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Measuring the Fender Bassman 20

Discussion in 'Amps and Cabs [BG]' started by corsa, Aug 3, 2012.


  1. corsa

    corsa Supporting Member

    Jul 12, 2010
    Fort Wright, KY
    A little over a week ago, Fuzzy Beard brought his Bassman 20 over for the amateur RTA treatment and here's what we found. As I stated the last time I did this with my 6G6-B Bassman, please feel free to comment, critique, offer interpretations etc. These are my amateur untrained observations. (Also, please ignore the 60 Hz spike of my mains)

    The Bassman 20 has just four controls - Treble, Mid, Bass and Volume. We started out by profiling the individual controls to get an idea of the frequency range that each one affected. The following graphs were captured with a speaker-level direct box sitting between the amp and cabinet and feeding into TrueRTA. We discovered in this process that setting any of the tone controls to 1 effectively disables that control with the response so far down in the noise as to be unnoticeable.

    This first graph combines the Treble and Mid control measurements. Settings are as follows:

    Orange line - Treble 1, Mid 5, Bass 1
    Blue line - Treble 1, Mid 10, Bass 1
    Purple line - Treble 5, Mid 1, Bass 1
    Yellow line - Treble 10, Mid 1, Bass 1

    [​IMG]

    As you can see, there is considerable overlap of the Mid and Treble controls. The Mid set to 10 brings the high end up almost to the same level as when the Treble is set to 5. I was also struck by how identical the curves were as the controls were increased. With the exception of the Treble curve flattening out above 1.5kHz when the dial was set to 10, the curves remain fairly uniform and seem to raise all frequencies in the spectrum about the same amount. I also noticed the roughness of the bass frequencies when Treble was set to 10.

    Having looked at Mid and Treble, I was surprised at what we found when profiling the Bass control. Settings are as follows:

    Green line - Treble 1, Mid 1, Bass 5
    Purple line - Treble 1, Mid 1, Bass 10

    [​IMG]

    The Bass control only affects frequencies up to around 200 Hz.

    For our last control profile, we measured the Volume control. This was especially puzzling to me since, above a setting of 2, the Volume only seemed to affect the bass frequencies. In this graph, the controls are set as follows:

    Treble 1, Mid 6, Bass 4 for all measurements.
    Purple line - Volume 2
    Each line above Purple is the Volume raised in increments of 1.

    I'm not sure how to explain this result.

    [​IMG]

    For our next measurement, we raised all the tone controls together in increments of 1 starting with all tone controls set to 2. The Purple line is Treble 2, Mid 2, Bass 2. The yellow line is Treble 3, Mid 3, Bass 3 etc.

    You can see a fairly consistent curve with the high end flattening out above 1.5kHz at settings of 9 and 10.

    [​IMG]

    Finally we measured the speaker response against the amp output curve. In this graph, the Green line is Fuzzy's preferred tone setting of Treble 1, Mid 6, Bass 4 and a volume setting of 4. The Purple line is the speaker response, smoothed by 1/6th octave, measured with a calibrated measurement mic at 36", with the amp on a stand in the middle of the room. The room is untreated and there are artifacts galore but you get the general idea.

    [​IMG]

    I was struck by how evenly the speaker response rose with the amp. As you can see, the speaker was done above 3kHz.

    This is a very cool little amp. We noticed the same bump around 100 Hz or so that I measured in my 6G6-B Bassman but it was more pronounced in the Bassman 20. After that, the similarities end. The amp sounds fantastic but I'm not sure I fully grasped the interaction of the tone controls. Even after all this measurement, I don't think I could dial in a specific curve using the numbers without a lot of trial and error.

    Also, the amp is working against a couple of disadvantages. First, at 18 watts, it's not very loud. Second, it runs a 15' speaker in a VERY small enclosure. A better speaker in an enclosure of suitable dimensions would make this an entirely different amp.

    I neglected to do an SPL measurement and I'm still trying to figure out if the Volume control measurement had a mistake in it somewhere but I had a blast working with this amp.
     
  2. Any thoughts out there?
     
  3. I have some thoughts about this -

    Love the tone of these - was lucky enough to have a friend that had one and at the same time I had a Bassman 10. We would set them up with the 20 on top of the 10 and play through both at the same time. That sound is burned into my brain.

    Sold my 10 (most stupid thing I have ever done in my entire bass life) and my friend sold his 20.

    One day if I have the money and time to pursue it, I would like to buy both a 10 and 20 and keep them until the day I die.

    (how do you like my thoughts so far?)

    About all the graphs and stuff - I find it interesting.

    And another thought that I have is this: Do you realize how few people let alone TB members have acutally had the luxury of playing this understated little tone monster?

    I think they should bring both the late 70's 10 and '82-'83 20 back into production in the new Pawn Shop line - and manufacture them exactly like the originals! Then old guys like me would snap them up in a heartbeat.

    Enjoy that 20. And don't ever sell it (unless you sell it to me).
     
  4. This amp is my love! When i had to sell most of my gear this is the one i kept! Sold the 65 Ampeg Sb-12, sold the 69 Ampeg B25, and the 71 Kustom 200. They were all great rigs! But just somthing about this one! The tone is me. Dont know if that makes sense?
     
    GreaserMatt likes this.
  5. No amp buffs out there?
     
  6. i like it, pretty much what you'd expect from a fender tone stack!
     
  7. nouroog

    nouroog

    Jan 14, 2010
    Paris, France
    I wish this could get more comments, because I find these results interesting and a little intriguing, even knowing how a passive tone stack works.
    Thanks Corsa and Fuzzy Beard for showing this !
    I wish I could have the opportunity to play a Bassman 20 once, it looks like a really unique little beast :)
     
  8. will33

    will33

    May 22, 2006
    austin,tx
    They are super cool little amps. I got to play a Musicmaster or whatever the 12 watt version preceding the bassman20 was, but the guy wouldn't sell it.

    Notice how low the treble kicks in, what we would call "upper mids".

    The volume level chart is the perfect example of why people say don't pay attention to the numbers on the dials. 1/2 way up doesn't mean 1/2 power, etc.

    Cool thread. Would like to see more of these of various amps.
     
  9. bobcruz

    bobcruz

    Mar 10, 2004
    CA
    +1. Very useful and informative. More please! :cool:
     
  10. Cool to see. The passive tone controls on my Studio Bass are the same no doubt?
     
  11. corsa

    corsa Supporting Member

    Jul 12, 2010
    Fort Wright, KY
    In the course of my trying to figure out my Bassman, a TB user pointed me to this tone stack calculator.

    http://www.duncanamps.com/tsc/

    It requires you to have reference to your amp schematic and it doesn't like to exit cleanly on my Windows 7 machine but I did eventually have limited success modelling the tone stack in my Bassman.

    It might be worth a shot for you.
     
  12. Found a Fender Bassman 20 locally. Will check it out after work, if I like it, will be adding it to the arsenal of cool bass amps!
     
  13. Well, I checked out the Fender Bassman 20 last night and it is really a great amp, IMO!

    I bought it, and it is in great condition. It has the blackface cover, and sounds wonderful with my Hofner basses and Kay reissue bass.

    Too bad they don't make these anymore. I have read how people would love to have an all tube combo, well this is it! And it weighs less than 40 lbs!!!
     
  14. I just joined the Bassman 20 club and man-O-man does this thing sound great! Mine is in near mint condition. All original tubes, never modded.

    I picked it up a week ago from TB'er Jim C, but just now got it home and plugged it in.

    Here she is:


    bassman.


    I am in tube heaven!
     
  15. Jim C

    Jim C Is that what you meant to play or is this jazz? Supporting Member

    Nov 29, 2008
    Bethesda, MD
    While I haven't used that amp much in the last 10 years, and now that I see it, I'm stumped why I sold such a clean tube bass amp.
    Glad it went to a great home though!
     
  16. xk49w

    xk49w

    Apr 13, 2008
  17. Hi.

    112?

    Insane price when compared with anything from that era/with tubes over here.
    I'm envious at You guys.

    Regards
    Sam
     
  18. That is a very good question. Maybe someone out there has an answer. Did they make these with a smaller speaker?? Or was this one "customized?"
     
  19. That one for sale at GC is a Bassman in a Champ (guitar) cabinet. The chassis is the exact same size.

    Guitarists love this amp but often mod the amp and change the speaker (or in this case, the entire cabinet)

    The Bassman 20, in original form, comes only with a 15 inch speaker in a closed back (sealed) cabinet.

    (Some folks also mount the amp in a head case and use a separate cab)


    It is our duty to claim all un-modded Bassman 20's before the guitarists get their hands on them!
     
    GreaserMatt likes this.

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