Tubescreamer input buffer problem

[tinlee]

Hi guys!

Recently started to get into pedal building and after messing around with a few different electra distortion builds I decided to make a tubescreamer. I like to breadboard circuits before making them to have a play around with different components etc before soldering a perf board or making a PCB.

Anyway I followed the schematic on Tonepad for the tubescreamer (http://www.tonepad.com/project.asp?id=1) and everything works perfectly apart from a problem. Whenever I turn the volume up on my guitar to full the signal starts cutting out, almost like a noise gate that is turned up too high. But I don't have this problem when I turn the volume down to just below full. I have narrowed it down to the input buffer stage by bypassing the clipping, volume and tone stages of the circuit and it still does exactly the same thing just clean. Please watch the video below and you can hear what I'm talking about (Excuse the bear dressing gown hehe  ):

I have tried:

Replacing all of the components in this stage (It's rare that a component would fail but better safe than sorry!)
Using two different transistors (same problem)
Adding a High Pass filter before it hits the Base of the transistor (corner freq around 500Hz) I had a thought it may be that too much low frequency was overloading the transistor.
Using different guitars.
Using different Amps.

If anyone has had a similar problem to this with this pedal or indeed any other build, then any help would be much appreciated! Thanks

[karbomusic]

Is the buffer biased properly? Assuming about 4.5 VDC at the buffer input pin?

[tinlee]

Is the buffer biased properly? Assuming about 4.5 VDC at the buffer input pin?

I wouldn't know how to check if the buffer is biased properly! I'm getting a reading of 4V from the voltage divider that is connected to the buffer via a 510k resistor! Is there anyway to check that it is getting the right bias with a multimeter?! There should be a schematic on the tonepad page that I linked previously!

Thanks for your help!

[R.G.]

What are the measured DC voltages on all three pins of the input buffer transistor, and also the measured DC voltage on the bias divider on the NON-transistor end of that 510K bias resistor?

[tinlee]

What are the measured DC voltages on all three pins of the input buffer transistor, and also the measured DC voltage on the bias divider on the NON-transistor end of that 510K bias resistor?

1 being the Collector
2 being the Base
and 3 being the Emitter

Over pins 1-3 is 4.8V
Over Pins 1-2 is 3.2V
Over Pins 2-3 is 0.58V

The voltage over the 510k is 0.2V - not sure how to measure just one end of a resistor! Do those measurements mean anything? Thanks!

[tinlee]

Ok so I learned how to measure voltage at the points you were describing, doh. Sorry I haven't trouble shooted anything like this before! heh

Collector is 8V
Base is 5.1V
Emitter is 6.9V
510k resistor is 4V

Should anything be different to what I measured?!

[R.G.]

Should anything be different to what I measured?!

There are still some issues there. For measuring voltages for this purpose, all the measurements have to be made with the meter set to DC volts, on a scale that's only modestly bigger than the maximum voltage, so about 10-20Vdc max for the scale, and the black/negative/common lead needs to be clipped to the signal ground. One easy way to get signal ground is the bushing of an input or output jack.

[M23Bomber]

Hey all,

Check the video:

https://www.youtube.com/watch?v=D6Pvvv1H53A

Does any one do the same? If yes How?

Regards,
M.

[tinlee]

Should anything be different to what I measured?!

There are still some issues there. For measuring voltages for this purpose, all the measurements have to be made with the meter set to DC volts, on a scale that's only modestly bigger than the maximum voltage, so about 10-20Vdc max for the scale, and the black/negative/common lead needs to be clipped to the signal ground. One easy way to get signal ground is the bushing of an input or output jack.

Yes this is how the second lot of measurements were taken! Ant idea what needs to be done/what the problem is! Thanks again for your help!

[tinlee]

So I re-built the buffer stage and the voltages are now reading:

Collector is at 9.3V (I changed the battery)
Base is at 2.7V
Emitter is at 3.2V

510k is at 4.6V

But I still have the same problem 

Here's a picture of the Breadboard, I'm 100% sure that everything is in the right place though! Any other ideas would be welcomed! This is driving me a bit insane!  (should be noted that i had to use a 47nF cap instead of the 27nF as the schematic suggests)




I noticed something though. I decided to try my 7 string guitar which has lower gain pickups and I didn't have this problem when using the bridge PU. But then the same problem came back when I switched to the neck PU on the 7. Does this mean anything? Like there's too much signal going into the base? No one else on tonepad seems to have had this problem so I'm pretty sure the schematic is okay! Any other ideas?! Thanks again for your help, much appreciated!

[induction]

I'm 100% sure that everything is in the right place though!

Your 510k resistor attached to the base should go to Vb. Instead it is going to ground. Fix this and you'll probably be sorted.

[tinlee]

The ground rail on the left is my Vb rail. So the 510k is going to Vb - Voltage reading is 4.5V

[induction]

Right. I should have read closer.

It appears that your input cap is shorted. Or am I seeing that wrong?

[deadastronaut]

'' I decided to try my 7 string guitar which has lower gain pickups and I didn't have this problem when using the bridge PU. But then the same problem came back when I switched to the neck PU on the 7.''

hmmmm....(interested as i have a 7 string too) just haven't time to breadboard this at the mo..

[duck_arse]

if induction is seeing right and the input cap is shorted, the pickups would pull the base voltage funny. pull that 1k and see what happens to the base voltage.

[tinlee]

Its just the angle of the of photo, the input cap is not shorted Pulled the 1k and base remains the same at 2.7V Anyone else got any ideas?! Might just give up on this schematic and try something else hehe

[induction]

Ok. It looks for all the world like the leads of the input cap are plugged into 2b and 2c of the breadboard, but if you're sure they aren't I'll shut up about it.

It also looks very much like your op-amp is reversed. There should be a notch between pins 1 and 8.

[R.G.]

Just to check the obvious again - what transistor are you using, and have you checked its pinout against what you think you have?

It is quite odd to see a 510K resistor drop that much voltage in supplying the input bias to the base. I am disregarding the actual base reading, because the impedance of the meter will mess that up. But the emitter sits at 3.2V . It is likely **if you have the pins identified correctly for the transistor** that the real base voltage is 0.5V higher at 3.7V. Still, that means some 4.6-3.7 = 0.9V is being dropped across the 510K, and that's 0.9/510K = 1.7uA. There is 3.2V/10K = 320uA flowing through the emitter. That makes the gain about 320/1.7 = 188, which is not all that bad, but still the emitter looks low.

Here's another thought: I quit using breadboards because the contacts get dirty and intermittent.

And finally, the input may simply be BIG. Do your guitars have onboard electronics?

[Frank_NH]

Its just the angle of the of photo, the input cap is not shorted Pulled the 1k and base remains the same at 2.7V Anyone else got any ideas?! Might just give up on this schematic and try something else hehe

Those transistor voltages look funny to me.  Check your ground wire - any problems there?  In fact measure voltages everywhere - the problem should reveal itself eventually.

I'd be tempted to put a 2N5088 in there - after all, it's just a buffer, but the way it's currently biased it seems to be acting like a switch with your 2n3904.  Also, did you try connecting your input downstream of the buffer (just pull the transistor and set up the input before the 1 uF cap)?  There are plenty of bufferless tube screamers out there, so that should work if the rest of the circuit is OK.  Good luck!

[tinlee]

Ok. It looks for all the world like the leads of the input cap are plugged into 2b and 2c of the breadboard, but if you're sure they aren't I'll shut up about it.

Yes they are, the schematic says to put them in series! :p

It also looks very much like your op-amp is reversed. There should be a notch between pins 1 and 8.

There isn't a notch on this IC instead a Dot where pin 1 should be then, you work your way around! The IC and clipping work fine, it's just the Buffer!

Just to check the obvious again - what transistor are you using, and have you checked its pinout against what you think you have?

I'm using a 2N3904 as the schem suggests, Yes pinnout is correct according to http://www.futurlec.com/Transistors/2N3904.shtml. Have even tried wiring it backwards to double check!

Here's another thought: I quit using breadboards because the contacts get dirty and intermittent.

Yes I'm starting to think I may think that way as well haha, although it's a brand new board with only a few previous builds in it before, so it should be okay!

And finally, the input may simply be BIG. Do your guitars have onboard electronics?

This is what I first thought to begin with, seems when the input gets too large it just kind of fluffs out! This would explain why this happens with my SD invader as the output is huge, but still doesn't explain why it still does it with stock ibanez neck pick ups! Is there anyway to reduce the input signal to a smaller amount?

I'd be tempted to put a 2N5088 in there - after all, it's just a buffer, but the way it's currently biased it seems to be acting like a switch with your 2n3904.

I have tried the 5088 and 5089 but still the same problem :p

Also, did you try connecting your input downstream of the buffer (just pull the transistor and set up the input before the 1 uF cap)?  There are plenty of bufferless tube screamers out there, so that should work if the rest of the circuit is OK.

Yes I did and it works perfectly! I might just forget the input buffer stage! What is it used for anyway?! It's just annoying as hell as it isnt working as it should do hehe.

Thanks for your help everyone! I'll going to call it a day on getting it working properly and just bypass the buffer stage for now! If anyone else has any other ideas then please post again! hehe Thanks