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Thread: Bass pre-amp

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    Cool Bass pre-amp

    Hi there,

    I've been thinking about building a bass pre-amp to amplify my signal to line level . I am going to use it for recording, going into an EMU 1820.

    I've found the Alembic F2-B schem at Geofex, and would love to build one. The only thing is, I can remember reading about a problem with this schematic.

    Would the Alembic be powerfull enough to feed into my soundcard (line level), and is the schem correct? Also, if any bass player here has built a nicer pre- feel free to say so, as long as you post the schematics as well.

    Link to schem: http://www.geofex.com/FX_images/alembpre.gif

    Rob

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    very good start

    The audio signal looks like a great start though you'll get plenty of voltage gain but possibly not enough current gain.

    The filiment is suspect, it looks like they are rectifying it and maybe floating it but with such a small unit you could just regulate it at 12v and eliminate a lot of hum.

    I tried something similar and built the power too close to the tubes and it caused hum.. I just prototyped a walwart power supply using a 500ma 18vac supply and a cascade rectifier to get 120vdc plate voltage.

    I used a 12vdc regulator for the filiment and haven't yet ordered the 120vdc voltage regulator for the plated but the hum was a lot better.

    I think my next proto will use a cathode follower and only one stage of gain since mine seemed to not drive the next thing in the pathway very well.

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    Yeah I was looking at the powersupply, I thought I might use a 12V AC wallwart, and split it, one way rectifying it and regulate 12v for the heater, the other into a transformer 12 to 220 or 230VAC, rectify for high B+ voltage. should be around 300VDC right?

    Rob

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    math

    If my math is correct rectified DC is 1.4 times the ac.

    If you get to 150 and above your golden though.

    I'll try and post the cascade rectifier here... I think it sucks a lot of current but the plate doesn't need hardly any so its okay..

    Ya the transformer is a good idea too but I would worry about EMF hum introduced into the circuit. I did the cascade for that reason but its not perfected so anything could work. If the unit was in a larger metal and you mount the tranny on top, no problems there I'll bet.

    The cascade...


    Thanks to Radio Shack's 'Basic Electronics' for that..

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    Lifetime Member Rob Mercure's Avatar
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    Chad,

    The "cascade" is more commony referred to as a "multiplier" (or simply a doubler, tripler, quadrupler, etc., for a few stages) - where in the hades RS got "cascade" I dunno but in close to 40 years of electronics I've not encountered the term. Like I suggested on another post it makes it easier on others who are less experienced to adopt the commonest terms (hey, I had to learn to say "long tailed pair" instead of "cathode coupled grounded grid" PI so we can all learn). Oh, and the circuit can't "suck" current but instead you have to divide the original available maximum current by the number of multiplier stages to derive resulting available current - TANSTAAFL ("there ain't no such thing as a free lunch" - R. Heinlein '"The Moon is a Harsh Mistress"). And while peak voltage is 1.414...times RMS - and a good thing to remember when sizing capacitors - it drops under load. But in this case with such a light B load you may wind up with something pretty close to peak.

    The "Alembic" preamp screams "Fender" all through the design so if it ain't whatcha want reviewing Fender schemos may provide fertile ideas.

    And I wouldn't worry about any extra EMF in this case unless one is truly trying to cram too much in too little space (guilty as charged, yer honor) - while any capacitive/electrostatic coupling is a function somewhat of voltage levels what we usually fight with trannies is the electromagnetic field which is a function of current and number winding turns (and if you use a sausage for your core you get a "turn for the wurst" <grin/groan>) and the heaviest current flow is in the heaters which you "wind" up with no matter how you derive your quite low current B+. And the heater is neither rectified nor floated - the rectifier "past" the heater connection just provides for an LED pilot light (which might just "rectify itself" with careful choice) while there is a 250 ohm pot with its wiper grounded to reference the heaters - but in practice one might have to still swap the tranny heater leads to find the least hum connection, I dunno.

    Rob

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Rob Mercure View Post
    The "Alembic" preamp screams "Fender" all through the design...
    That's exactly what it is. They have even said they took the front end of a Fender amp and put it in a rack enclosure.

    Would the Alembic be powerfull enough to feed into my soundcard (line level), and is the schem correct?
    I can't say if the schematic is correct, but you can drive a power amp with the F2-B, so I don't see where you would have a problem recording with it.

    If you want to hear what it sounds like used for recording, just listen to any Stanley Clarke album. School Days is a good example. He runs his bass through the F2-B and into the console.

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    'mmmmuuulltttiiply' + viability of a wallwart [ac adapter] supply

    Ya, cascade looks like an old 30s term based on a google search.

    He could just use the transformer for the plates and rectify and regulate the wallwart AC votaget to run the heaters.. Keeping noisy wall voltage clear away from the unit.

    With a 12 volt regulator I believe you would only need 150ma, giving more wall wart options after you 'loose' current in 'multiplying' the voltage.. [you guys are picky]


    Rob, the filiment supply still doesn't look functional to me. I see that the led could go off and indicate by itself but the IN914? The scheme doesn't show where they hook up the filiment, maybe after the 68ohm resistor.

    So what is the equation for current loss in a multiplying rectifier?

    If we start with 18vac @ 500ma and we multiply it 7 times to achieve apprx. 126 [this would rectify upwards though I'm not sure if the 1.4 would apply, might act more like a half wave rec???] . Then divide 500/7 leaving 71ma.. wait, actually youll run the heater before the multiplier so lets take 150 ma off the top and 350/7 = 50ma for the plates which should be plenty for just two stages I believe. Rob hows the math on that? I'll bet we loose a couple more ma in the circuit but with enough left over for a 18vac 500ma wallwart to actually run a bass pre.

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    Last edited by Chadwick; 03-27-2007 at 10:33 PM. Reason: math

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    It does show where to hook up the heaters, just to the left of the 250 Ohm pot, pins 4,5 and 9 (CT). Or am I just loopy?

    Rob

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    ya, your right

    ya, that makes a lot more sense.. those diodes were bugging me but if you hook it up just where it says its all good.

    not loopy, right on the money.

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    Pickup Maker David Schwab's Avatar
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    By the way, Alembic has some info on the F-2B:

    Circuit detail

    The F-2B was inspired by the Fender Dual Showman amplifier. In the late '60's, we routinely added preamp output jacks to Dual Showman amplifiers and substituted an external power amplifier for the built-in quad 6L6's, most usually a MacIntosh 75.

    The F-2B came about to provide the same preamp functionality in a rack-mounting package, as the Dual Showman was considerably wider than the standard 19 inch rack.

    We took the opportunity to make a small improvement in the input jack circuit, while providing identical functionality. Plugging into Jack 1 alone gives full sensitivity, while Jack 2 alone is attenuated 6 dB. If signals are plugged into both jacks simultaneously, mixing resistors combine the signals and give isolation to the two input sources. In the original circuit, the mixing resistors are always in series with the grid of the first stage, and contribute a small amount of Johnson (thermal) noise. Our jacks have additional contacts which completely switch out the mixing resistors when using Jack 1 and the ultimate quietness of the tube is preserved.

    As in its predecessor, the instrument signal is amplified by the first stage before any volume control. Without the wide dynamic range provided by the 300 volt supply, the first stage could be easily overloaded by large transients which are characteristic of electric instruments.
    ...
    Seems like the schematic is at the very least missing the other input jack. And what kind of power transformer did it use?

    Here's some pictures of the insides:

    http://www.alembic.com/club/messages...tml?1076361570

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    Last edited by David Schwab; 03-28-2007 at 01:06 AM.
    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. — Albert Einstein


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    Quote Originally Posted by RPmanen View Post
    Hi there,
    I've found the Alembic F2-B schem at Geofex, and would love to build one. The only thing is, I can remember reading about a problem with this schematic.
    Rob
    It amazes me a little how full of gossip the internet is. Several people have complained that I didn't specify a power transformer for that schematic. I couldn't - the original was a custom unit, made for the box. And there is almost never a mention of that schemo without someone vaguely remembering something somewhere was wrong with it.

    It does match the box I traced, if that's any help.

    If I was building that, I'd get two 120:120:6.3:6.3 toroids at Digikey, and power the mess from one of them with paralleled 120V primaries (I'm in the USA), run the filaments from the center of the series secondaries, and hook up the 6V windings of the second toroid to the 6V windings of the first one to power the two 120V primaries of the second toroid in series. This gives something less than 240Vac, which can be full wave rectified to around 300Vdc.

    You could go nuts with DC filaments, elevated filaments, etc, but just running the 12AX7s from 12Vdc centertapped and grounded will be pretty good.
    I'll try and post the cascade rectifier here.
    Google "Cockcroft-Walton" and "multiplier".

    With a 12 volt regulator I believe you would only need 150ma, giving more wall wart options after you 'loose' current in 'multiplying' the voltage.. [you guys are picky]
    It turns out that you have to be picky on this one because all of the losses add up to drop that secondary voltage.

    Rob, the filiment supply still doesn't look functional to me. I see that the led could go off and indicate by itself but the IN914?
    Trust me - it's functional. The 1N914 is a 100V/100ma diode. LEDs conduct with forward voltage of 1.5 -2.5V but they don't withstand reverse voltage well at all. The 1N914 keeps the LED from breaking over in the reverse direction and being damaged.
    The scheme doesn't show where they hook up the filiment, maybe after the 68ohm resistor.
    Actually, it does. You see that 4, 5 and 9 to the left of the 250 ohm hum balancing pot? Those are the pin numbers for the heaters on a 12AX7. Notice that the schemo says that only one channel is shown. The second 12AX7 will have the same pins connected to the same places.

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    Noodle of Reality Steve Conner's Avatar
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    OMG, Stanley Clarke! I'm about to try and build a bass preamp to go with a Class-D power amp I recently acquired, and I can't decide whether to try the Albert Kreuzer JFET preamp, or one channel of the Alembic F2B. I'm a big fan of Stanley Clarke, though I think my favourite tone is one I heard on a live recording of "All Blues" and I have no idea what he used for that. Probably mostly raw talent

    If people complained about the F2B lacking output, I guess it would be because the output jack is fed straight from the tube plate in the published schematic. That gives a very high output impedance, which would be loaded down a lot by the input impedance of whatever you connect. A cathode follower stage, or the guts of an active DI box, would help. I see Alembic built the guts of a DI box into the F1X.

    PS: I wouldn't care to try using a Cockroft-Walton stack to get the B+ voltage from the filament voltage, if that was what was being suggested. It might be feasible to use a C-W stack in a hybrid amp that has, say, an 80V CT transformer to power its solid-state output stage. (And it would look even more attractive in my Class-D amp with its +/-60V switching power supply, since C-Ws work a lot better at high frequencies, and there is plenty of HF voltage there to be tapped into.)

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    Last edited by Steve Conner; 03-28-2007 at 01:36 PM.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    If I was building that, I'd get two 120:120:6.3:6.3 toroids at Digikey, and power the mess from one of them with paralleled 120V primaries (I'm in the USA), run the filaments from the center of the series secondaries, and hook up the 6V windings of the second toroid to the 6V windings of the first one to power the two 120V primaries of the second toroid in series. This gives something less than 240Vac, which can be full wave rectified to around 300Vdc.
    Sorry, not quite following you . why not use a single 120:120:6.3:6.3 and rectify the 120, and use one 6.3 secondary per tube (when using 2 channels)?

    Rob

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    Quote Originally Posted by RPmanen View Post
    Sorry, not quite following you . why not use a single 120:120:6.3:6.3 and rectify the 120, and use one 6.3 secondary per tube (when using 2 channels)?
    Rob
    For safety issues primarily.

    You could do what you suggest. However, you would be using two 120V windings which might be wound side by side. This is OK for normal primary use, as the voltage difference between any two points is never more than about 350V peak. However, if you then use one of those windings as a secondary, the two wiring insulations can't be counted on to absorb 1kV to 2.4kV line transients without puncturing and exposing your own personal body to the AC line. Bad juju indeed.

    This may seem like quibbling, but let me tell you about my friend Ivan. He was the worlds champion Russian Roulette player. His career record was 453 - 1.

    In additinon, you'd still need to double the 120Vac winding to get up in the 300V range. Not as good as full wave rectification.

    In addition, you really don't need to worry about using separate filament supplies per tube. They'll work fine connected to a single 12Vac filament supply. Or to a single 6.3V supply.

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    Good Point

    However, where I live, we've got 230VAC @ 50Hz. What would you do in that case? Since I already have 230 to begin with, I don't need douple 120 secondaries.

    Rob

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Steve Conner View Post
    OMG, Stanley Clarke! I'm about to try and build a bass preamp to go with a Class-D power amp I recently acquired, and I can't decide whether to try the Albert Kreuzer JFET preamp, or one channel of the Alembic F2B. I'm a big fan of Stanley Clarke, though I think my favourite tone is one I heard on a live recording of "All Blues" and I have no idea what he used for that. Probably mostly raw talent
    Steve, Stanley always plays through the F2B. That's part of his sound. He runs each pickup into a channel. I've read he then runs those into an SWR power amp and cabs. I'm a big Clarke fan.

    I've looked at that JFET preamp... look pretty good, except he copied the tone stack from an Ampeg... I never liked their tone controls. I dislike them even more than G-K's! Ugly center frequencies. I owed an 800RB for years and ended up using a Yamaha rack guitar preamp with it most of the time!

    Quote Originally Posted by Steve Conner View Post
    If people complained about the F2B lacking output, I guess it would be because the output jack is fed straight from the tube plate in the published schematic. That gives a very high output impedance, which would be loaded down a lot by the input impedance of whatever you connect. A cathode follower stage, or the guts of an active DI box, would help. I see Alembic built the guts of a DI box into the F1X.
    My BlueTube pedal has the same problem. I have to plug it into a high impedance source, like an effect pedal, to get any level out of it.

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    Noodle of Reality Steve Conner's Avatar
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    Well, if the F2B is good enough for Stanley Clarke it's good enough for me!

    If I used the Kreuzer preamp, I was planning to just use the Baxandall bass and treble, and replace his mid EQ with a parametric EQ. Likewise, with the F2B, I'd probably put a parametric midrange control after it.

    Recording is one thing, you can EQ and compress the bass with plugins on your computer, but at rehearsals and gigs, sometimes my midrange just sucks. My old bass had a real midrange growl around 400Hz that needed cut, but my new one just wants to boom and twang, and I have to boost it.

    Maybe I should make a parametric EQ as a stomp box instead.

    BTW, David, what do you like to use as a preamp? I got the impression you were an "Active bass straight into power amp" kind of guy.

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Steve Conner View Post
    Well, if the F2B is good enough for Stanley Clarke it's good enough for me!
    Of course playing an Alembic bass through the thing helps!

    Quote Originally Posted by Steve Conner View Post
    If I used the Kreuzer preamp, I was planning to just use the Baxandall bass and treble, and replace his mid EQ with a parametric EQ. Likewise, with the F2B, I'd probably put a parametric midrange control after it.
    Yeah, those Ampeg Ultra-Hi and Ultra-Lo switches always annoyed me. I loved my old B-15N, but the later Ampegs just had annoying tone controls. They always mess up the tone too much. It's all variations of the same Ampeg sound.

    Quote Originally Posted by Steve Conner View Post
    Recording is one thing, you can EQ and compress the bass with plugins on your computer, but at rehearsals and gigs, sometimes my midrange just sucks. My old bass had a real midrange growl around 400Hz that needed cut, but my new one just wants to boom and twang, and I have to boost it.
    You know, when I was using EMG pickups, I could never EQ them and make them sound different. I had to do all these drastic mid boosts and stuff.

    Now I mostly just plug into the board and maybe goose the lows or mids a bit. And some compression of course.

    Quote Originally Posted by Steve Conner View Post
    BTW, David, what do you like to use as a preamp? I got the impression you were an "Active bass straight into power amp" kind of guy.
    Right now I play through a Trace Elliot combo. I've been on this search for a transparent amp. I spend a whole afternoon trying out amps last year and hated almost everyone of them! Almost every amp I tried did something to the sound of my bass that I didn't like, and couldn't undo. The most accurate amp was (surprisingly) a SWR LA12 combo (and not the LA10 or 15)... tiny little thing. Too small to use as a main amp. Runner ups included a Peavey and Harkie amps. Of course I didn't get to try every amp made... no Edens for example. My main gripe is that most amps seem to impart too much of their own sound. This is great for guitar, but it's not what I want from a bass amp. I want to start off pretty neutral, and then I can EQ or dirty the tone up as needed... not being stuck with the same type of tone all the time.

    I used a GK for years, and then picked up a Mesa 400+ (back when I was in my vintage tone period... complete with flatwounds). I sold the GK and bought the TE because the Mesa is too damn heavy. Sometimes I plug directly into the effect return on the TE, just because the preamp section never seems to go high enough.

    I was thinking of building that preamp and picking up a power amp, and making a new 2X12 cab or something.

    I want to check out those MarkBass amps and the new Roland DBass combos.

    I have to say though that bass amps these days in general are better than when I started playing bass!

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    Noodle of Reality Steve Conner's Avatar
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    Yay, thanks for all the info. So you don't like the Ultra Hi and Ultra Lo's? I've never used an Ampeg long enough to really figure out what they do. Whenever I plug into a SVT at a rehearsal place or whatever, I just fiddle with the volume and tone knobs at random until the rest of the band stop yelling at me to turn down. The volume is usually around 2 by then

    About compression, do you think a compressor should be an integral part of a good bass preamp? Or do you prefer to use a stomp box for it? What kind of characteristic do you like? I ask because I tried to build a bass compressor before, but it never seemed to sound right. The tube compressor from the Trace Elliot V-8 looks interesting, but I'm struggling to understand how it works or find a clear schematic.

    I have a Marshall ED-1 compressor stomp box that works OK, but I think it's really meant for guitar rather than bass.

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    Better Tone thru Mathematics bob p's Avatar
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    back in the RTF days i saw Stanley playing in a small venue. of all things, he was plugged into an Acoustic 136 solid state combo, 100W with 1x15. that surprised the hell out of me as i had the same amp. that is not to say our tone was at all similar ... no matter how hard i tried we sounded different.

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by Steve Conner View Post
    Yay, thanks for all the info. So you don't like the Ultra Hi and Ultra Lo's? I've never used an Ampeg long enough to really figure out what they do. Whenever I plug into a SVT at a rehearsal place or whatever, I just fiddle with the volume and tone knobs at random until the rest of the band stop yelling at me to turn down. The volume is usually around 2 by then
    The ultra switches shift the frequency of the controls, so the ultra lo makes it really deep. But you lose the mids. It's just too gimmicky sounding to me.

    Quote Originally Posted by Steve Conner View Post
    About compression, do you think a compressor should be an integral part of a good bass preamp? Or do you prefer to use a stomp box for it? What kind of characteristic do you like? I ask because I tried to build a bass compressor before, but it never seemed to sound right. The tube compressor from the Trace Elliot V-8 looks interesting, but I'm struggling to understand how it works or find a clear schematic.
    The multiband compressor in the Trace is really nice. I have that on all the time. You don't actually hear it though. Sometimes I like more squash, you know, like using the compressor as an effect, so I have an old MXR AC Limiter that I use. When I'm recording I usually either use the compressor in my Roland VM-3100Pro digital mixer, or an Alesis Nanocompressor.

    Quote Originally Posted by Steve Conner View Post
    I have a Marshall ED-1 compressor stomp box that works OK, but I think it's really meant for guitar rather than bass.
    That can be a cool sound, as long as you get good low end. I used to use a lot of effects... I had this elaborate signal chain.. my bass went to the MXR Limiter, and an old script logo Phase 90, and then into the GK-800RB head. The effect send on that went to a Peavey biamp chorus pedal, that had a crossover that split the signal and applied the effect on the high frequencies only. It had a high and low out. The low channel went to an Electro Harmonix Microsynth, mostly for sub octave, and the high frequency channel went to a Blue Tube pedal. The outs from these went to a two channel mixer I made, and then back to the GK! Whew!

    I used the same setup for recording, using the direct out on the GK head.

    Now I just plug into the amp. Every now and then I use some chorus, but I've been letting our singer use it on her acoustic guitar...

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    Pickup Maker David Schwab's Avatar
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    Quote Originally Posted by bob p View Post
    back in the RTF days i saw Stanley playing in a small venue. of all things, he was plugged into an Acoustic 136 solid state combo, 100W with 1x15. that surprised the hell out of me as i had the same amp. that is not to say our tone was at all similar ... no matter how hard i tried we sounded different.
    I had the head version of that amp! I used to use it with my old B-15 cab loaded with a JBL. That was a great sounding amp. When I saw him with RTF (Romantic Warrior tour) and on the School Days tour, he had an Electro Voice cab.. I think it was a 3-way setup. Damn they were scary good! I didn't know if I wanted to go home and practice, or quit! lol

    The closest I got to his tone was my Ric 4001 with a Bart Hi-A pickup in the bridge, TC-1 preamp, into my old Peavey "Bass" amp (the first one they made). I called that Ric my "baby Alembic" lol

    Being the Stanley fan that I am, I modeled my basses loosely on Alembics. I can kind of get that tone, but there's something about his high end that has a certain character that's hard to nail down.

    He also likes to put his action really high when he slaps. He said it was so high when he recorded Silly Putty, that he almost couldn't play it! You can hear him slip up a little in the intro too.

    I like my action really low. But the lowest action I ever felt was Jeff Berlin's Dean bass! Everyone who tried it said "oh shit"! It was crazy. It buzzed quite a bit, but you couldn't hear it when he played. he probably has a light touch, as compared to me... I pluck hard. I like that Stanley snap!


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    It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. — Albert Einstein


    http://coneyislandguitars.com
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    Anyway, I've been looking around for toroids thatare available over here, and found one with 230V/60mA - 6,3V/2,5A secondaries, 230V primary, would this be ok or would it be a hazard (Ivan was good point)? Its one made by amplimo, I've heard good things about them.

    Rob

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    Quote Originally Posted by RPmanen View Post
    Anyway, I've been looking around for toroids thatare available over here, and found one with 230V/60mA - 6,3V/2,5A secondaries, 230V primary, would this be ok or would it be a hazard (Ivan was good point)? Its one made by amplimo, I've heard good things about them.

    Rob
    That would be fine. You'll get a bit less than 230*1.414 = 263V for your tube plates, and the 6.3V/2.5A is plenty. That transformer will put out far more power than the circuit really needs, but that's fine - a transformer that will do the job that you can actually get is worth 12,347 transformers that are perfect, but unobtainable.

    Be sure to do your AC power wiring properly and safely. The real primary side can make an Ivan out of any of us if we're not careful.

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    Better Tone thru Mathematics bob p's Avatar
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    Quote Originally Posted by David Schwab View Post
    I had the head version of that amp! I used to use it with my old B-15 cab loaded with a JBL. That was a great sounding amp. When I saw him with RTF (Romantic Warrior tour) and on the School Days tour, he had an Electro Voice cab.. I think it was a 3-way setup. Damn they were scary good!
    Those EV bass rigs used either 1 or 2 EVM-15L in a Thiele-Small array, plus a VMR vented midrange driver that was originally designed for the Interface D studio monitor. The VMR made the popping sounds HUGE!

    When I first discovered Stanley I removed the crappy 15" Eminence speaker from my Acoustic combo amp and replaced it with an EVM-15L. I was in heaven! The problem was that the huge vented reflex Acoustic "combo" was about 3-1/2 feet tall, about 30" wide, and it weighed more than a Twin Reverb. The amp had casters, but it had ONE HANDLE on the side, like they expected The Governator to carry it for you!

    To manage getting my bass rig into and out of a 2-door car, I had to put that head into a homemade head-only cab, and I needed a small/portable a 1x15 cab. I knew some of the engineers at EV from a separate Hifi project I had been working on, so they helped me by drawing up plans for a little 1x15 cab for my EVM-15L and I built the prototype.

    As it turns out, I was a little ahead of the curve. A year or two later was when EV came out with those Thiele-Small / VMR bass rigs, and later on EV marketed the original T-S cab design that we had worked on to Mesa. Although the original EV rig with the VMR never seemed to catch on, Mesa hit it big with the EVM-15L/TL-606 cabinet as one of their compact bass rigs. The EVM design with the VMR was better, but the Mesa rig without the VMR fared better because it was less expensive and it because was aggressively marketed. EVs marketing for MI has always seemed half-hearted, IMO.

    That was back in 1976 or 78. I've still got that head/cab setup and I still using it for practice and small venues 30+ years later...

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    That would be fine. You'll get a bit less than 230*1.414 = 263V for your tube plates, and the 6.3V/2.5A is plenty. That transformer will put out far more power than the circuit really needs, but that's fine - a transformer that will do the job that you can actually get is worth 12,347 transformers that are perfect, but unobtainable.
    Not trying to be a pain, but 230*1.414= 325, and running the numbers through duncans PSU designer, I get around 300V (although I'm not sure how much ripple to allow).

    Sooo, am I missing valuable psu info? Also, how much ripple do you guys allow/ how much capacitance/RC stages do you use?

    Love the forum btw, lots of good info

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    Shameless kick

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    Quote Originally Posted by RPmanen View Post
    Not trying to be a pain, but 230*1.414= 325, and running the numbers through duncans PSU designer, I get around 300V (although I'm not sure how much ripple to allow).

    Sooo, am I missing valuable psu info? Also, how much ripple do you guys allow/ how much capacitance/RC stages do you use?

    Love the forum btw, lots of good info
    So your point is I can't multiply? You're right, I can't.

    230*1.414 is actually325.22, as you correctly note. You'll get peaks of around 325, less the losses in the wire resistances and diodes. 300's a fair guess.

    Ripple is the enemy. Keep ripple under 5-10% on your first capacitor. That's 0.05*325 = 16.25V to 32.5V. Then put in another section and reduce it again.

    To compute the reduction, consider the R-C section as a voltage divider. The lowest frequency going through is 120Hz or 100Hz if you live in a 50Hz country. So the DC drop is the I*R across the resistor R, but the ripple is reduced by the ratio of |Xc|/(R+|Xc|) approximately. Dial in a 10% DC loss, about 32V and ...er... maybe 6ma, for 32/0.006 = 5.3K, then a 47uF cap for Xc = 1/(47E-6 *2 * pi *100) = 33.9 ohms, or the attenuation of 100Hz ripple is 33.9/(5134) = 0.0066 = 6.6E-3, 50+ db of attenuation. A couple of stages of this will get the ripple right out of what you feed to your input stage.

    32V of ripple becomes 211mV - which is stlll too much for your first stage.

    Your first stage will have a gain of maybe 30, guessing, so your 100mV guitar signal will get up to 3V. 211mV of ripple, which rides right through, it's on the plate, becomes a hum only 0.211/3.00 = 0.07 of the desired signal peaks, about 30db down. That's way too much ripple. You need it down another 20 db at least, preferably 40, so you need a minimum of two 40-50db ripple reduction sections if you allow 5-10% ripple on the input filter.

    If all that sound chaotic, it was stream of consiousness as I thought and calculated what would be needed.

    You could get the necessary ripple reduction in one stage with a MOSFET source follower regulator to knock a lot of the ripple off.

    You can let the ripple run high on the first rectifier, then reduce it a lot with a couple of R-C stages.

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    Thanks for that, it was a bit chaotic, but at least now I know where to start designing my psu. I'll play around with some numbers in PSUII later.

    ROb

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    Quote Originally Posted by R.G. View Post
    It amazes me a little how full of gossip the internet is. Several people have complained that I didn't specify a power transformer for that schematic. I couldn't - the original was a custom unit, made for the box. And there is almost never a mention of that schemo without someone vaguely remembering something somewhere was wrong with it.
    R.G., that would be my doing. I sent you an email immediately after noticing that the Plate and Cathode numbers were reversed on one of the triodes. You never fixed it afaik.

    Beyond that, I found an at TubeCAD about the voltage doubler circuit as shown on the GEO schematic. Iirc, the concept is something like magnetically unbalancing the PT. A different configuration with the exact same parts, pulls current through evenly in each direction.

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    TubeCAD article covers voltage doublers. There's a side-bar on page 15, about magnetizing the core. This article and explains why the Alembic design is inferior and provides a better circuit.

    http://www.tubecad.com/january2000/page14.html

    I just looked at the 1999 schematic at Geo. Still wrong for the first triode. Beyond that, regardless of what Alembic did, the fuse and power switch should both be on the hot line. The schematic infers that the fuse on the neutral line.

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    http://moosapotamus.net/IDEAS/F2B/alembic.htm

    Maybe we should take a look at this schematic.

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    Moosapotamus has a nice power supply, Its a lot like what R.G. described in his first post.
    What kind of power rating do these transformers need? 30VA? I'm asking because like Charlie, I would like to build it in a 1U rack space, and it looks like cascading transformers would make things fit easier. Basically, the lower the power rating, the more space I can get between the transformers and the audio path.

    Rob

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    I built this preamp using a pair of 500mA trans wired back-to-back. But the output is overdriven.

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