Le Craquiement Thermonique (SP?) debugging

[jbord39]

Hey guys.  I am working on the Le Craquiement Thermonique pedal with superhot/hot additions.  Here is the picture:



Here is a list of the modifications I had to make to the layout:

I am using a 7805 with two diodes connected to ground, raising the output voltage to 6.2V (Verified with multimeter) instead of the 7806.
I am using a 5.1V zener instead of a 9V zener.  I would guess that this is the problem, except that the problem occurs later in the circuit.

My problem is that the output signal is extremely attenuated; about 5mV p-p; although it is following the input signal and changing with all the buttons/switches.
From the tone potentiometer; I am getting a good; approximately 10V p-p waveform; however from there on the signal is greatly attenuated and very quiet.  I am measuring the signal from the tone pot at the top node (.0047 uF and 47kOhm in the same node).  The tube glowing a little orange; and seems to be working.

Thanks for any help,

John

This is my first time using a tube and most likely that is the problem.

[PRR]

DC voltages on tube pins 1(G), 8(K), 7(G2), 5(P) ??

[frequencycentral]

Got the MOSFET the right way around? I believe the BS170 has different pinouts by manufacturer. The zener shouldn't be a problem - it will work with no zener, or with a 1n4148 - is the zener the right way around? Though I guess if you're getting good signal between the tonestack and the tube, the issue must be with the tube? got the the tube pinout correct? What layout are you using? I haven't had the chance to verify my layout yet, the board is populated but not in a pedal yet.....

[jbord39]

Thanks for the replies.

The pin readouts are:

Pin 1, 5, 7: 13-14V DC readout using DMM.  Using oscilloscope, I can see a 10 mV p-p signal on each of these.
Pin 8: few mV

From the tonestack, there is a capacitor leading into pin 1.  Taking the voltage across the capacitor, it is a few volts (around 6) peak to peak with the waveform.  The rest of the waveform is attenuated across the resistor.  I tried replacing the capacitor but it didn't help. 

I also checked the mosfets datasheet and the pins seem right.

I am building the layout on a breadboard; would a picture of the breadboard help?

[frequencycentral]

Yeah, photos would help. Also, are you sure the 1M at pin 1 actually is a 1M (not a 1K !).

[jbord39]

Yeah, I've checked every resistor value; and double checked the nodes for shorts.



Here is a picture of the setup.  The part on the PC board is everything up to tone potentiometer, which has a black lead (bad choice, no other wire) into the capacitor on the breadboard.

The two wires on top of the breadboard (red, orange) are linking ground and V+ on each side of the breadboard.

The top left of the breadboard is the 7805 with two diodes boosting it's output to 6.2V.  This is working.

The pins from the 5840 are spaced two nodes apart, I know it's hard to see.

I am pretty sure that up to the tonestack is correct; I am getting strong waveforms.  For some reason pin 1 of the 5840 seems like it can't be supplied more than a few mV.  I tried removing the capacitor (which was taking most of the AC voltage for some reason) and the waveform shrunk.  I also tried placing an LED from pin 1 to ground, and the LED would only barely light up, even with the capacitor removed.

Thanks for all the help; I really want to try this pedal!

John

[jbord39]

Update:

I tried applying a 10 V p-p signal directly to pin 1; with a resistor from pin 1 to ground.  The signal was attenuated to about 2V.  I also tried applying the signal from the tonestack to a resistor connected to ground.  It was a solid, good waveform; so I am pretty sure that up to the tonestack things are working.  I also tried applying various other signals in different combinations to pin 1; and it seems like all of the signals were attenuated into the 10's of mV range.

Thanks again,

John

[frequencycentral]

It's hard to tell from the photo, but it looks like the 7805 is feeding pin 2?

One of the 'building blocks' of this circuit is this tube boost: http://www.diystompboxes.com/smfforum/index.php?topic=76356.0

As you have got everying else working, it might be worth your while building the tube boost section in isolation, and then applying the rest of the circuit to it's input.

[jbord39]

I checked the pin, and it's connected to 3.  Is it possible that my tube could be bad?  I tried building just the tube boost; which I also double checked everything for--the same problem happens.

Thanks again,

John

[frequencycentral]

Hmmm, pins 2, 4 and 8 are internally connected, anything externally connected to pins 2 or 4? A bad tube is unlikely but possible I suppose.

[jbord39]

Here are some pictures of just the booster; along with an oscilloscope frame.





The yellow waveform is taken at pin 1.  The blue waveform is taken at the input.
The output is the same as the yellow waveform with the potentiometer turned all the way up.

The input is hooked up to a function generator with a 560 Hz sine wave.

V+ is being powered by +12 V.

I can't figure out why the capacitor is taking so much of the AC... shouldn't the capacitor take the DC voltage and pass the AC component?  The signal is being attenuated from about 7V to like 40 mV.

I have double checked everything twice now...

Thanks for all your help,

John

[frequencycentral]

I'm still not convinced that you have the pinout correct. Can you post some photos of the tube not taken from above, so I can confirm the pinout? It still looks to me like you have the 7805 feeding pin 2, with pin 7 connected to ground.

[jbord39]

Here are pictures taken from each side.  The bottom of the tube is supposed to have a larger gap than the other spaces, correct?





From my first picture of the tube from above, I am assuming the pinout is like this:

  4 5
 3   6
 2   7
  1 8

In these pictures, a white wire connects 6 to ground.

Would I need to be worried about breaking the tube if I just flip it 180 degrees to see if that works?

[PRR]

>> DC voltages on tube pins 1(G), 8(K), 7(G2), 5(P) ??

> Pin 1, 5, 7: 13-14V DC readout using DMM.  

Pin 1, Grid, MUST be zero volts.

Pin 8, Cathode, is likely something under 100mV.

That both are up near a 12V level says that you don't have a ground here.

Pinout:

Is the tube lit-up? (might be dim; turn off the lights.) Or does it get warm? If not, then you do not have 6V across the correct pins 3 and 6.

You have to see the bottom of the tube, not shown in your shots. It would be a 9-pin circle except one pin is missing. Looking at the bottom, pins are numbered from the gap, clockwise.

Also: pull the tube, ass-ume a pin number plan, and start poking with an ohm meter. Pins 2 4 and 8 should be dead-short to each other. If not, but some other three pins are, you mis-numbered, try the other way. Then pins 3 and 6 should be near-short, around 100-200 ohms. All other pins should not connect to any other.

[jbord39]

The tube does light up, and get warm.  Using the pinout you're describing (the same one I am using on the breadboard, too), pins 2, 4, and 8 are all shorted.  Pins 3 and 6 have a resistance of 5.2 ohms--not 100-200.  Could this be the problem?

Also, how can pin 1 be zero?  Isn't the input signal supposed to be applied to pin 1?

Here are the readouts with no input signal on the boost.

1: 10.3V
2: few mV's
3: 6.3V
4: few mV's
5: 10.3V
6: few mV's
7: 10.3V
8. few mV's

Thanks again

[frequencycentral]

From my first picture of the tube from above, I am assuming the pinout is like this:

  4 5
 3   6
 2   7
  1 8

You have the pinout incorrect. From above it should be:

    5  4 
6        3
7        2
  8      1

Check out the 'from below' (ie looking at the pins) diagram in the box (bottom right) on the schematic.

[jbord39]

Thanks so much!!! It's working!

John

[frequencycentral]

Thanks so much!!! It's working!

John

And it sounds..................? 

[PRR]

Pins 3 and 6 have a resistance of 5.2 ohms--not 100-200

Lift one of these leads. I bet you measured in-circuit, and were measuring the voltage regulator more than the filament. But if it is clearly lit-up, don't even worry about it.

> Isn't the input signal supposed to be applied to pin 1?

Pin 1 is (should be) at zero volts DC plus the audio signal.

Since the DC voltages look wrong, don't apply audio now, it just confuses things. We want to get near zero on Grid, tenth-volt on Cathode, 4V-11V on Plate.

[jbord39]

It sounds really good.  Thanks for all the help.

I have studied transistors in college but we do not cover cathode ray tubes.  Is there a way that they relate, ex. Does the gate correspond to any part of the CRT?

-
John