Is it supposed to sound really transistor-y?  It doesn't seem to boost the mids as much as make the guitar sound thin.

Has anyone built one using the combined buffer/booster PCB from the .pdf?

What are the transistor voltages supposed to be?

Thank you.

Can you post a link to the schematic?

Here you go, sorry.  I don't think I can direct link it.

http://www.blueguitar.org/new/schem/_gtr/ec_midbst_all.zip

If not, you'll need to go to

http://www.blueguitar.org

Then click on schematics on the left side, then find "Misc Guitar Wiring", and then click the ec_midbst_all.zip

It's not a direct schematic, but the .zip file.


Wow, I just had a DejaVu moment with posting this, or something wierd just happened in my brain!

Thanks![/url]

Another thing I noticed is that there doesn't seem to be an appreciable increase in 60Hz hum that some users complain when using this circuit with SC's.  Does anyone have the correct transistor voltages?

Thanks.

I have the circuit somewhere in a box, and I'll see if I can find it tonight or tomorrow to check that.

I remember I was disapointed when I tried it into my guitar. So that's why it's not in it anymore. But I don't remember why.

The guitar I normally use for that doesn't have any neck for now, so I can't try anything with it right now.

Gilles

I'M SURE i'ts there>>.sorry i couldn't see it.

The 250K pots are not there, but otherwise it's complete

http://www.finitesite.com/gtechblues/photos/ECschem.JPG

Added:Here is where to find the original drawings on The Blue Guitar site

http://www.blueguitar.org/new/schem/_gtr/

Btw, I never saw the original drawings until today...

Here are the voltage readings I took:

Q1:  B - 1.15  C - 0.68  E - 0.56

Q2:  B - 3.73  C - 9.40  E - 4.43

Q3:  B - 4.95  C - 10.00  E - 4.43

Q4:  B - 9.40  C - 4.96 E - 9.99

(Q4 is a PNP, so the emitter switches places with the collector)

I also took the Base-emitter voltage difference RG recommends:

Q1:  .59
Q2:  .54
Q3:  .52
Q4:  4.43

Q4 looks off.  I'm 99% sure I have the orientation correct - legs facing down, flat side facing me, 2n5087 should read left to right EBC.  And the schematic has this transistor with the collector where the emitter usually is drawn, and vice versa.  

What am I doing wrong here?  I'm still getting a horribly gated effect.

Thanks

OK, I can see a couple of things wrong.

Quote from: Craig VHere are the voltage readings I took:

Q1:  B - 1.15  C - 0.68  E - 0.56...

Here the base and emitter are right but the collector is almost at ground and the transistor is saturated it should be somewhere around 1/2V+. Check to make sure that you have a 22k on the collector. This is almost certainly your source of gating...

QuoteQ2:  B - 3.73  C - 9.40  E - 4.43

Q3:  B - 4.95  C - 10.00  E - 4.43

Q4:  B - 9.40  C - 4.96 E - 9.99

(Q4 is a PNP, so the emitter gets the voltage from the battery)

I also took the Base-emitter voltage difference RG recommends:

Q1:  .59
Q2:  .54
Q3:  .52
Q4:  4.43

Q4 looks off.  I'm 99% sure I have the orientation correct - legs facing down, flat side facing me, 2n5087 should read left to right EBC.  And the schematic has this transistor with the collector where the emitter usually is drawn, and vice versa.  

What am I doing wrong here?  I'm still getting a horribly gated effect.

Thanks

Q2, Q3 and Q4 form a discrete opamp. To understand it better, Q2's base is the positive input, Q3's base is the negative input and Q4's collector is the output. The stuff between Q4's collector and Q3's base is the normal feedback stuff you would find on any non-inverting opamp configuration.

Your voltages for the opamp portion actually look OK. The Vbe on Q4 is actually .59V you measured either base ot collector or emitter to collector. All in all the opamp portion looks good.

Check the collector on Q1, that is where the problem is...

Good luck,

Jay

I simulated the circuit and it works perfectly, so triple check your soldering and all your wiring.  Voltages should be:

Q1 - C = 4.7V, B = 1.5V, E = 0.9V
Q2 - C = 7.8V, B = 5.5V, E = 4.9V
Q3 - C = 9.0V, B = 0.3V, E = 4.9V
Q4 - C = 8.3V, B = 7.8V, E = 0.3V

Oh yeah, here's the schem I used:

http://www.blueguitar.org/new/schem/_gtr/ec_schem_fact.jpg

Thanks for the replies!

I do measure 10V on one side of R4 (measured resistance at 21.5K) but at the R4-Q1-C4 junction I still get .68V.  

I don't get where the voltage is going.  Is 22K too big of a value?


Is the purpose of C4 to act as a voltage blocker for the volume pot, so it is normal to measure 0V after C4?  Would an error after this cap effect the voltage reading on the transistor?  (I have checked and rechecked and so on.... everything looks good)

THanks for the help

How are you getting 10V?  You're using a 9V battery, right?

22k should be fine.  The transistors are known good ones, right?  Or in other words, you've tried replacing the transistors?

Double check the values of the resistors on the voltage bias string to Q1. the 1M5 and the 33k on the base. Something may be wrong there in those values.

I do indeed get 10.02 V from the 9V battery - it's brand new.

I haven't tried switching the transistors because these are new, and I've learned time and time again that parts don't cause a failure - it's always an error on my behalf causing the failure!  I will switch them to test if needed.

On the base, the 1M5 checks alright, but I don't use a 33K - I use a 330K.  Is there a discrepancy between the schematics?

For C6 I used a non-polar electrolytic 10uf instead of a polarized.  Would this effect anything?  I'm also using trimpots (250K and 50K) but that shouldn't be a problem.

Thanks for the help.

Quote from: Craig VOn the base, the 1M5 checks alright, but I don't use a 33K - I use a 330K.  Is there a discrepancy between the schematics?

Man, gotta be careful about those typos. I meant 330k. Make sure that the solder joint between the 330k and the base is solid, that could cause your problems.

I promise you something is wrong with that first stage. Double check all the values and then the pinout of the transistor, you could have it in backwards. If need be, swap out the transistor and see if you get the same voltages.

Good luck,

Jay

Make sure that 21.5k resistor on the C of Q1 is actually around 21.5k, and that it's not open or shorted.  Your voltages on Q1 correspond to a short or too large a resistor.

Hi guys,

I went through and resoldered everything on the buffer side, and I found what looked like a hairline copper trace to ground between R6 and C3.  I used my Dremel to break the connection up, and now I get:

Q1:  B - 1.31  C - 5.19  E - 1.04

Now it doesn't gate at all, but doesn't seem to really drive the amp like it should.  It has a little boost, and the Mid pot doesn't change anything except a slight tone change at one extreme of the pot. Something still may be wrong.

Here are the other transistors:
Q1:  B - 1.31  C - 5.19  E - 1.04
Q2:  B - 3.72  C - 9.31  E - 4.38
Q3:  B - 4.91  C - 9.71  E - 4.38
Q4:  B - 9.31  C - 4.91  E - 9.91

cd's simulation shows:
Q1 - C = 4.7V, B = 1.5V, E = 0.9V
Q2 - C = 7.8V, B = 5.5V, E = 4.9V
Q3 - C = 9.0V, B = 0.3V, E = 4.9V
Q4 - C = 8.3V, B = 7.8V, E = 0.3V

I'm still off on a number of things.  

Q2 on mine has the base lower than the emitter. Q4E is way too high.  Does the .6V Base-Emitter rule apply here?  The simulation doesn't adhere on Q3 and Q4 - can the circuit function with those readings?

The battery I'm using is still at 9.9-10.0V so some voltages should be higher.

I appreciate your help!

I built the booster in a pedal, but did not use the Buffer section, Plus i used NTE tranny's, mine works great, i did change the cap that control's the mid's, i increased it for a tad more mid's, I can't help you on my voltage's because i've loaned the pedal out, by the time i get it back you will probably have your's figured out.
Johnny

Whoops, looking at your latest post it seems I posted the wrong voltages on Q4.  Should be:

Q4 - E = 8.9V, B = 8.3V, C = 5.4V

BTW in the sim I'm using 2N3904s instead of 2N6429s (as per Fender factory schem).  There should be no difference compared to 2N5089s or other NPNs though.

There should be plenty of volume boost with the mid pot turned completely one way.  Something like this:



The upper wave is the boosted (full mid) signal, the lower wave is the stock signal (440Hz, 55mV sine wave).