Tube Screamer voltages

[Greenballs]

Hello all.

So, I've built a bufferless Tube Screamer and it transpires that I've got some voltage issues. I'm using a TL072 and the voltages are as follows:

P1 - 4.69
P2 - 5.2
P3 - 1.92
P4 - 0.01

P5 - 4.68
P6 - 4.68
P7 - 4.68
P8 - 9.4

Both clipping diodes (1N4001) measure 4.68

The obvious inconsistency is pin 3. I have a 470k biasing resistor coming off pin 3 which for some reason is only measuring 250k (yes, its definitely a 470k resistor!). Before I start switching parts out, would I be right in assuming that this is where the problem begins and ends?

Many thanks in advance.

[GibsonGM]

Can you post a schematic of what you're doing? 

A resistor measured in-circuit will be dodgy and may give strange numbers, to get an accurate read of it you may need to lift one leg.

Does it pass sound?  Output voltage is ok.    Your meter may be loading the pin and giving a BS reading.   I'm assuming the voltage at the bias network is ~4.6V, just before the 470k?   

[antonis]

Can you post a schematic of what you're doing?

+1..

P.S.
pin 2 DC voltage can't be higher than pin 1 one..

[Greenballs]

The voltage at the bias network is 4.66v, yes. Measured from the output pin on the 3PDT to ground, the output voltage is 0v (is that what you meant?). It passes sound, it just doesn't sound good unless the gain and vol controls are cranked and even then it could be better!

I'll post a pic of the schematic when I get home later but its basically the Lovepedal bufferless tube screamer - the Eternity I believe? - with a couple of minor tweaks. Sounded fine on the breadboard!

Just out of interest, what could be the cause of the pin 2 DC voltage reading higher than pin 1?

[antonis]

From DC point of view, grey framed items (C2 & R2) are considered out of circuit so Green, Red and Blue point must exhibit the save voltage 'cause op-amp is configured as unity gain follower..
(C5 blocks DC hence R2 in gain branch appears as of infinite value..)
So, in case of Green point indeed sits at 1.92V (OK a bit higher actually due to meter loading effect..) Red and Blue points should also sit at 1.92V..

In case of pins 1 & 2 measurements were reversed, I'd say that there is some kind of short between R2 and GND (Red point a diode forward voltage drop higher than Blue one..) while Green point measurement is fallacious..

[Greenballs]

From DC point of view, grey framed items (C2 & R2) are considered out of circuit so Green, Red and Blue point must exhibit the save voltage 'cause op-amp is configured as unity gain follower..
(C5 blocks DC hence R2 in gain branch appears as of infinite value..)
So, in case of Green point indeed sits at 1.92V (OK a bit higher actually due to meter loading effect..) Red and Blue points should also sit at 1.92V..

In case of pins 1 & 2 measurements were reversed, I'd say that there is some kind of short between R2 and GND (Red point a diode forward voltage drop higher than Blue one..) while Green point measurement is fallacious..

Ok, so thanks Antonis and GibsonGM for your replies, much appreciated.

This is the schematic that I'm working with. Its pretty much identical, with a few tweaks in values here and there. The main differences are the 2.7k biasing resistor off pin 5, which I put there to slightly reduce the gain (maxed out the overdrive was a bit much for my tastes and this seemed to do the trick) and the 100n cap/3.3k resistor in series are switched around. Why? I honestly don't know but I think I've seen other schematics with this arrangement so I was more than likely following one of them.

My question is now, how do I get a consistent voltage across pins 1, 2 and 3 if that is what's required?

[antonis]

The main differences are the 2.7k biasing resistor off pin 5,

Try to lift one leg of 2k7 resistor up..
(don't worry about pin 5 bias - it's directly biased  from pin 1 output..)

Now I see the reason for pin 1 apparently correct DC output..

[Greenballs]

The main differences are the 2.7k biasing resistor off pin 5,

Try to lift one leg of 2k7 resistor up..
(don't worry about pin 5 bias - it's directly biased  from pin 1 output..)

Now I see the reason for pin 1 apparently correct DC output..

Ok, will do. I'll report back with my findings later. Thanks Antonis!

[Greenballs]

The main differences are the 2.7k biasing resistor off pin 5,

Try to lift one leg of 2k7 resistor up..
(don't worry about pin 5 bias - it's directly biased  from pin 1 output..)

Now I see the reason for pin 1 apparently correct DC output..

Yep, worked a treat! Now, how does that old saying about not fixing things that aren't broken go again?

[antonis]

The main differences are the 2.7k biasing resistor off pin 5, which I put there to slightly reduce the gain (maxed out the overdrive was a bit much for my tastes and this seemed to do the trick)

You could raise 3k3 resistor value (and lower 100nF by the same proportion to maintain 482 Hz HPF corner frequency)
By placing 2k7 there, you didn't affect IC1 gain - you divided IC1 output amplitude going to IC2 non-inverting input with (1k +2k7) / 2k7..

and the 100n cap/3.3k resistor in series are switched around. Why? I honestly don't know but I think I've seen other schematics with this arrangement so I was more than likely following one of them.

The order of series connected items doesn't matter at all..

[Greenballs]

Many thanks Antonis!

[merlinb]

Pin 3 will always measure lower than the true voltage, because of meter loading. Don't worry about pin 3.