Bass: more tightness = less beauty, it seems.

Discussion in 'Shock Brother's DIY Amps' started by LetItGrowTone, May 22, 2019.

  1. LetItGrowTone

    LetItGrowTone TDPRI Member

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    I'm new to bass and new to circuits that would sound the way I want bass to sound.

    My cloning journey, this time, started with the idea of something like an Ampeg B-15, which is like an early '50s Pro with an extra gain stage and a very lossy tone stack. Since my chassis had the right sockets for the latter, I started there.

    Without a tone stack (just putting a voltage divider there, and another before the PI, so I could hear the amp), and with no NFB, cathode biased, and with all of the preamp triode cathodes unbypassed, it was at its *most* beautiful, but of course the bass was way too strong.

    I then went through my options for rolling off that bass.
    I found that I needed steep rolloff to try to get the low notes to behave practically (not *too* touch sensitive), while also allowing more singing on higher notes.

    I arrived at solid, well-behaved low notes but then the highs got too raspy.

    To make a long story short: The ouput section is now almost all the way to AA371 (but it still has cathode bias and reduced screen voltage, see '50 RCA Manual). 40Hz is very solid. It is still beautiful in a different way. By setting the volumes right, and by changing the NFB amount, there is versatility too. It's usable. EDIT: Haven't tried the third preamp gain stage yet.

    But maybe you will recognize this sentiment: I'm haunted by the memory of its earlier tweedish tones, but it wasn't practical. I'll have to get over it, I guess. I never "finished" an amp for 6-string, but I'm determined to finish because I really want to study bass.
     
    Last edited: May 22, 2019
  2. Les Gear

    Les Gear TDPRI Member

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    45-year bass player and occasional guitar-amp-builder here.

    Critical Fact #1: How it sounds in your practice or recording studio, how it sounds to you with a band and how it sounds to the audience with a band are three totally different things. That beautiful sound -- unsullied by any lossy EQ -- is statistically likely to sound thudlike onstage when situated next to someone smashing bronze platters with broom-handles.

    That's particularly true of the B15. I played a checkerboard B15N for several years and it just didn't have enough power to fall downhill. It's perfect for recording but simply cannot keep up in public.
    .

    I hope that this is some help somehow. Good luck with the build!
     
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  3. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Thank you for your reply.
    So right, though I haven't thought about that lately. Yes, I experienced how the delicious low end of my 6-string amps disappeared when I played with friends. So discouraging. I realize now that what I wanted then (playing mostly by myself) was something that sounded like a 6-string and a bass at the same time, and it was impressive *by itself*.

    I'll keep this issue in the foreground, and arrange a way to check it out.
     
  4. tubeswell

    tubeswell Friend of Leo's

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    ^What Les said^

    I used to play bass in a regular band for a few years in the '80s. My favourite live setup was a Jazz bass through a 300W solid state Bass amp (with a 5 band EQ) into a 250W 15" JBL Bass speaker in a ported cab. I prefer cleaner bass sounds without too much cluttery boominess. I set the 5-band EQ for mid scoop, but kept the bass dialled back in the tone stack, and got a predominantly middish punch that was good for slap bass and filling in the low background for rhythm section.

    For tube bass amp, that sort of sound can be delivered by the big Ampegs, Sunns or Fenders
     
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  5. LetItGrowTone

    LetItGrowTone TDPRI Member

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    tubeswell, you said "solid state".
    Received my bass and didn't have an amp. I used a solid state microphone amp (not designed to distort, and with input and output level controls), with good headphones.
    I was surprised at how beautiful it sounded, especially the lowest notes. To me, it just needed a little tube flavoring on the high notes. My appreciation of this clean amp is what guided me toward the kind of circuit I'm working on now.

    I don't intend to buy a solid state bass amp, but because of this experience my "religious" objection to it is gone.
     
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  6. philosofriend

    philosofriend Tele-Holic

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    As my friend told me, "My Les Paul has two pickups: one by the neck to play in my living room and one by the bridge to play at the gig."
     
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  7. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Final circuit is clone of Bassman 10, "Studio Channel", except:
    Cathode bias, 270V screens, 8400 ohm load mean the current draw doesn't change much so sag is very low and it's unusually tight. It does as I asked: solid lows and increasing tube flavoring on the highs. This output stage is HiFi circa 1950. Low power, home use only.
     
  8. LetItGrowTone

    LetItGrowTone TDPRI Member

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    I worded the thread title badly.
    I'll try again to tell you what I was thinking:

    After getting the first version of the amp working, and gaining experience with the needs of bass amplification (or *my* bass amplification needs I should say), I began reducing distortion because three things were bothering me:

    1) too much rasp. This was easy enough to fix.
    2) too much intermodulation distortion (or deciding how much is enough, or how many stages may compound it). Just *this* is what I meant by "beauty" in the thread title.
    3) poor speaker damping. But *this* is what I should have said, as this is what was killing what I meant by "tightness". This was the biggest problem. After a lot of study (and because I don't have an UL transformer to try that first), I have wired the 2*6L6 output as triodes. Practice amp power only.

    Now the speaker damping on the lowest notes I can say is "good enough" (also blocked ports in the cab).
    Rasp is zero.
    Amount of IM distortion is easily controlled with NFB.

    I'm certain that I always want well-defined lows. So in the future, if I want to sound more like circa '68 hard rock (through most of the bass-appropriate spectrum but keeping good speaker damping of the lows), I will use a DI and/or crossover of the lows into a SS amp.

    I want to study bass, not be an amp hobbyist, so I intend to button this up and use it now.
     
  9. strat a various

    strat a various Friend of Leo's

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    AFAIK, that Ampeg amp was designed with String Bass in mind. I play String Bass, and I will tell you, at small combo volumes, the Ampeg flip top sounds perfect. Recording on Bass ... whew, the most prolific studio Bass player, Carol Kaye, used a guitar amp.

    For venues, the requirements are all over the map. I use a 68 Bassman for Fender Bass, with one of four cabs, depending on the gig. None of these rigs will cover a loud venue, so it's up to the sound engineer to do his/her magic. The best sounding Bass amp I ever had for electric Bass was a SS Acoustic Control 150, a guitar amp. If I still had that Acoustic amp, I'd use the Bassman strictly for guitar.

    Amps need to match the type of instrument, playing style of the player, and intended use.
     
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  10. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Yes, I hope I'm converging to that.
     
  11. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Edited:
    Power tubes now have separate, unbypassed cathode resistors.
    Unfortunate that 6550s draw too much current for this circuit, because they are definitely more solid in the lows.
     
    Last edited: Jul 13, 2019
  12. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Over these several months I've learned a lot about what I want from a bass amp. I'm sorry it's just my own taste, and anything I say might not apply to you.

    I already told the story of how I basically followed in the footsteps of Fender, from '50 Pro to the cleanest Silverface Bassman. I stopped at the point where they made it UL, because I didn't have a UL transformer.
    I did study UL operation. But something I could do cheaply was to go "all the way" to the best speaker damping possible: wiring the output tubes as triode.

    Now, when you're doing traditional distorted 6-string circuits triodes can sound funny, but when you're going for clean they drop out and become mostly "transparent"; they don't sound untraditional, just clean.

    Above I said that I had left the power tube cathode resistors unbypassed, but I learned that that increases their output impedance in a big way (for example twofold according to the original Mullard EL34 data) and decreases speaker damping, so today I added 100uF each. Also switched to 6550A's (at the limit of my filament supply now), after learning that they needn't be biased as hot as I thought. Now the strength and clarity of the bass is probably close to as good as it's going to get with these transformers, and for the first time ever I can now play a good-sounding (low E + B on A string). It's just a test point it had never been close to passing.

    **
    Continuing on the subject of clean bass amplification: I had no idea how much I loved the Acoustic 360/361 (solid state). I also never knew how many of my favorite tones were recorded "direct". For SS amps this avoids the speakers and room acoustics etc, but for tube amps it also sidesteps the speaker damping problem.

    I'll be using speakers, and push-pull triodes do a good job driving them. The next step cleaner, if needed, would be a SS power amp, and the Acoustic 361 power amp is not impossibly complicated. I did build a simple SS amp in the past, a three transistor RF preamp.

    Never guessed that I would end up *here*: if I need more clarity I will go to a solid state power amp!

    **
    And speaking of the Acoustic 360 (SS preamp), I learned that it shares some magic ingredients with the SVT (and many others): switchable series LC filters for the "mid" control. While the P bass has some LC edge in it I had overestimated the amount, and I now believe that Acoustic 360 and SVT (and others) were *more* responsible for this edge than I had known. I think most people aren't aware of this.

    The 360/SVT mid control *idea* could be borrowed. But if needed, in the future, I won't forget that the 360 has only three transistors in the power supply, only seven in the preamp (and the rest in the fuzz section which I would omit).

    Never guessed that I would be here.
     
  13. strat a various

    strat a various Friend of Leo's

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    For a long time, my Bass amp was an Acoustic 150 guitar amp. With a good 1x15 cab, it covered all bases. Steve, (whom I met in the 80s), the main designer, went on to start SWR, if I'm not wrong. I have one of his SWR amps as well.
     
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  14. LetItGrowTone

    LetItGrowTone TDPRI Member

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    Should not be necessary:

    My two beautiful GE 6550As (wired as triodes) weren't matched well enough, so I switched to EL34s. Their plate resistance as a triode is only about 50% higher than 6550s, so low bass is almost as good (hard to tell), and I have three well-enough-matched pairs.

    If I want low bass to be even clearer (remind you I'm talking about a bass amp), I believe I know exactly what to do: KT120s, as triodes, have a plate resistance about 1/6th that of EL34s (don't quote me, as it's an extrapolation). I don't know how well speaker damping tracks with that, but is probably several times better.
    I also don't know how 2 PP KT120s would compare to 12 PP EL34s (same plate resistance) with regard to speaker damping, as the latter would enjoy a much lower turns ratio, but the form factor of the former appeals to me more! It would require new transformers so I won't be doing it now.

    ***
    I told you that I was admiring some very clean bass tones that had been recorded with SS and/or were recorded DI (which I had never realized before), and I was wondering how in the world they could sound so good, when I suddenly realized/remembered that they probably went through a tube console/mixer. Some of these were: John Paul Jones on Ramble On (SS amp, isolated bass available on Youtube), Carl Radle on Layla (a few seconds of which, almost isolated, can be heard on Youtube (never realized it was played with a pick)) and Let It Grow from 461 Ocean Blvd. (probably a tube amp but probably DI).
    So, there's no need (for me) to build/buy a SS amp.

    My amp is sounding very clean AND very beautiful, and I'll post audio someday soon for your critique. But it's mostly just equivalent to a Bassman 10, Studio channel, with output tubes wired as triodes. So, nothing original.
     
  15. LetItGrowTone

    LetItGrowTone TDPRI Member

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    That was silly. I just provided bias balance (for zero hum at idle), and all is well. Wired as triodes, bass clean and strong.

    ***
    I have grommets coming today. Chassis never had them in three places since I've owned it. As it has accumulated some low-spec wiring over the years, and as the circuit is nearing its final configuration, it's a good opportunity to install the grommets! And, oh yeah, finally new tube sockets too! (The sockets I installed in this chassis around '88 were already old at that time, and endured heavy experimentation since then.)

    Agonized over wire selection (as I agonize over everything). Couldn't resist the 200C temperature rating of silicone, and 600V, so chose that. Goal is to finally be free of worrying about that.
     
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