasons I stopped the search for info was that I picked up a Barber Barb EQ, which is an awesome pedal for this amp as the 70 is super clean with so much headroom. I also jumped the instrument channels which allowed me to shape the tone to a much greater degree. So my base setup is the TIM with the BBQ in the TIM's effects loop. This along with my new Tele has provided countless hours of entertainment as I have so many tonal variations at my fingertips. The Tele is the plus model with the coil splitter at the bridge.

    When I do dive into the Bassman, I believe that the Tweed voice is where I will begin. I will start by posting photos from inside the chassis.
     
  6. Screamin Willie

    Screamin Willie Member

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    Jul 1, 2007
    abovethewave, thanks for the feedback and good luck with all your projects.

    drbob1, I love the 70 the way it is for clean, but, I'm on the fence with the mods as I just can't seem to find an OD pedal I'm completely happy with so I'm leaning in that direction.

    Willie
     

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