Tube boost + overdrive running off a 9 volt battery

[Dongle]

Yes, I really did that mistake!!
Thanks a lot for pointing me at this!!

[Dongle]

By the way: Is the TL072 appropriate for that purpose? I just had it there, so I used ist...
Does it make sense to use the second OPA for an "Input-buffer"? Or does it make no sense at all?

[merlinb]

By the way: Is the TL072 appropriate for that purpose?

Yes perfectly!

Does it make sense to use the second OPA for an "Input-buffer"? Or does it make no sense at all?

It would make sense to use the second opamp as an output buffer. But you don't have to use it.

[Dongle]

Yes perfectly!

Thanks!

Does it make sense to use the second OPA for an "Input-buffer"? Or does it make no sense at all?

It would make sense to use the second opamp as an output buffer. But you don't have to use it.

Sorry, I did not get this fully. Does an Input buffer or an output buffer make more sense?
I thought an output buffer would be crucial, but an input buffer not?

I understand your post in the way, that it is the other way round...

[merlinb]

Does an Input buffer or an output buffer make more sense?

Well, it would be nice to have an input buffer and an output buffer. But if you have to choose, then I guess an output buffer is more useful in this instance, as it eliminates tone sucking due to cable capacitance and reduces noise after the pedal.

[BarnyardBill]

Result: NO sound.... But if I disconnected the resitor, I do get a sound - but a very "bright" (in the negative sense) one...

The resistor needs to be connected to "Vr" = reference voltage that is about half the supply voltage. Did you accidentally connect it to ground instead?


I would suggest you add one more resistor and turn it into this buffer:

my valvey is still homeless .  But my real question is about the buffer you posted.  Where would the buffer go in a standard vavlecaster layout?  I am still new at this, so I hope this isn't a stupid question.  I saw that you said it would be best to at least have an output buffer, but where do i connect it to?  Does it go right before the 1/4" output jack?  

[merlinb]

 Does it go right before the 1/4" output jack?  

Yes, so the opamp will ultimately be driving the following cable.

[pazuzu]

what is the purpose of the buffer?

[Dongle]

The buffer does in generall compensate for differemt impedances. The output of the tube has a very high impedance, which is not good for a good signal transfer.

To make it simple: think of impedance as a resistor.
Both, your "source" (valvecaster) as well as the next thing in on your effect board (or the amp), has a "resistor": the valvecaster at the end, the amp at the beginning. If they would not have these resistors, you would not be able to put a voltage inside...

Now imagine, that your valvecaster has a very big resistor, but the next thing a very small one.
The valvecaster now tries to establish its normal output voltage, but is not able to do so, because the resitor at the other side is very small: thus, a very high current happens, but not a high voltage... you cant really use the full potential of the valvecaster.
On the other hand, if you have a high input resistor at the other side, you would get a high voltage -> more volume and different sound...

In principal, you would have to adapt the output resistors to every different device. As this is too complicated, you can also build such a simple buffer as shown before. The buffer is a "follower": it puts out the same voltage as it gets in the input, no matter how much current is flowing...

To summarize: a buffer
1. presents a controlled and constant load to the exit of the tubes
2. Sends the same signal as the input signal, no matter which device follows


EDIT: PLEASE corret me, if I was wrong. I am not so sure about all this stuff, but thats how I explained it to myself...

EDIT 2: The ideal effect would have a very high input impedance, so that it is very easy for the effect before (or guitar) to establish the voltage signal.
And it has a very low output impedance, beacuse otherwise the high input impedance of the next effect does not help... and it prevents the treble loss metioned by merlinb

[merlinb]

The buffer does in generall compensate for differemt impedances. The output of the tube has a very high impedance, which is not good for a good signal transfer.

Further to what Dongle said, an output buffer also eliminates treble loss due to cable capacitance. The stock valve caster (and many DIY pedals) has high output resistance, so when loaded by some cable capacitance you have an RC filter that cuts treble. The greater the resistance, the lower the cutoff frequency of the filter. A buffer prevents this because it has a very low output resistance.

What's more, if you have a high output resistance then it is very easy for external hum to couple into the cable. A buffer eliminates this too, because its small output resistance shunts any such noise to ground.

[PaulBass]

If using a buffer there are a lot better opamps you can use than the TL071. like the OPA134, LT1351, LT1028, OPA1611, or TLE2071. current draw wouldn't be an issue since everybody is using wallwarts or power supplies

[Dongle]

Thanks for the hint!
Why are they better for that purpose? I will try them...

[merlinb]

Thanks for the hint!
Why are they better for that purpose? I will try them...

The main advantage is that some (all?) of them don't suffer from sudden phase inversion if the input signal swings close to the rail, unlike the TL07x, which could potentially happen in this pedal. Also, some of them are stable even with large capacitive loads, so could potentially drive super long cables. But then, so could a TL07x if you add a small resistor in series with its output.
Apart from that there aren't any other real advantages that I can see; gain, noise, bandwidth, slew rate etc are all either no better or irrelevant in this application.

[pazuzu]

so this will cut down on signal noise than?

[merlinb]

so this will cut down on signal noise than?

Adding an output buffer to a stompbox will reduce sensitivity to external noise, yes (mainly mains hum pickup). It can't remove noise that is already on the signal, of course. And unless you adapt the rest of the FX circuit, it can't improve the noise of the pedal itself. However, a sprinkling of buffers throughout an FX circuit may allow the designer to use much smaller resistances than usual, which is the number one way to improve the overall signal/noise ratio.

[pazuzu]

hmmm. ok thank you. my pedal is probably not gonna get much better, but i am planning to do a double like the on in your sig, and this time i will be breadboarding instead of just making it right out.

[Dongle]

Hi guys!
Another question:

We does everybody control the gain pot at the anode and not at the cathode? You could also replace R2 with a pot and control the gain, right? What is the disadvantage here? Is there a different in sound?

Regards
Dobgle

[Renegadrian]

The gain pot is already at the cathode, as you can see here - never seen a gain pot at the anode...

[Dongle]

Alright.. I mixed both up...
But the meaning is the same: I could also put a pot at the anode and control the gain with it. If I raise resistance, gain raises - only point is: If I have zero resistance, I have no sound... so, the gain pot should be in line with a fixed R...

[zambo]

i guess you could techniclt put a resistor in series with a 100k pot or something to the plate and vary the gain that way. The best way seems to be to put a pot in between the gain stages of the triodes. it goes right after the decoupling cap and into the following grid.