F-style preamp pedal

[Isaac313]

Hey, this is my first post on this particular forum, I'm a member on many many other ones, but I am pretty new to stompbox design. I am quite experienced at guitar tube amp building and design, but this is for pedals.   I took a F-style blackface circuit (ab763) preamp and replaced the tubes with two sides of a JRC4558 (i use the 4558 because I have those in my drawers)  But I only designed this on paper so far. So heres the deal. I took the preamp from this AB763 design and replaced the triodes with an IC, I haven't put in any switching, power supply, or led stuff, yet. This is only 30 minute form the time I started the design.  So heres a scan of the schem i drew,  It has some notes, and there is one scribble... (ignore it)  and there is a drawing on the other side of the paper which shows through a little (ignore that too. Its kinda hard to see anyway)
  Hope this link works, I couldn't figure out how to upload an image, and It wouldn't let me copy and paste it. So i shared it on google+ and this should work.    https://lh3.googleusercontent.com/-X1tkGUnjzJc/UVEhpqtMPvI/AAAAAAAAAHE/JsL9E5HwXII/s977/2013-03-25

What do you guys think, will this work, or if it works do you think it'll work well, or what can i do to make it nicer, or whatever your thoughts may be.  I have some more thoughts for it, like boost and stuff but that is for after we get the preliminary stuff figured out.

[Davelectro]

It won't work. You missed biasing resistors and coupling caps.

[Isaac313]

Can you elaborate what exactly, where and what values?

[PRR]

Op Amps For Everyone, 2MB PDF file

[PRR]

Additionally:

Stage One of the tube amp has gain of 50 and a 300V power supply.

If you set-up your opamp 1 for gain of 50, but only 9V supply, it will be OVER-driven on all but the lightest strums.

You need to estimate the gain and maximum output of those AB763 stages, then RE-work the stage gains to pass signal well with the lower supply voltage. (And of course learn how to set an op-amp's gain; see book above.)

The tube output impedance is around 40K; the opamp just a few ohms. The tone network impedance drops from over 1Meg in deep bass to under 100K in treble. The response is (slightly) different when driven from 40K ohms or 4 ohms.

Tube distortion differs from opamp distortion.

There's reasons Fender didn't chip-ify all their circuits the year opamps got cheaper than tubes.

There are MANY MANY opamp guitar-amp designs. Some owe a lot to AB763 roots, but significantly re-designed for the lower supply voltages. Plagiarism is the best research.

[Isaac313]

Thanks for the help, Can you show me what you would do to make this work, I just pulled this together really quick since I was reading about lots of diy pedal stuff, so i thought I might be able to do this, thats why I posted here.

[Isaac313]

I thought that the opamp ran on 9v similarly like the tube runs on upwards of 300, honestly I think a 12ax7 in this case actually only runs like 250v max, but that's unrelated, anyway, again, I thought a opamp in a this scenario would run 9v with gain of 50 as a triode would do 250v-ish with same gain ratio.

[Bill Mountain]

You can surely set your opamp up for a gain of 50 but the problem you run into is that 50 times the average voltage of your signal is likely to be higher than the headroom afforded at 9 volts.  An opamp at 9 volts may only have 6 volts max of clean headroom.  Plus...if you try to have your other gain stage multiply by 50 then you will surely run into distortion (not the good kind).

You will need to establish the average voltage of your signal.  Then establish the max gain in each opamp stage to avoid clipping.  Or you could raise your supply voltage.

[Isaac313]

I'm sure my unknowingness in this field and my same questions pretty much are annoying, and I'm sorry for that, but I simply dont understand "all" of what you guys are saying. I get some of it, just not all.  Can you give me a small drawing or something to elaborate. And I do want this box to distort a little, to make for a small fender-style overdrive pedal. Thats my idea.  But thanks for being so responsive guys.   I could probably raise my supply voltage to 18v tops.

How do I do this,  "You will need to establish the average voltage of your signal.  Then establish the max gain in each opamp stage to avoid clipping."?

[garcho]

Check out rog

Lots of pedals modeled more or less after tube amps, mostly using discreet FETs. A few op amp designs too.

Tubes and op amps don't have a linear relationship. Better to take PRR's advice and see what others have done.

[Isaac313]

I have said this already but noone has provided one yet. Sorry if I sound desperate or disrespectful I dont mean to, I just want to figure this out and learn new stuff.   Can someone provide me with a schematic or modded version of  mine that should get me to something that at least works a little?  I want to understand so bad but cant.  So I need some stuff.  So again can someone give me a schematic or modded version of mine?

[slacker]

Here you go, have a look at this http://www.runoffgroove.com/tonemender.html it's similar to what you're wanting to do. You could splice your tone stack in place of theirs and your gain control could replace the 1M resistor after the tone stack.
Their design has the first op amp set up as a buffer but you could make it into a booster similar to their second stage.

How do I do this,  "You will need to establish the average voltage of your signal.  Then establish the max gain in each opamp stage to avoid clipping."?

Say your tube is running at 250 volts and has a gain of 50, your guitar signal will be something between 100mV and over 1Volt on peaks depending on how hard you play, pickups etc. Take the maximum of 1 volt your tube can easily apply a gain of 50 making it 50 volts without any distortion, 50 being much less than the 250 Volt supply.
If you try and apply the same amount of gain to a 1 volt signal in a circuit running on 9 Volts, it can't make it be 50 Volts, the absolute max it can do is 9 Volts, it will still apply a gain of 50 but the signal will be clipped hard. Like Bill said many op amps will clip below the supply Voltage. So if you want a clean boost you can only get away with a gain of around 5, anything over that will clip.

I'd try Something similar to what I posted above and play about with different gains, to get an idea of what's going on, this will also show you if you like opamp distortion or not, a lot of people don't.

Hope that helps.

[Isaac313]

Thanks, Your post has been very helpful. If you have any other ideas dont forget to share. ;-)

[Ben N]

Fender did a number of "chipified" BF/SF preamps. Look for the schematic for the Champ 25SE; I'm sure there are others, too. Also, pretty much any Music Man preamp is along the lines of what you want. None of them run at 9v or anything close. (I think the MMs gave the preamps +/-15 VDC, IIRC.)

[defaced]

What do you guys think, will this work, or if it works do you think it'll work well, or what can i do to make it nicer, or whatever your thoughts may be.

I'd go a different route entirely.  High voltage MOSFETs are available which can do this task in a pedal size with the only real obstacle being generation of the high voltage supply. Search these forums and the internet at large for "LND150".  KMG (a user here who has posted some of his work online) has some great information that will help you achieve what you're after.  I've also made a project using that part, which includes a Fender style clean channel, and I will say it sounds very good.  If you do a search, you will find my work (which has progressed, but is not finished since that post was made).   

[Bill Mountain]

If you want a little overdrive I think your best bet is to do a JFET or MOSFET style preamp.  Check out the designs by Runoffgroove.com which I believe was linked above.

JFETs can be used to replace tubes in circuits but they require more attention to biasing.  Runoffgroove has a tone of great overdrives and some of them are modelled after Fender preamps.

Opamps won't clip musically without a lot of extra design that is beyond the abilities of most budding hobbyists.

I would however read this page:

http://sound.westhost.com/dwopa.htm

It was most helpful to me when I was learning how to use opamps.

[Isaac313]

Thanks, you guys have been helpful.   I think I'll go with the JFET route if I can get around to starting this project. Yea, some of the cicuits I found on runoffgroove.com   inspired me to do this, before anyone else told me to visit there.   especially the marshall 18w pedal, since I am VERY familiar with that amp. I have built several.  But I decided that I liked that other circuit and wanted to build it so I decided I'd ask.

[garcho]

But I decided that I liked that other circuit and wanted to build it so I decided I'd ask.

Why? And what other circuit are you talking about, the Fender preamp? What do you like about it? What musical goals do you have? There's nothing to this circuit. It's two gain stages with a tone stack in the middle.

Keep in mind that the venerable TL072 is a FET op amp. You know, if you got something for FETs. High input impedance, good for guitars, usually.

Yea, some of the cicuits I found on runoffgroove.com   inspired me to do this

And you didn't notice that op amps have two inputs?!      Take PRR's advice (he's a titan) - do your homework and plagiarize, ain't no shame!

[Isaac313]

Why? And what other circuit are you talking about, the Fender preamp? What do you like about it? What musical goals do you have? There's nothing to this circuit. It's two gain stages with a tone stack in the middle.

Keep in mind that the venerable TL072 is a FET op amp. You know, if you got something for FETs. High input impedance, good for guitars, usually.
I know op amps have two inputs, thats how I got the two stages from one op amp.  Yea the other circuit is the fender preamp. I Just installed that in a marshall 18w instead of the normal 18w preamp and it sounds nice and I know some other good mods for it that I didn't include in the schematic i set, but anyway, If anyone wants to make a schematic that is similar to what I got but a more usable design, please share.
my goal for this pedal was a tonestack and boost pedal.
Do you guys have any REALLY good examples of something similar that i could plagiarize from?   I really like the tonemender tonestack and boost on ROG.

[garcho]

Each op amp has two inputs, an inverting and non-inverting input. No offense, but I believe you're confusing the fact that most DIP op amp chips are dual op amps, meaning there are two completely separate op amps on the same chip. Op amps 'work' by how the inputs are manipulated. Keep in mind they're originally designed to be used with bi-polar power supplies, so there is a 'negative' voltage, but it's still DC. 9V pedal builders deal with this by creating a half-way point between lowest and highest voltage, then biasing the op amp as if the PS was bipolar.

two inputs:



one of many standard applications: