Ampeg Based Piezo Preamp Build

[terminalgs]

XLR ground lift.



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).

[blown240]

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!!!

[RicharD]

The million dollar question for the DI is, "What transformer are you using?"

[blown240]

I dont have it yet, but it will be close to this:

https://taweber.powweb.com/store/balun.jpg

[blown240]

Here is where I am at now:



Will I need some sort of resistor for the B+ to the CF now that I am using both sides of the 12AU7?

[blown240]

Here is a quick rough layout idea.  Its based on a 16 x 8 inch chassis.

[terminalgs]

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...)

[RicharD]

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.

[birt]

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.

[blown240]

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

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!

[birt]

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)

[blown240]

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.)

[blown240]

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?

[RicharD]

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...

[blown240]

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?

[blown240]

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:



What am I missing?

THANKS!!

[RicharD]

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.

[blown240]

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)

[RicharD]

That's an awful lot for 2 clams. What PT do you have?

[blown240]

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.

[RicharD]

I sketched up a topology. 

[terminalgs]

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.

[terminalgs]

you might KISS, and simply add stereo option like this:



Then, later,, when you are happy with that, and you have some money, do something like this:



EDIT:: don't let your guy charge you $10/12au7, you should get 2-3 for $10.

[blown240]

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:

[blown240]

Here is my idea for the stages:



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?

[RicharD]

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.

[blown240]

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!!

[RicharD]

>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. 

[blown240]

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!!

[RicharD]

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.

[blown240]

OK, here is where I am at this point:



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.

[blown240]

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.

[RicharD]

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.

[blown240]

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]

[RicharD]

Here ya.  I got bored last night.  No notch filter or effects loop.

[blown240]

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!!

[RicharD]

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.

[blown240]

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!

[RicharD]

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.

[blown240]

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.

[bakerlite]

did you ever get to building this?

[tdvt]

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?

[Willabe]

He might be out on the road playin?

[blown240]

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.

[tdvt]

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.

[Shrapnel]

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.)

[blown240]

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. 

[Siku Music]

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   

[eleventeen]

" 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.

[DummyLoad]

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