Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blown240 on October 29, 2011, 11:49:19 am
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I am on vacation for a week and I am starting to plan my next build. I play an upright bass that has a dual piezo pickup. It came with a preamp that sounded so bad I literally threw it away. What I want to build is an Ampeg b15n based dual channel preamp that I can use to plug into house sound, my regular bass amp, or use to record direct.
My bass has a stereo jack on it, so the preamp will have a single stereo input. It will have separate volume, treble, and bass controls for each channel, and I will use push/pull tone pots for the bass and treble boost.
I want to make the preamp circuits as close as possible to the original B15n but obviously setup for a piezo. Also, when playing at louder volumes, or when the acoustics of a room are terrible, my bass feedsback at about 100hz. So I want to do a notch filter on each channel that drops 100hz, but leaves everything above about 120hz and below 80hz. I will have a separate notch for each channel that is switched with a small micro switch.
Here is a link to the pickup I am using:
http://www.kksound.com/bassmasterrb.html (http://www.kksound.com/bassmasterrb.html)
Here are some questions I have:
1. Input Resistor – the pickups input resistance is 1m per channel. Should I match that, or should I got with something like a 4.7m input resistor like a lot of the peizo preamps I see online.
2. Rectifier – I want to use a tube rectifier, but I am not sure if the tranny I am using will let me, or what rectifier tube to use. Here is a pic of the small trans that I will be using (tranny on the left):
(http://i268.photobucket.com/albums/jj26/thebattens/Philco/2011-09-20_185049.jpg)
3. Notch Filter – No idea how to do this. Any ideas?
4. Is there anything else I need to do to the basic b15n preamp circuit to better work with a peizo?
5. I want to make sure I have loads of clean headroom, so anything that will help that...
Thanks for any input!
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Here is a schematic for a b15n for reference:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/Ampeg_B15N.jpg)
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if you make only one input for each channel (the one with the 100K for example) then you don't have the 47K to ground from the other input. this means the 5M6 resistor is more or less the input impedance. good for a piezo.
this will also boost the signal a bit when compared to the original circuit. you might use a lower gain tube so it will be cleaner. it doesn't matter for the volume because the FOH mixer has loads of volume for you ;-)
can you post what is written on top of that transformer that you have?
you might want to look into good passive DI transformers so you can have a balanced XLR output without having to use a separate DI. add a jack output too and you can plug into a poweramp on stage at the same time.
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I am only going to have 1 input total. It will be stereo. So I guess the 47k to ground wont be an issue.
I will post whats on top of the trans in a week when I get home. I got it a few years ago from Hoffman. Its just a basic preamp transformer. What is a FOH mixer? Sorry, but I am still pretty new at this.
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FOH = Front Of House
you plug your preamp into the system of the house so they will provide all the volume you need in monitors and PA for the audience.
i'm so used to the term FOH from my job that i forget to write it out on a forum like this.
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Thanks!
Ya I really want something that will bring out the sound of my upright bass, and its hard to go wrong with the portoflex sound. So this preamp hopefully will get me that sound no matter what I am plugged into. Plus it will give me the impedance I need to make the piezo sound better and louder.
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Here is a schematic I did to start with. I still need to make sure the input impedance is correct for a piezo. And I need to figure out where to put the notch filter(s) and exactly how to do them.
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreamp.jpg)
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The two 270K mix network is just strong enough to run inside the box, maybe thru a short cord.
You need more beef to drive modern power amps and long runs to sound system.
Don't over-think the input impedance. After all, you can always change it on prototype.
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I never even considered that. What should they be? 100k?
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Changing the mix resistors is not the answer. Put a cathode follower between the mix resistors and the output jack.
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Ok, Thats easy enough. Something like this:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/cathodefollower.jpg)
What value should the load resistor be? And what tube should I use that will color the sound the least. And does it matter what filter cap to get I get the B+ off of? I can add another since I was going to get cap cans anyway and would have an extra.
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That's basically it, but you need a wee bit more to bias the CF and block the dc voltages. This should work. The input cap can be a small value .001µF to .047µF) since the bootstrapped grid circuit is very high Z. The output cap should be larger (0.1µF to 0.25µF) since it's in a much lower Z circuit. Caps are easy to play with if you don't like those values.
Some of the other guys can put you on a tried and true CF circuit. You may even consider a MOSFET source follower.
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Thats looks pretty good. I will add that to my schematic and get a few extra caps when I order my parts. I dont really like the idea of a mosfet when its so easy to do it with a tube.
It seems that some like the 12AU7 for this...
What are the thoughts on a "twin T filter" to give me the 100hz cut? I still need to figure that part out.
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Here is an updated schematic:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate1.jpg)
I was looking at a pic I have of the trans that I going to use. There are only 4 wires coming out of the secondary side. 1 pair is the heaters. I am not sure on the power of the 2nd pair. I may have to go with a SS rectifier because of this. Which is fine I guess.
So I still need to figure out:
1. Notch filter at 100k
2. What tube to use for the CF
3. If my trans will work with a tube rectifier
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Don't over-think the input impedance. After all, you can always change it on prototype.
Don't we need to over-think it just a little. I don't think he can just copy the B15 input circuit because...
I play an upright bass that has a dual piezo pickup.
He may need some kinda small buffer amp right at the instrument, to give a ~4-5M load impedance for the piezo. No bass might be a bad thing for an upright bass pickup.
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Wouldn't I have 5.6m input impedance with the schematic I have? Thats what I got from the post from birt.
But maybe there is more too it than that. I would prefer not to have anything else mounted on my bass, but instead to integrate it into this preamp. Thats half the reason I am building it...
Would a 15-20 foot cable really change things as far as having the buffer right at the bass or first thing in the preamp?
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Wouldn't I have 5.6m input impedance with the schematic I have? Thats what I got from the post from birt.
Yes, but...
But maybe there is more too it than that. I would prefer not to have anything else mounted on my bass, but instead to integrate it into this preamp.
Would a 15-20 foot cable really change things as far as having the buffer right at the bass or first thing in the preamp?
Have you noticed how almost every guitar using a piezo pickup has an onboard preamp? It's only partly about convenience. A piezo element might as well be a capacitor; in fact, the mechanics that make it like a capacitor are the same that allow it to produce an output signal.
Have you ever used a condenser mic? They need phantom power because there is a preamp internal to the mic. The old-school name for "capacitor" was "condenser" and this type of mic operates on the same basic principle as a piezo element. Anyway, for several reasons, the pre-preamp is mounted in the mic, and presents an impedance of more than a gigohm.
Anyway, a classical-style guitar from the late-60's that introduced the first piezo pickup (the Baldwin Prismatone pickup), and considered to have one of the best available sounds, has a small internal preamp mounted directly on the jack in the guitar. There can't be much to it except a couple of FETs and a couple of resistors, because it's encased in a glob of potting material maybe 1-2" long. It uses a stereo jack, so that power for the FETs is carried on a different conductor than the signal, and is powered by an external power supply.
So it can be done. I don't know anything specific about your pickup. I can't tell you if it will work fine with the preamp you have in mind. If it were my instrument, I'd look to have some kind of simple onboard preamp, perhaps even one small enough to mount on the jack.
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Perhaps I should build this preamp, then see how it is.
Then if I need to I can build one of these:
http://www.diyguitarist.com/Guitars/PiezoBuffer.htm (http://www.diyguitarist.com/Guitars/PiezoBuffer.htm)
Either way, this preamp will get me alot of what I am looking for.
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Sounds like a plan.
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I still need to see what exact trans I have to determine if I can go with a tube rectifier or not.
Plus I need to figure out the notch filter and see what tube to use for the CF.
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There's no advantage when using a tube rectifier for this type circuit.
I don't have any ideas on a notch filter.
Wire the CF for a 12AU7. If you don't like it, swap in a 12AX7, 12AT7, 5751, etc.
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Sweet. Thanks!
Now I only have to figure out the notch filter.
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Here is the updated schematic:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate2.jpg)
Couldn't I just use a cap in parallel with a resistor to make a high/low pass? Use the cap to cut out everything below 120hz and the resistor to cut everything above 80hz. It seems to me that in the end there would be a cut from 80-120hz. Maybe put this in series between the 270k resistors and the CF, and use a double pole, double throw switch to take it in/out of the circuit.
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take a look on how the filters in a graphic EQ work. i'm sure you can find schematics for that. there's also stompboxes lie the Boss GE-7 that you can look at.
and don't forget you will need a DI when you plug this into the sound system of a venue. you might as well add a transformer and XLR output to your preamp.
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Couldn't I just use a cap in parallel with a resistor to make a high/low pass?
for hi-pass 'yes', for low-pass, no. for low-pass, you can use a cap, or a set of parallel caps and resistors to ground.
BIRT has a good suggestion about the XLR balanced output.
I've built a few tube pre-amps, here are some thoughts:
1.) have a ground lift switch, if you plug this into an amp with unbalanced signal (1/4 inst. cable) and both the preamp and the amp have 3 prong plugs, you can get ground loop noise. the shield on the inst. cable will provide path to ground from with the chassis. This is different than a ground lift switch on a XLR/balanced output box, but you can have a ground lift there as well (for pin 1).
2.) re-evaluate the tube rectifier. pre-amp tubes won't draw enough current to ever get a tube rectifier to cause sag, and I'm not sure there is any reason beyond that to use them. If you are modeling your engineering of Ampeg, they switched to SS as soon as they could.
3.) Triode transformers makes a nice Dynaco pre-amp PA211/PA522 that is a 330-0-330 and 12.6v/900ma filament transformer that works well for this application, tho its 12.6v, not 6.3v and I see you'd like to use 6SL7s. If you reconsider and decide (1) use 12SL7s or (2) decide to use 9A based tubes (12ax7/12au7 etc), this is a perfect tranny. I've used the International version PA522 (which has 120v and 240v windings) by using the 240v windings with 120v USA wall power to produce 165-0-165 and 6.3v secondaries.
4.) you've got stereo inputs, you could add extra switched output jacks to have optional stereo outs, pre-mixing resistors...
5.) you've got a 12au7 enlisted for the CF, you've got another section there for another CF out (maybe for the other channel, or to drive a transformer XLR/balanced out.
6.) that preamp has enough gain to hit a power-amp section, you might want to tame it down with lower Mu-triodes (6SN7s, or 6072As, 6211s, 12AU7s). If you go with 6SL7s (or 12ax7s), your usable gain will be between 0 and 3 on your volume knobs, I suspect).
7.) 9A based tubes will give you a lot tuning of the overall gain, with an 8BD pinout, you are limited to 6SL7s and 6SN7s, with 9A's, you can you use 12au7s and 12BH7As all the way up to 12AX7s (there are about 10 different Mu tubes in between).
Piezoelectric pickups in acoustics produce sound a lot different than magnetic pickups. Besides the much much high output impedance (which you might have to phtuz with on your 1st stage to match), you might find that the tone stack in a traditional amp (like the one you are looking at) change the sound of your Piezos differently than magnetic pickup guitars/basses. Be prepared to have to change the tonestack a bit. also look at the bass tone stack on an SVT.
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Thanks for the reply terminalgs! You have some good ideas.
1. Ground switch - will do, thats a no brainer.
2. Going to use a solid state rectifier.
3. Transformer - Hopefully the one I have will work. If not my local amp guy has loads of them that he sells CHEAP to his buddies.
4. Stereo out - I may do this after the initial build is done.
5. How could I use the other half of the 12AX7 to drive a transformer XLR out? What would the circuit look like?
6. I will see how it sounds with the tubes and switch them out of need be. Another advantage of my local amp guy!
7. I really want to try the 6SL7, if that sucks, then I may switch to 9 pin.
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http://www.national.com/ms/LB/LB-5.pdf (http://www.national.com/ms/LB/LB-5.pdf)
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>ground lift switch
Last several pres I've built, I've blown off the switch and just simply wired the output lifted. You run it grounded unless it it buzzes, then you lift it.... which never buzzes. I 100% think you should conjure up a post EQ direct out. Your instrument, you're the player, you should be in control. I have a tried and tested bass preamp circuit that everybody who has used it loves it. Drop me a line if interested.
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http://www.national.com/ms/LB/LB-5.pdf (http://www.national.com/ms/LB/LB-5.pdf)
Thanks! Thats exactly what I am looking for, except how can I get it from 60hz to 100hz? I assume that it would sit between the 270k resistors and the CF, is that right?
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I got your PM and I have attached the last (I think) schematic. This circuit is loosely based on the STV Channel B preamp. This is the side without all the cool inductors which in 2011 are unobtainable. (L to R) V1b was changed to a plate follower to reduce gain and distortion. S1 inserts a RC circuit that cuts the lows @ 20ish Hz. It also loads V1b but doesn't really cut volume all that much. It's the magic button when feeding a Big BooM PA. R6 through V2b are a direct copy of the SVT sans the summing network R19 & R14. V2b is the HiZ output (J2). I added V2a to drive a DI transformer. Yup, I know you don't wanna add another transformer but life is cruel that way in the world of tubes. It's designed around a cool old Peerless 15095 transformer. 15k:600 ohm plate load. We had a few of them and they're too cool not to use. The Edcor XSM10k/600 will do the job just fine. http://www.edcorusa.com/products/160-xsm10k-600.aspx (http://www.edcorusa.com/products/160-xsm10k-600.aspx) It's $15.00. The B+ is pretty straight forward SS full wave recto. I did 12VDC regulated filaments. The 555 timer is simply a 60ish second delay that keeps the outputs disconnected (via a very poorly drawn relay) until the filaments are hot and the circuit is quiescent. The little relay and timer thang is good to have. This circuit goes NASTY for about the first 5 to 10 seconds. It's all food for thought.
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This might be of interest to you.
http://personalpages.tds.net/~fdeck/bass/hpfpre.htm (http://personalpages.tds.net/~fdeck/bass/hpfpre.htm)
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Thats an interesting circuit. There are some aspects of it I really like. It definately gives me some food for thought.
Also, I was looking at the schematic for the Boss EQ pedal. There is an EQ circuit for 100hz that I may try to incorporate right before the CF in my preamp.
The like for the peizo buffer cool as well. I will have to go back to that once I see how the preamp effects my overall setup.
Thanks again guys!
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I use that same K&K preamp and actually like it. It's beat everything else I've used to date.
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The best pickup I have ever used was a King Doublebass Doll Head. It was AMAZING. But with them being out of biz, and the fact that so few were sold, I was offered a TON of money for it, so it went...
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http://www.national.com/ms/LB/LB-5.pdf (http://www.national.com/ms/LB/LB-5.pdf)
Thanks! Thats exactly what I am looking for, except how can I get it from 60hz to 100hz? I assume that it would sit between the 270k resistors and the CF, is that right?
Figure 1 of the sheet tells ya. It's all based on R*C.
But since you'd like to know now, leave the resistors alone, change C1 & C2 to 150pF each, and change C3 to 300pF (not a standard value; use two 150pF caps in parallel). Those values land on 106Hz, but the original values land on ~58Hz, so I'm close enough.
You likely want to build the variable-Q version.
You could trim the resistor values down and cap values up, but if you build what the sheet says, you know it'll work right. Plus, larger-valued caps cost more.
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HotBluePlates - thanks for the reply. Thats exactly what I needed to know. I appreciate it!
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OK so here is where I am at. I have added the balanced DI out, the ground switch and the variable HZ cut.
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate3.jpg)
What do you guys recommend for:
1. The first resister on the XLR DI out. It can be 5k to 100k. What should I start with?
2. will the SPDT switch work for the notch filter, or should I use a DPDT to completely remove it from the circuit?
Thanks!
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1. The firs resister on the XLR DI out. It can be 5k to 100k. What should I start with?
Probably a 50k trimpot in series with your actual level pot.
I don't know what signal level your line out needs to supply. We can guess your preamp should output 1v or less of the normal signal, and if we knew the required output for the XLR, we could figure a ratio.
But it's easier to just add a trimpot, turn your level full up, then adjust the trim for the max signal you'll likely need. Button up, and your done.
2. will the SPDT switch work for the notch filter, or should I use a DPDT to completely remove it from the circuit?
DPDT.
The variable Q pot of the notch is a 50k load that is fine for the opamps, but too low for the tube stage ahead of it. You want to be able to completely bypass the notch filter when it's off.
And a question you didn't ask:
Don't do the ground-lift the way you've drawn. The 3rd prong should always be attached to the chassis of any amp/preamp. If B+ is shorted to the case, you want a breaker to trip, not you to trip after you get zapped touching the case. I've grabbed ahold of 450vdc before... it's not fun.
If you really want to incorporate a ground lift that works, but is safe, you need to use isolated jacks, and have a ground buss on the circuit board for everything with a ground symbol. 3rd wire of the power cord gets connected to the case. Connect a 100pF cap and 50k resistor in series. Connect this resistor/cap combo across a SPST switch, which has wires running to your wired ground system and the chassis (close to the 3rd prong wire).
It works like this:
If the switch is closed, the circuit ground is connected to chassis ground, which is hardwired to the 3rd prong.
If the switch is open, circuit ground is disconnected from chassis ground. 3rd prong wire provides shock safety. 100pF/50k combo connects the circuit ground to chassis ground at radio frequency, which helps keep RF buzz/noise out of your circuit. This particular combination is -3dB down at about 31kHz.
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So a 50k trim pot, sort of like a bias pot. What does that resistor actually do? is it even necessary with the level pot?
SPDT - will do, makes sense.
Ground Switch. Is it even worth the hassle? Seems like if it had a ground loop thru a cable to a power amp, it wouldnt matter if there was a chassis ground thru the power cord. It would ground to the power amp thru the cable.
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don't rely on a cable between devices to provide a ground path like that. (it can be helpfull in some situations but since you are designing something you better go for the safer way)
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Good Point. I suppose an easier way to do it would to use a non conducting chassis. I had planned on a 16aug steel chassis, because I have a ton of 16aug sheet metal. OR I can isolate the pots and jacks from the chassis. I guess that isnt too bad either, just a bunch of nylon washers...
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Here is where I am at now. I have isolated all the grounds after the standby switch and have them going thru a ground switch. Also added a DPDT switch for the notch, and a 50k trim pot for the DI.
Here is the schematic. The isolated ground path is in green:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate4.jpg)
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Re input impedance matching: I'm not sure what your input impedance is. It looks like the incoming signal is split so there's about 2X 6M in parallel = 3M. Piezo's usually have about 6M output impedance, so 3M input impedance is rather low. It might work well enough though.
FWIW Merlin recommends a 12AX7 cathode follower (not preceded by a gain stage) as the input stage for a piezo PU. It has an effective AC impedance of 6M. Designing Tube Preamps for Guitar & Bass. This equals the input impedance of hi-end SS acoustic guitar preamps.
Another alternative is an outboard SS preamp/buffer designed for piezo's, in front of the amp.
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> It looks like the incoming signal is split so there's about 2X 6M in parallel = 3M.
No, he's got _2_ piezos (bridge and fingerboard), wired with one 3-conductor cable and jack. Each piezo sees 5.6Meg.
He can readily try 2Meg or 22Meg just by changing resistors.... no need to fret about it before the Build.
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You're missing the boat on the ground lift. I don't think it's safe IMO to float a HV circuit. All you lift is pin one of the XLR output. This prevents the possibility of a loop through other equipment down stream.
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OH YEAH, I missed that one for sure. Well thats a whole lot easier!
Thanks Richard!
Something else I am considering:
If I wanted a 1/4 inch output jack on the front AND back of the preamp, can I just run 2 jacks in parallel? One on front (easier when plugging into some amps), and one on back (for running to house sound/recording).
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I'd use the other 1/2 of the 12AU7 for the DI A phase switch is awfully good to have. If you're gonna run with the 6SL7, why not use a 6SN7 in place of the AU? How are you powering the OpAmp? Can it run on 12V? Tis a good excuse to go with DC filaments which puts you back in the Ax -n- AU tubes.
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What is the advantage of using the other half of the 12AU7 for the DI? I am totally for it, just curious if there is an advantage.
6SN7 - I am not agianst using it, I have no idea what it is. I only picked the 12AU7 because I have seen others use them. I do want to stick with the 6SL7.
I didnt know I had to power the OpAmp, I know absolutely ZERO about them. Apparently they need 12-15v
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The advantage of using separate triodes for the 2 outputs is so that the 2 loads don't affect each other. You could easily stick a pot and another iso cap in front of the 2nd cathode follower to get a separate level control for the DI. You've got the triode, why not use it. The 6SN7 is for the most part the octal equivalent of the 12AU7.
Yes the opamp has to have juice. If you use the PA211 DynaClone PT, you've got 12.6 VAC so you could copy my regulated filament supply. Stack the 2 SL filaments in series and stick with the 12AU7 and wire it for 12V filaments.
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> It looks like the incoming signal is split so there's about 2X 6M in parallel = 3M.
No, he's got _2_ piezos (bridge and fingerboard), wired with one 3-conductor cable and jack. Each piezo sees 5.6Meg.
He can readily try 2Meg or 22Meg just by changing resistors.... no need to fret about it before the Build.
Ok. Then that's as good as it gets!
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XLR ground lift.
(http://i.imgur.com/06YdA.png)
as other said: separate 1/4 jack and XLR, unless you use a switched jack and you want to make it an "either or" output. having two output each driven by a 1/2 12au7 CF would be a feature. I don't think you need the pot in front of the OT, you've got level pots back in the early stages of the preamp, I'd let the CF drive the OT directly. If you want a post-EQ level pot, i'd do it before the CF.
you might even need a coupling cap to keep the DC you need for the transistors off the tone stack. If it were me, I might start with a set of switched 1/4 jacks as an effects loop and let an EQ pedal handle it for now. Let the integrated notch filter be a "coming soon" feature.
probably no need for a standby switch. make it out of steel, use isolated input jacks, use low capacitance cable and keep the box close to you on stage (15ft cable instead of 30ft).
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Wow, 2 more great post with suggestions. I will defiantly use the the other half of the 12ax7 for the DI.
And thanks for the idiots guide to a ground lift!!!
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The million dollar question for the DI is, "What transformer are you using?"
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I dont have it yet, but it will be close to this:
https://taweber.powweb.com/store/balun.jpg
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Here is where I am at now:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate5.jpg)
Will I need some sort of resistor for the B+ to the CF now that I am using both sides of the 12AU7?
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Here is a quick rough layout idea. Its based on a 16 x 8 inch chassis.
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/UprightPreampLayout.jpg)
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I dont have it yet, but it will be close to this:
https://taweber.powweb.com/store/balun.jpg
not so fast! frequency response on that little fellow is 80Hz to 20kHz, isn't a lot of what you guys play on those big basses below 80Hz? like 40Hz??
if you want to do the XLR out, you'll need to get a trans that has a good freq. response for everything you want to run through it. also, its a 150ohm:150ohm, 150ohm pri won't work behind your CF. The CF is lo-Z, but not that low. notice that weber uses it coming off the speaker out. someone mentioned this cheapo transformer earlier in the thread, do they drive it with a tube? or like weber suggests, from the "hot speaker jack"?
you might have to peel off another $20 and get one from edcor, but call them/email them and tell them what you are doing and you'll need freq. response down to 30Hz or so (whats the low -E on that thing?)
also.... if you want to drive both a 1/4in out unbalance signal, like, to an amp,, and drive a line level out "DI style" via the XLR,, And you only have a signal level control at the beginning of the preamp,, you are going to have trouble getting the right level on both the unbal. 1/4in out and the XLR balanced out at the same time. You can't do it the way you have it (someone tell me I'm wrong).
lets say, edcor sezs these two are good for your application::
* 2-1/2W 10K Ohms to 600 Ohms (4.1:1) line level matching transformer.
* 2-1/2W 15K Ohms to 600 Ohms (5:1) line level matching transformer.
you want to send the unbalanced (1/4" out) to the amp, at say 0.5v p-p (point to point), thats -12dBu. If you send the same 0.5v to a 5:1 transformer, you'll have .1v p-p on the balanced (XLR) out,, and the soundman will say "wut? turn it up", because he is expecting +2dBu? or 0dBu into his board if its a DI. (do we still have a soundman reading this thread?) thats 2v p-p (0dBu).
sooo.,, with a 5:1 OT, working backwards, you'd need 10V p-p on the primaries of the OT.
options:
(1) turn the CF for the OT into a gain stage,, maybe a 12au7 gain stage with 100K plate/2K cathode might turn a 1v into a 10v (probably closer to 7v ??)
(2) make the 1/4" out and the XLR an "either or" but not both... with a 6SL7 you'll have gobs of gain on the 2nd stage to put all the juice that OT would need (actually, you'll have your volume knob on max=4 for the XLR ,and about 2 for the 1/4"-out... thats my guess...
(3) maybe try an attenuation/level pot before the 1/4"-out CF... let the 10v p-p come out of the preamp for the OT, and attenuate the 1/4"-out signal down to 1v.
(4) or maybe, if you want both 1/4" out and the XLR: best option might be to.... add another twin triode (sorry!) and buffer output level pots before each of the two..
(anyone feel free to step in on this one...)
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My circuit, I ran with terminalgs option 1. My one regret with the circuit is not having separate levels for the hiZ and loZ outputs, but they are well balanced. You do want to use a quality transformer of good frequency response.
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but if you make the triode in front of the OT a gainstage you probably need an other OT than you would use with a CF since the output of the tube will have an higher impedance.
option (5):
what advantage do you have with a tube driven DI output? if you make it with opamps it might be easier to add a gainstage and volume control for the balanced out (there's even IC's that have a balanced output where you don't need a transformer anymore) and it will all fit on a very small print.
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Wow, thats alot of info first thing in the morning!
OK here it goes:
1. Low E string is 40hz when tuned to E. So a better transformer will be necessary.
2. I do want to have both the 1/4 inch AND XLR at the same time.
3. A volume pot for the 1/4 inch out is no big deal.
4. I dont mind adding another tube, I just need to make sure the PT I am using will work. I really want to use the one I have. I dont mind spending some $$$ on this, but I dont have a whole lot to throw around.
5. I really dont want to add any more OpAmps. At this point I may even turn the Notch into a effects loop like what was mentioned earlier in this thread. I dont want to have another trans just to get 12v to power the OpAmps.
5b. Of Course if I do use OpAmps for the DI, I can ditch the DI trans and replace it with a 12v PT. That may be a better option. Something like this: http://www.frys.com/product/1922620?site=sr:SEARCH:MAIN_RSLT_PG (http://www.frys.com/product/1922620?site=sr:SEARCH:MAIN_RSLT_PG)
6. Can I use the 1/2 of the 12AU7 as a gain stage just for the XLR and still have the first half as a CF for the 1/4 out?
I am just glad to have all this input from you guys. Its WAAAAY better to overthink it now, than to have it not work once its built!
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6. Can I use the 1/2 of the 12AU7 as a gain stage just for the XLR and still have the first half as a CF for the 1/4 out?
yes.
oh and this might be a crazy idea:
if you find a circuit where you can put a volume control on the DI output it doesn't have to be a pot. the only thing you want to do with it is make sure it's a bit balanced with the normal output. how about an optocoupler to ground instead of a volume pot? you make the volume control in the preamp a double log pot and use one of the sections to control the led part of the optocoupler. this means that the more you turn up the volume, the brighter the led goes and the DI output stays constant. it might take some fine tuning but i think this might work. (you put the preamp volume as low as you would ever go when you play and then some more. at this point you want the balanced output at a reasonable level. with a series resistor you make sure this is the point where the optocoupler starts working when you turn up the preamp volume)
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Wow birt, that was WAY over my head, but I think I know where you are going. Seems like a volume control pot on the back for the XLR and one on the front for the 1/4 jacks would be alot simpler and user friendly.
I am glad I can use the 12AU7 for 2 different things. I could almost build this thing without the DI and Notch, get it dialed tone wise, and then add the DI and the notch filter. (I would drill the chassis and install the switches and pots, but not wire them for now.)
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Here is another feature I would like to add:
An Effects loop with 3 switched posisitions:
1. Effects loop off
2. Effects loop on
3. Effects and clean thru, with a blend pot so I can blend the clean signal with the effects signal.
How would I do this?
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Look at the Channel B schematic I posted several replies back. I used the other 1/2 of the 12AU7 as a gain stage feeding a 10K:600 ohm balancing transformer. Very similar circuit sans all the 2 pickup summing and the OPamp thingy. Might look at the 12VDC supply too. nudge nudge...
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OK so what is better:
Using the other half of the 12AU7 as a gain stage for the DI, or trying to do it with OpAmps and no transformer?
The DI is a really important part of this build because we are getting ready to record and I want to be able to go direct into protools.
And how should I do the effects loop? Do I need to add another tube for the loop?
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OK, After doing some more reading I decided that I want a tube gain stage for the DI. I figure that this build is important and I want the best results possible.
RicharD - I am having a bit of trouble with your schematic. I think the relay is throwing me off. I want to borrow/copy your idea for the DI gain stage, and I have a start to it, but I need your help getting it correct. Here is a pic of where I am so far:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/DIGainStage.jpg)
What am I missing?
THANKS!!
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My vote is all tube. It's all a question of dollars vs. sense/cents. You could do the whole project in opamps for pennies on the dollar vs. tubes. I like tubes -n- transformers. What sorta jack you willing to throw at this thang? If this is a first project you may wanna KISS and build a little 2 to 1 summer and be done with it. Hang all the bells and whistles off the single output. If you want to build the "glory" box, then you really need to rethink your topology. You mentioned ProTools, right there tells me this device should be capable of stereo through & through with L-R pan. A 2 x 2 mixer if you will.
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Its not my first build, but I am definately new at this. I want this thing to be all encompassing, so it won't be simple. Stereo thru to the XLR would be nice, But not at all necessary for the 1/4 inch. Can a single xlr do stereo?
I guess if I could have everything I want, it would be:
1. All tube, with a portaflex feel/sound
2. Stereo XLR for recording, switchable to Mono for playing shows thru house sound
3. 1/4 out mono for plugging into an amp
4. Effects Loop - that is switchable between series and parallel so I can blend the clean and colored sounds
5. notch filter - cant be tube, but oh well
6. I would like to do it for under $200, but that may not be possible.
The chassis is free, because I already have the metal to make it. The PT is free is the one I have will work. If not then my amp guy sells me trannys pretty cheap. The tubes I can get from my guy for ~$10 each (good, used ones)
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That's an awful lot for 2 clams. What PT do you have?
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I will have to see what it is when I get home tonight. I will post it as soon as I know.
I know its alot for $200, but alot of it I can get cheap. If it costs more, than it does. But the goal is $200.
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I sketched up a topology.
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2. Stereo XLR for recording, switchable to Mono for playing shows thru house sound
to be clear, if you want stereo XLR, you need *two* XLRs and *two* OTs. even tho the XLR has two signal wires, it is mono signal! You did away with stereo at the two mixer resistors when channel No.1 and No.2 came together.
The best approach is probably what RicharD suggests: KISS.
* Phase 1: preamp with 1/4" outs, no balanced output.
* Phase 2: dial in the tone stack that works with your bass
* Phase 3: XLR balanced out (or outs).
start with a chassis with room to grow, drill extra holes for some extra noval sockets and XLR connects.
pick a PT that you hopefully add a few tubes down the road with.
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you might KISS, and simply add stereo option like this:
(http://i.imgur.com/I5ptjl.png)
Then, later,, when you are happy with that, and you have some money, do something like this:
(http://i.imgur.com/GDxNMl.png)
EDIT:: don't let your guy charge you $10/12au7, you should get 2-3 for $10.
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Ya I like the idea of building it in stages. I just want to have a complete plan for it.
Here is my transformer, I seriously doubt it will work:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/2011-11-04171400.jpg)
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Here is my idea for the stages:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/Stages.jpg)
1. PLAN
2. Channel 1 & 2, Dual CF, stereo and blended 1/4 out.
3. Effects Loop
4. Stereo XLR
5. Blended XLR and Notch filter
So in an effort to figure out chassis size, I will need:
5x tubes - 2 preamp,1 effects loop, 1 cathode follower, 2 XLR gain stage
5x Trans - PT, XLR balance x3, 12v for notch filter
3x xlr jacks
5x 1/4" jacks - 1 input, 2 for stereo out, and a front and rear blended out
9x pots and knobs - 6 preamp (2 volume, 4 push/pull tone), 1 notch filter, 1 blended XLR volume, effects loop blend
6x switches - power, standby, notch, effects loop, XLR ground lift (Can all 3 xlr use 1 ground lift? 3pdt), effects loop 3 way
?? Filter caps. How many will I need?
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It'll actually work perfectly using voltage doublers. 120VAC into a doubler will give you close to 300VDC. You have to figure 50% for current. Your PT is rated for 140mA so we get 70mA out of the doubler. I doubt your circuit will hit 1/2 of that so you're cool with room to grow.
Filaments..... I'm gonna push 12VDC for the filaments. First off you need 12VDC for the Opamp. Secondly DC is quieter. Finally this is not a PP amp so you don't get any canceling. With proper layout, AC filaments would be fine but...... trust me on this one. The Dc might come in handy if you do any relay switching. 6.3VAC into a doubler will give you about 14VDC. Hit a little 12V regulator and call it good. So you have about 750mA available. 2 6SL7's in series = 300mA. Each 12A*7 tube at 12V = 150mA so right now you're at 600mA. That leaves 150mA before you snag a handful of mA each for LED's, Opamps, relays -n- such. Call it just enough filament supply.
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Thats awesome, except I have 3 12A*7 tubes. But hopefully it will be close enough.
I wil still need 4 trannys though, 1 power and 3 balance.
A few questions:
1. 12vdc heaters - they all go to ground?
2. Balance Transformers - will they cause HUM in the preamp like a PT?
3. How many filter caps for all this?
THANKS!!
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>1. 12vdc heaters - they all go to ground?
2. Balance Transformers - will they cause HUM in the preamp like a PT?
3. How many filter caps for all this?
1. Yes. Your 2 6SL7 filaments stack in series to =12V. Current stays the same in series. Load = 300mA. Each 12A*7 tube draws 150mA. You do not connect pin 9 to anything. You're limited to 4 tubes no matter how you slice it with the PT you have. You could always use a 2nd filament power transformer or compromise.
2. No.
3. To be determined. You need 3 right off the bat to make a voltage doubler. Add 1 more for each voltage node. Truth be told, it could all hang off 1 node, but isolation is isolation. For right now lets look at it as 1 node.
See attached.
This is a voltage doubler full wave rectifier. I'm ball barking 25 to 10mA. The current transition is graphed at 5 seconds. Call B+ = 315V. This swing does not occur in real time but is simply to demonstrate the loading of your transformer. 3%, don't lose any sleep fretting about this.
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I could have one of the XLRs switched so that it is either blend or one of the side channels. Seems like a SPDT would take care of that. Then I would only need 4 tubes.
On your voltage doubler schematic. Where the 2 circles come together on the right side, that is where I get me B+ from, correct? So I would do 2 of these, one for high voltage, and 1 for 12v heaters. They also take care of my rectifiers...
So I would need 6 filter caps for the doublers and figure 1 for each tube. So 10 filter caps, or 5 cans...
Thats alot of filter caps!!
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The 2 circles represent the current load. The top of that is your B+.
You'll need 3 caps for the doubler itself, and 1 additional cap for each voltage node that follows. I'm guessing 2 more nodes. Node A = cathode followers, Node B = DI gain stages, & Node C = 6SL& preamps. 5 caps. You don't need to use those spendy boy can caps. Not really a big concern at this stage of the design. The 12V supply will need 3 or 4 caps but 25V electrolytics are way cheaper than 450V counterparts.
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OK, here is where I am at this point:
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate6.jpg)
Still need to figure out the correct way to do the gain staged for the 2 DIs and the power rail. But I really like the way this is coming together.
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In this schematic I added inputs for channel 1 and channel 2 separately. I figure why not have the option if I want to plug a couple acoustic guitars or something else in. I still have the "stereo" input for my bass.
Also, I did the power rail and the 12v heaters, and got my front and rear blended outputs back.
I still need to get the exact setup for the DI gain stages. And I think I am going to have to add a 12v tranns for the other tube and the OpAmps.
(http://i268.photobucket.com/albums/jj26/thebattens/Upright%20Preamp/PiezoPreampupdate7.jpg)
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You drew both of your voltage doublers shorted. Gonna wanna use a regulator in the 12V circuit. You need to design in compartments, preamp 1, preamp 2, summing, effects loop, notch EQ, direct outs, B+, filaments, and controls.
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You drew both of your voltage doublers shorted.
So the line that parallels the 220uf isnt supposed to be there?
Gonna wanna use a regulator in the 12V circuit.
Ya, Or I may just do with a 12v trans, I have to have 5 tubes anyway, so the one I have wont be enough.
You need to design in compartments, preamp 1, preamp 2, summing, effects loop, notch EQ, direct outs, B+, filaments, and controls.
You mean break down the schematics into sections so they are easier to read? Like a MESA schematic?
[/quote]
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Here ya. I got bored last night. No notch filter or effects loop.
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Wow!! Thanks so much. Thats Perfect!
And once I get to the point where i add the effects loop and notch i can easily integrate them with their own 12v transformer. Thanks Again!!
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It'd be a good idea to completely review this schematic. It was late and I was drinking. There should be enough 12VDC for the opamp circuit although the LM102 is an antiquated part. I think pretty much any old opamp should work. You may want to think about replacing some switches with 12V relays but I think your $200.00 budget is already shot. While on the subject, a time delay and output relay is also in your best interest.
I question summing off the 2 tone stacks. Yes I realize this is how Ampeg did it but it seems like the 2 tone stacks will interact with each other. There will be some loading factor, ie. when switching to mono, the output will be somewhat diminished. This may be the "magic" of the circuit. I didn't bother to recalculate the gain of the previous stage with the additional loading, but I'd bank on losing at least 3dB if not more.
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OK cool. I will review the schematic and compare it to the Ampeg. I really do appreciate you taking the time to help me!
The effects loop has a tube in it too, so I will have to add the 12v trans. No big deal though, I would rather have the option of adding more tubes later if I come up with some other features. I thing that the chassis is going to be about 18x10 or so, and maybe 4-5 inches tall, so there will be room to grow!
Who knows, Maybe in a few years I will add a power section to it. Maybe it will replace my 300ps. (though I seriously doubt it!)
Now I need to get the bugs worked out of my Bassman that I built from an old radio. Then I can start on this bad boy!
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This is a fairly thick project. I know exactly how easy it is to get excited and put the cart in front of the horse. You really need to enforce the 6P's. (Prior Planning Prevents Piss Poor Performance) Don't start buying stuff and poking holes until you've thought it through -n- through and created a layout.
I'm happy to help out. IIRC, Tubenit equated doing layouts to working crossword puzzles. I couldn't agree more. Designing is fun brain candy. Beats watching TV.
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I agree totally. I wont buy any parts until I have it all in a layout. Its a fun thing to do while I watch the 3 year old and the 1 year old destroy the house. :icon_biggrin:
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did you ever get to building this?
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did you ever get to building this?
Good question!
Glad you found this & dragged it back up, I posted in your other thread last week regarding a bass-pre & I think I had skimmed a section of this thread a while back, but had forgotten about it.
A single-channel version of this is EXACTLY what I would like.
Wonder how it turned out?
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He might be out on the road playin?
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I havent gotten to it yet unfortunatly. My work schedule has been crazy and when I do get time I have been playing around with some simple plate starvation circuits and making a couple pedals. But the good part is that I have been learning a bunch of troubleshooting, and my soldering is getting much better. So when I do have time to start this, my final product will be much better.
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Thanks for the update, you guys did alot of sorting already, just through this thread.
Other than the piezo input, it's just what I had in mind, so I might try a version once I finish the AB763 I have going.
Please let us know when you do have it up & running.
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Wow, thats alot of info first thing in the morning!
OK here it goes:
1. Low E string is 40hz when tuned to E. So a better transformer will be necessary.
I believe it's about 32Hz on a 5+ string bass with that low B added. (Someone correct me if I'm wrong.)
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Thanks for the update, you guys did alot of sorting already, just through this thread.
Other than the piezo input, it's just what I had in mind, so I might try a version once I finish the AB763 I have going.
Please let us know when you do have it up & running.
I will definitely let you guys know when I get to this. I actually think its going to get a basic dual 6v6 power stage to.
In the mean time I am having fun with plate starvation preamps and 1/2 watt SS power amps.
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Here ya. I got bored last night. No notch filter or effects loop.
Hey, Great schematic Richard!
I have ordered all the parts for a single channel preamp section but, still have not decided on a driver tube for the cathode follower / DI section. I'm leaning towards a 6SN7.
I presume your measurements for the coupling cap and cathode resisters would still be the same with a 6SN7 tube as its very similar to the 12AU7?
My apologies for the *green* questions as this is a my first post.
Jeff
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" 6.3VAC into a doubler will give you about 14VDC. Hit a little 12V regulator and call it good."
Betcha that won't work. I could be wrong, but my experience says you will have problems.
First, any kind of 3-terminal regulator needs a pretty solid +3 volts over the output to work, though maybe a 7805 could work with +2 volts. So we think "15" as in "barely enough". But that's for teeny solid state type loads.
The kind of load tube filament(s) present to a regulator....I assume you are thinking a 7812...I would be surprised to find out you can get away with less than 20 volts input volts and that's going to require a non-trivial heatsink and very frankly, that type of scheme essentially eliminates your current power transformer from consideration.
I don't think you're gonna make it with a doubler off a 6.3 VAC winding feeding a 7812. In fact, I'll *promise* you won't make it. not if you're running tube heaters. My experiences trying such arrangements have not worked out at all. YMMV.
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use a low dropout regulator - operates down to 1V over dropout. so a 12V output requires 13V input to the regulator.
ex. of LDO regulators are: LT1083, LT1084, LT1085, LT1086. each part number differs based on output current. they are all available in fixed or adjustable configurations. LT1083 is 7.5A and LT1086 is 1.5A
use the adjustable version and set Vout to 11.5V for greater dropout margin. 12V tubes and 12V relays will operate fine with 11.5V.
--pete
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I hope I'm not "hijacking" this thread as I'm not concerned with a 12V regulator. I'm not familiar with LT voltage regulators...
This is what I have so far. Please disregard the power supply section as I've decided to go with a power supply from London Power.
My concern is that I have the load properly calculated for the cathode follower section. Is this right?