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Vintage Fender Bassman questions

Discussion in 'Amps [BG]' started by Eric_Allard, Feb 12, 2014.

  1. Eric_Allard

    Eric_Allard

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    I just bought a 1969 "Drip edge" silverface Fender Basman amp. Before everyone chimes in that it's not really a bass amp and all that: I have a GK 700RB-II with a Neo 212 and Neo 410 cabinets so I have decent bass amplification for live use; I was looking for something smaller for my living room that could hopefully sound good for both guitar and bass. The original 2x15 "tall" piggy-back cabinet was included in the sale, however, as expected it sounds awful for bass (though surprisingly good for guitar) and somewhat contradicts my goal of a smaller living room amp.

    Now to my questions (hopefully the electronics wizards will chime in):

    I have identified the circuit as the AC568. I know our six-string brethren are fond of modifying these amps. What do I have to gain modifying it as a bass player and which mods might be recommended? The main thing I'm aware of is changing the bias circuitry to a proper bias rather than the balance circuitry that is in there now. Beyond that, of all of the modifications suggested elsewhere, which are worth doing? My concern is that many of the sources for these mods are more guitar oriented and I wonder if their intention is to give the amp less clean headroom rather than more.

    Also, regarding the electrolytic filter capacitors. I have read that these tend to go bad after 15-20 years. In this case, they exhibit none of the telltale signs of failure (hissing and crackling through the amp, bubbling, leaking, discolouration, etc.). Is it necessary to change them? What would be the consequence of failure? Would it take out a bunch of components with it? I have ordered replacements, but am more curious than anything.

    Is there anything that can be done with that speaker cabinet? Would it sound decent with more modern, efficient 15" drivers? Using Bassbox Pro, I modeled the cab with Eminence Delta Pro 15's and found that it would get an F3 of approximately 80-85 Hz and approximately 12dB/octave low frequency rolloff and efficiency of approximately 102-103 dB. On paper, the low frequency response wouldn't be much worse than say an Ampeg 8x10 Classic (who really believes the claimed F3 of 60 Hz, anyway) with significantly better sensitivity. Like I said, on paper it sounds like it might work, but I'm curious if anyone has done it and their results. 2x15's aren't really my thing (not so much the sound, more so just the awkward lugging and weight), but this head and cabinet have been together for 45 years, it would be a shame to separate them now. I would likely change the particle board baffle with plywood prior to performing the speaker change (that thing would probably explode with modern drivers).

    I'll post more questions as they come to me.

    Thanks in advance for your help as I refurbish this thing (though it doesn't need much refurbishing, the head is near mint). I would post pictures if I could figure out how...

    Eric
  2. 4Mal

    4Mal Supporting Member

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    Your bassman is an awesome bass amp, just not very loud. I would have a well regarded tech run through it and see that it is tip top. The cab... Is pretty close to worthless as a bass cab. Put it in storage and locate a clone of the Bandmaster 2x12 sealed cab. This is the small cab with tilt back legs. Load it with JBL d120 eqivalents (Weber California's for instance) or EV's or Altec's if you can find them. It will weigh a metric ton but if it is studio/ rehearsal space then you won't move it often.

    Eagle Works (now defunct) in Portland OR made very nice copies of the small box 2x12. I ran one loaded with Altecs under the Bassman in my Avatar. Not loud but gorgeous tone.
  3. nashvillebill

    nashvillebill

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    As far as the filter capacitors, as they age and dry out their failure can be manifested in several ways. Sometimes their failure is so gradual people don't realize it. When they really get bad, there can be a lot of hum, and reduced power output, but even before that, in some amps the reduced power supply filtering can cause certain notes (like the low A) to have weird overtones.

    As electrolytic cathode bypass caps fail, the frequency response of that gain stage will suffer, meaning less treble gain. Some folks forget to change these.

    Coupling caps, which are generally not electrolytics, can start leaking electrically in older amps; their symptoms can be crackling...or toasted tubes.
  4. Eric_Allard

    Eric_Allard

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    Thank you both for your responses.

    I actually have been planning to build a 2x12 cabinet for this thing. I have a pair of Eminence Delta Pro 12A's I'm not using. They're known for being very similar to an old EV model (the EVM12L, I think). They sound great for bass when tuned correctly and I've since read that a lot of guitarists like them too. Their efficiency will help me get the most from this low wattage head. I plan on matching the fender piggy-back cab aesthetics. I think I'll make the cabinet 34" (or 36") by 22" by 16", that way, I can use it as a tall cab or a short cab like the old 2x15 was designed. I have them built into a cabinet now and plan to test them with this amp as soon as I can wire in a 1/4" jack (I only have a speakon in there now).

    I think that you're right about the 2x15. While it's cool that I have the original speaker cabinet that came with this amp, I shouldn't waste any money on this thing; just store it.

    Regarding an amp tech, I'm hoping to read up enough that I feel comfortable enough to do this work myself. I feel vintage amps are kind of like vintage cars: use them if you want, but don't go running to a mechanic everything something goes wrong... you have to fix these things yourself. Although I know a few good amp techs that will give me a primer to get me started and bail me out if I need it.

    All of the research I have done so far say that it's pretty much imperative that I change the filter capacitor and the bias supply capacitor. Many sources also indicate that the capacitors Fender was using for coupling capacitors and signal capacitors tend to drift over time and should be replaced (yes, my amp has the notorious "dog turds" capacitors).

    What really surprised me was the tubes: it still has the factory original 45-year-old RCA tubes. I had them tested and one of the preamp tubes was weak, but the power tubes tested as "very strong". Plugged into an efficient cabinet (my GK NEO 410) this thing could easily keep up with a drummer and a guitarist playing a smaller amp like Deluxe Reverb.
  5. nashvillebill

    nashvillebill

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    I would not throw out that preamp tube yet! Many tube testers have difficulty testing 7025/12AX7 tubes "properly", especially if the tube tester hasn't been refurbished and calibrated.
  6. Eric_Allard

    Eric_Allard

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    I just put in in position 2 (the guitar preamp). I figured it might help get a little more breakup with the six-string.
  7. Jim C

    Jim C Supporting Member

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    Geat amps; I've had a few as well as a highly modified hot rod version.
    Not enough power for me for gigging and had less umph than a V4B.
  8. beans-on-toast

    beans-on-toast

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    Since you want to use the amp regularly, I would change all the electrolytic caps. In addition to the signs that you listed, hum and a lack of headroom are things to look for. A tech would measure their resistance with an ESR meter. High ESR is a sign of an issue.

    If the amp has been sitting a long time, the caps might check out, but when you start using the amp and pushing it, they could deteriorate in short order.

    This is a very nice amp. Connect it a bass cabinet and it will sound good and serve your purpose.

    As for mods for bass. I'd get to know the amp as is. If you find it lacking, you could try optimizing the tone stack. Compare what you have to a bass amp for guidance. Some people would add a bit more capacitance to the power supply: the two 70uF/350 caps could be increased to 80uF-100, the 20uF/525 after the choke could be increased to 40uF-47uF. This would give you more low end and headroom for bass but would sound good with guitar as well. About 47uF on each side of the choke would be a good target. Then there is the possibility of a beefier output transformer that will help with less distortion in the low end. www.mercurymagnetics.com sells oversized transformers but this can get expensive.

    Whatever mods you do, they are reversible. The amp can serve as a fun testbed for tweaking.
  9. Eric_Allard

    Eric_Allard

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    It is often suggested with these amps to modify the preamp and power section to the AB165 (early silverface) and AA864 (blackface) circuits, respectively. With my AC568, it should be relatively simple to perform these tasks, but what will it do to the tone and headroom when used as a bass amp?

    It sounds really good as is without modifications, but I can't help but wondering if there are significant gains from doing this. It appears to be pretty much universally acknowledged as being an awesome mod for guitar use, but I'm hoping someone can comment on the difference between the blackface and silverface tones as well as available headroom for bass use.

    Thanks again for the help.
    Eric
  10. Eric_Allard

    Eric_Allard

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    I think I figures out pictures:

    [​IMG]
    Here's the head cabinet after about a days worth of scrubbing filth out of the tolex texture, gluing down all of the lifted edges, scrubbing the rust off of the chrome, etc.

    [​IMG]
    Here's the head and cabinet.

    [​IMG]
    An obligatory gut shot.
  11. Eric_Allard

    Eric_Allard

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    Crap...I guess not.
  12. hennessybass

    hennessybass Supporting Member

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    I have a 70s Bassman 100, and gig it regularly. Started gigging it right when I got it, but went to the tech about two or three times during the first year I owned and was gigging it. By now, all the caps have been replaced, power tubes, and most (I think all except one) of the pre tubes have been changed. I gigged it for well over a year before one of the original pre tubes started to make any noise.

    I'm with Beans-on-Toast when he says, "get to know the amp as is." To me, tube amps have a personality, and like an instrument, you can get to know them, and figure out how to "play" them. After gigging mine for quite a while, I talked to my tech about what I though was a pretty massive low end, and he changed a resistor value that "tightened up the low end, and let a little more mids come through." Perfect.

    Enjoy that amp, and don't let anyone tell you that it is not a bass amp. It's one of the best.
  13. hennessybass

    hennessybass Supporting Member

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    My guit player uses a silver face bassman, and he had a tech mod it to the blackface circut and says that the amp breaks up a lot earlier now. Can't comment much beyond that.
  14. Eric_Allard

    Eric_Allard

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    Thank you both for the replies. The "the amp breaks up a lot earlier now" is kind of what I want to avoid. Maybe strategic a choice of preamp tube in V2 might help the guitar channel and keep the bass channel clean.

    It is a fun amp. I plugged it in the other day just to check if it works and I wound up playing for three hours without noticing. I love my GK, but I agree that this thing has some personality of its own. I usually aim for some midrange growl and a fairly restrained bottom with the GK, but with the Bassman, I gravitated towards really fat, smooth tones. I cranked the bass to 10 and set the treble to about 5 with the "deep" switch to on. With my flatwound equipped Precision, I was getting an awesome Jamerson-like tone.

    Speaking of which, what does the "deep" switch do? It just sounds like a midrange cut to me. Is that about right?
  15. beans-on-toast

    beans-on-toast

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    Your amp has some extra capacitors around the power tubes. This is typical of what you see when they are try to fix issues that make the amp dirty or less controlled. To tame the high frequencies that the blackface has that make it sound good when driven hard.

    The tone stage differences are minor tweaks. As you said, they are minor and it wouldn't take much to try them. I tend to sit there all day clipping in and out resistors while tuning a circuit. It is a personal thing. What I like might not be what someone else is looking for. So you have to try these sort of things with your bass.

    For getting the most increase in headroom, try the power supply changes that I mentioned.
  16. hennessybass

    hennessybass Supporting Member

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    Not exactly sure what the deep switch does,,, all I know is that I have it on all the time, and it sounds good! Seem to really tighten up, and fatten up the sound.

    Here is a cool graphic from Alembic. Just to give a visual idea of what you are hearing. I play the bass channel of my amp, so only have trebble and bass, and I usually play with the deep switch on, trebble on somewhere between 0-3 and the bass around 3-5. I guess that gives me a little boost in the low mids and a little dip in the upper mids. Sounds good to me... but the thing I like about my fender, is no matter where you set the knobs, it still sounds good. Different, but good.

    [​IMG]
  17. Eric_Allard

    Eric_Allard

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    Thanks, I'll keep all that in mind.

    So... how do I post pictures? I looked at the FAQ, it mentioned something about an attachment manager, but I couldn't find it anywhere.

    I'm extremely pleased with how this thing cleaned up, but nobody at home cares. I have to show it off to someone!
  18. beans-on-toast

    beans-on-toast

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    When you are posting, at the bottom there is a button "manage attachments" that you click on to upload a pic. The space allocated to attachments is limited. People use image hosting services such as www.photobucket.com or www.imageshack.com. There are many image hosting sites. Upload your images to one of them. Link to the image within your post using the inert link function, the blue globe in the tools above the composing window.
  19. hennessybass

    hennessybass Supporting Member

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    got my pic up in my previous post.
  20. will33

    will33

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    Disclosures:
    Use of this field for any other purpose is prohibited
    I'll leave the intracacies of amp modding to the real techs here.

    When I had my 135 watt Bassman, I too looked up stuff about the deep switch. Some stuff I found said mid-cut, others said 50hz boost. I don't know if it's one, the other, or both.

    Anyway, sweet, sweet amp.

    I will say your DeltaPro 12's are very nice dual-purpose bass/guitar speakers, especially with that amp as you can fatten up the bass channel and leave the guitar channel set brighter, and play both instruments at the same time through the same rig (although that may be easier with 2 people. :D ).

    Get all that sorted and you'll be filling your home with beautiful sounds. Also useable on bass for small, intimate gigs settings, and of course, will handle anything up to, and including, open-air festival stages on guitar. :)

    Hint: If you want a reverb for the guitar side, the Electro-Harmonix Holy Grail pedal does a pretty convincing spring reverb sound for fractions of the cost of a real, tube-driven Fender spring reverb unit.

    Another hint: if you want to experiment with different tones while playing a single instrument, trying jumping the channels. Instrument into input 1 of the bass channel, patch cord from input 2 of the bass channel to input 1 of the normal channel, then fiddle with all the knobs and switches.

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