Tube preamp inside a guitar

[sumgai]

dunk,

No worries, I know all about that grapefruit versus statins thing. It's not so much that it'll "kill you right now" or something, it's only that the acidic nature of grapefruit nullifies the good stuff in a statin drug. Too much of that, and you might as well have never taken the meds at all.

Kinda too bad, 'cause I did like a half-grapefruit, most mornings. I've been "making do" with oranges since then.

Thanks for the sentiments.


Standing by for the arrival of your goodies.




sumgai

[dunkelfalke]

Nah, doesn't have to do anything with acidity but with the fact that the grapefruit contains some flavonoides which inhibit an enzyme that metabolises certain medicines, thus actually amplifying their effect, not nullifying them but giving you basically an overdose.

Sorry, my mother is a medical doctor and that rubs off.

[dunkelfalke]

Okay, the parts have arrived, at last.

[dunkelfalke]

It looks that after rearranging the parts and some drilling I just might to smuggle in an AAA sized second battery. It is not nice, but certainly not as difficult as the phantom powered version.
Would I still need the filter caps?

Sorry that your work probably was for nothing.

[JohnH]

If you can get that AAA in, then I think your scheme gets alot simpler. Heres a data sheet on a Lithium AAA:

data.energizer.com/PDFs/l92.pdf

Interestingly, at 25mA (and presumably also below), the current/time capacity in mAh is the same as alkaline. But the discharge curve in terms of output voltage/time of lithium is much more consistent. So maybe with one of those, drawing 10mA for your heater, you might get 80 to 100 hours at close to the design voltage?

Go breadboard it!

John

[dunkelfalke]

It looks like I have lots of things to do before I can start with it - it turned out that my active pickups won't fit into the guitar so I'll have to route. And I need one replacement pickup since I have managed to break one pickup's mounting eyelets while trying to place it.

I can't even be absolutely sure yet that I'll manage to fit the second battery (and the arrangement will be non-standard, the selector switch and the volume control will have to exchange their normal places.

[dunkelfalke]

sumgai, here is a question especially to you:
is it possible to tap the roland gk-2a control unit to power the tube?

[sumgai]

Mar 16, 2010 4:30:32 GMT -5 dunkelfalke said:

sumgai, here is a question especially to you:
is it possible to tap the roland gk-2a control unit to power the tube?

Certainly! In fact, we'll have both positive and negative 7 vDC to play with. That means, if we're careful, we can get as much as 14 vDC on the tube's anode (plate), without any regulation devices at all. We need only take care about grounds and such, and we can make it all work together.

Do you already have such a beast, and if so, is it internal or external? I should mention that RMC, Graphtech and Axon all make similar units that will also work, if one of them should pop up on your radar. (Yamaha used to make a hex pickup that was licensed from Axon, so it should be good too.)

HTH



sumgai

[dunkelfalke]

In a way. On sunday I've damaged my Revelation ATX with a runaway router and today I've found (and ordered) another one on ebay, already equipped with such a pickup sold together with a VG-8 and since you've advised me to get one... well, goodbye €500, I just hope I can sell my Digitech GNX 3000 for good money.

Is it possible to get 9v out of the Roland unit? Because the pickups and the active electronics would still need that.

This is what the seller promises:

[sumgai]

dunk,

I recall a conversation with a fella, several years ago now, where he asked about finding 9 vDC within his VG-8 (not a VG-88). I thought that wasn't possible, but he poked and prodded around in there, and finally found a place where 9 vDC was indeed being generated.

Now, I had envisioned a not-nearly-as-complicated scheme as we dreamed up with that 48 vDC phantom supply (from a mixer). First, the plus and minus 7 vDC is already regulated to a high degree. All we need to do is wire up a voltage divider, and we can get 9 vDC from the 14 volt supply, no problem. But I say again, we do need to watch our ground circuits, they can't interact directly, or all bets are off.

However, if I'm correct about 9 vDC being truly present and ready for duty, then there's a blank pin/wire within that 13-pin cable, and that will get the 9 vDC up to the guitar for us. Interestingly enough, it's Pin 9 that's blank, and several schemes have been dreamt up over the years for making use of this wire. But there is a gotcha...... some companies have elected to sell aftermarket "13-pin" cables without a wire on Pin 9.   That puts a big damper on any plans for that pin, so before any further work, be sure your cable has continuity on Pin 9. (So far as I know, Roland's cables are complete in this regard.)

So, in the interest of getting you started on the proper foot, let me suggest a few websites and such. First, the granddaddy of 'em all, and still a good reference:

vg-8.com (no www in front)

Lots of info here, better pack a lunch, it's gonna be a long time before you return to the real world!

launch.groups.yahoo.com/launch/vg-8

Been kind of hanging fire since the VG-99 came out, but still useful for some things. Has a library of patchs and other files.

www.vguitarforums.com

Newest site on the web, already has an extensive (and impressive) roster of long-time users and experts, lots of patch sets and files, and they cover most of the modeller/MIDI world, not just Roland. (They even include loopers and such, like for those who do the "one man band" thing.)

If you're not yet into MIDI, you soon will be! Check around for a used Behringer FCB-1010 foot switch controller. That will give you a pedal (actually two of 'em) that you can use to control the volume and some other parameter, not to mention a way to quickly switch patches and/or other values. But if that's a bit rich for your budget just now, you can use any regular analog volume pedal, there's a jack in the back just for that purpose.

And if all else, fails, you can always eBay it back out the door! But I have a feeling that you'll give it a good trial run before you start thinking along those lines. If nothing else, the ability to re-tune in the blink of an eye will get your attention! Being able to drop down 5 semi-tones (thus imitating the lower 6 strings of a 7 string axe) really plays mind-tricks on the audience, lemme tell you! ;D

HTH



sumgai

[dunkelfalke]

Got it. Since I will someday move to a more modern VG, I think there would be better to get 14V from the control unit itself and use the 9.1V zener to power the pickups. It will take a while though before I start, because, as you say, I am kind of low on money and on time right now. I've been watching that Behringer MIDI foot controller for some years now, will buy one someday.

By the way, is it possible to use the VG-8 as a generic midi synthesizer? Say, for guitar pro or keyboard output?

Thanks for all this information.

[sumgai]

Mar 17, 2010 11:10:16 GMT -5 dunkelfalke said:

By the way, is it possible to use the VG-8 as a generic midi synthesizer? Say, for guitar pro or keyboard output?

Sadly, no. The reason is, none of Roland's VG-xx series do the "pitch to midi" conversion. To get this feature requires a GR-30/33/20 (complete self-contained units), or a GI-10/20 (pitch-to-midi conversion only). Axon and others also make pitch converter units that are totally compatible with your newly-purchased guitar and GK system.

To answer your question more correctly, this thing has nearly all of the MIDI command set, with the exception I've already mentioned, the inability to create a note and send it to some other generator like a keyboard (or to the computer). Neither can it receive a "Note" message and do anything with it, there's no separate tone generator on-board.

However, you might be interested to learn that the VG-8 comes with a patch that is a very convincing synth sound, quite reminscent of the old GR-300, which was used by Pat Methany. You can, of course, twist and tweak that (or any other patch) until it sounds like what you want.

HTH




sumgai

[dunkelfalke]

I have prototyped it today, with two batteries first. It did not work, only the 1.5v battery got very hot. Probably shorted somewhere, but I couldn't find where. Or maybe I've managed to damage the tube somehow.

I guess I'll try again some day later.

[bassxtreme]

Feb 12, 2010 13:25:43 GMT -5 sumgai said:

Feb 12, 2010 6:18:35 GMT -5 dunkelfalke said:

Can you explain it to me in layman's terms?

Certainly, be happy to.

Since you didn't link to the datasheet itself, nor the corresponding example schematic, I'll assume you're referring to the part numbers in your own schematic.

The tube itself is amplifying AC (your guitar signal), and that's getting back to the battery and the input of the LM317, via R3. To keep the input of the regulator happy, we need to filter this down as much as possible - that's C3's job.

R1 is a current limiter. Many simple regulator circuits rely only on this part, and have no filter cap (C2). In this case, C2 is used to, once again, filter out the amplified guitar signal from the regulator's Adj terminal. Without it, the DC voltage to the cathode/G3 would vary up and down in time with the signal, and that's not acceptable.

Transient response refers to the ability to follow the signal. In your case, it has to be large enough to handle all the possible frequencies that your circuit will likely see. I should think that 1µf would be sufficient for this task, but your diagram shows 10µf. That's a bit of overkill, but if you already have this value cap in your "junk pile", then go ahead and use it, it won't hurt anything.

C1 and C4 are quite different values, for a reason. C1 allows AC to pass to ground. Note the 47µf value - it effectively shorts all AC to ground, thus there is only DC on the cathode/G3 pin. I think this will affect the tone, but how much I can't predict. I'd say that you should experiment with the thing in the circuit, and again without it.... see which way sounds best to your ears.

(Advanced study: note that older tube pre-amps often used a 'bypass' capacitor in parallel with a resistor on the cathode. This was a "selective" AC grounding, meaning that certain frequencies were bypassed to ground, while others went through the tube just fine. I'd caution you not to get too obsessed here, because you can kill a lot of time, trying out various possible combinations of resistors and capacitors! My advice is to keep it simple at first, get the circuit working and sounding the way you like it, then maybe "hack" at it a little bit. )

Sometimes things go 'bump in the night', meaning that other signals get into your circuit, from outside souces. Things like AM radio stations are good source of this type of interference. C4 is intended to squelch that kind of noise. If you're environment is pristine, then you can eliminate this cap too. If you suffer 'CB' radio coming out of your speakers, this is the first place a technician works on, in order to get rid of that kind of crap.

There, I think that covered all of what you asked, anything else?

HTH




sumgai

I built this preamp awhile ago (without the opamp) and it sounds great. I added the C1 47uF cap and didn't hear any difference. since I never heard a CB radio or a high shrilly sound I didn't add C4

[exabrial]

Hey Guys,

My apologies to wake up a really old thread, but I'm interesting in building this as a replacement in my guitar. Two things:

First, does anyone have the final diagram?

Second, my guitar is a piezo pickup, so I've read that they have resistance in the 1m-2m ohm range and they need to be impedance matched to their preamp. What would need to be changed on the circuit?

Many thanks, this is perhaps the only discussion of this kind of project on the entire web!

[nikogo]

May 30, 2015 10:46:51 GMT -5 exabrial said:

Hey Guys,

My apologies to wake up a really old thread, but I'm interesting in building this as a replacement in my guitar. Two things:

First, does anyone have the final diagram?

Second, my guitar is a piezo pickup, so I've read that they have resistance in the 1m-2m ohm range and they need to be impedance matched to their preamp. What would need to be changed on the circuit?

Many thanks, this is perhaps the only discussion of this kind of project on the entire web!

I wonder why would anybody need a tube preamplifier?
The piezo pickup has higher output than magnetic and needs rather divider to  adjust the output level to the rest of pickups. A good piezo preamp should have about 5 MOhm of input impedance for good bass. The original tube scheme has 1 MOhm, and it is a pot that may be noisy. Such high impedance divider needs some treble compensation because the tube input capacitance will reduce high frequences in divider.
A regular follower based on low noise MOSFET transistor can do good job and have less problems with power supply.

[sumgai]

exabrial,


First, welcome to The NutzHouse!

Second, there's no need to apologize for "waking up" an old thread, in fact you don't hold the record for how far back one can go, trust me on that.

Third, your piezo's output impedance is indeed very high. But you don't need to worry quite so much about exactly matching the amplifier's input impedance to that piezo. In fact, your amp can have a much higher input impedance, and the piezo will be just as happy as if the two impedance values were exactly the same. We can discuss the reasons for that if you wish, but for now, that bit of information should make your searches/researches a bit easier.


Now I'm gonna address a few remarks to nikogo, these shouldn't be germane to your needs, but you're alrady here, so what the hey, sit back, pop a top, and keep yer thinkin' cap on.

May 31, 2015 6:14:17 GMT -5 nikogo said:

I wonder why would anybody need a tube preamplifier?

nik, I hate to have to say this, but you just pee'd in a new guy's Cheerios. That's not our usual modus operandi here in The NutzHouse - rather, we try to encourage people to experiment, no matter how simple, how complex, how much its been done before, etc. Yes, if someone is about to blow a large wad of cash on a Marketing Fad, or if they're about to do something completely dangerous, then that's a good time to speak up, even to a first-time poster. I didn't see that happening here, hence my admonition to you. In the future, please keep an open mind about what newcomers are asking for, and try to "let them down, gently", if you feel that's needed. Thanks.

The piezo pickup has higher output than magnetic and needs rather divider to  adjust the output level to the rest of pickups. A good piezo preamp should have about 5 MOhm of input impedance for good bass. The original tube scheme has 1 MOhm, and it is a pot that may be noisy. Such high impedance divider needs some treble compensation because the tube input capacitance will reduce high frequences in divider.
A regular follower based on low noise MOSFET transistor can do good job and have less problems with power supply.

All pretty much true, but why not take it a step further and either supply a diagram, or at least link to such on another website? Both the requestor (exabrial) and other casual readers might benefit from such actions on your part.   Again, thanks for thinking ahead.






sumgai

[nikogo]

sumgai,
thank you for peeing down gently on me.
The piezo amplifier is here:
guitarnuts2.proboards.com/thread/7444/490-fixed-tones-hh

 

[sumgai]

niko,


Ah, that's better. In fact.....



exabrial - take a close look at what nikogo just linked to. I've checked it over, and while I might use different values in certain places, it does look to me like would work as advertised. If you have any questions, you can ask anybody in general, or nikogo in particular.


HTH





sumgai