Tube Works Blue Tube 903 - no signal when engaged?

[kvandekrol]

Hey guys. I'm trying to fix a Tube Works Blue Tube pedal without much luck, and was wondering if I could get some input. I've never worked on a tube-based pedal before, so there are some parts of the schematic I'm not really sure how to read.

Anyway, the problem is this: the pedal bypasses signal fine, but when the pedal is engaged, it's quiet. The red LED turns on so I know the switch is working. I used an audio probe in drive mode and found that I get an overdriven signal on pin 7 of the signal IC as expected, and I get signal from the lugs of the Drive knob, but nothing from the Hi, Lo, Mid or Output knobs. For the tube, I get clean signal on pin 2 but there's no audio signal on any of the other pins. Based on the schematic it looks like the overdriven signal should be input to pin 7 of the tube and output to pin 1, but I get no signal on either of those - just that clean signal on 2.

Here's the schematic:

http://i731.photobucket.com/albums/ww317/MillsApparatus/903.jpg

Here are my voltages on the relevant pieces:

Q1: pin 3 0.3v on, -14.3 off. No voltage on pins 1 or 2.
Q2: pin 3 -14.3v on, 0v off. No voltage on pins 1 or 2.

Power/Switching IC:
1 17.3v
2 -14.63v
3 0v
4 -17.75v
5 0v
6 0.65v
7 -14.97v
8 17.95v

Signal IC:
1 .47v
2 .47v
3 .42v
4 -13.52v
5 0v
6 0v
7 0v
8 13.68v

Tube:
1 12.72v
2 13.76v
3 -9.08v
4 0v
5 0v
6 -12.75v
7 -12.41v
8 -5.67v

Before taking these voltages, I went through and replaced all the electrolytics just in case, but that didn't have any effect. Any help would be appreciated greatly! Thanks!

[PRR]

> Tube:
> 1 12.72v
> 2 13.76v
> 3 -9.08v
> 4 0v
> 5 0v
>  6 -12.75v
> 7 -12.41v
> 8 -5.67v

Pins 8 and 3 are connected together. How can they have different voltages?? Something is not connected according to this plan.

> Power/Switching IC:
> 4 -17.75v
> Signal IC:
> 4 -13.52v

Both IC pin 4s are presumably connected together.... how can they be different?

Tube-type is not specified where I can find it. I will ass-ume 12AX7.

Transformer voltage is not specified. For 12AX7 connected this way it should be 12V. I have ass-umed + and - 17V DC rails

Tube is, unusually, powered from + and - voltages. This makes old tube-heads dizzy.

Basic normal tube voltages. Grid is at the most negative. Cathode is slightly positive of grid. Plate is nearer but not at the most positive available voltage.

[Ice-9]

The different voltages on pin 4 of each Tl072 look like they can be accounted for by them taking power from different points from the PSU. A+ and A- for one IC while the other is powered from B+ and B-.

You have a signal on pin 7 of the valve, but no signal on pin 1. Is this correct. If so the problem is around the valve or associated parts. As PRR suggests it's not possible for pins 3 and 8 to be different voltages if the schematic is to be trusted.

I would try and bypass the valve part of the circuit and connect pin 7 of the valve socket via a capacitor to the input of the tone stack and see if signal passes to the output (remove the valve).

Check for a voltage across the valve heaters, it looks like pins 4 and 5 have 0v , That sounds wrong as the schematic clearly shows the heaters wired in series so 12v should be acroos the heaters. (or if they are wired parrallel then 6v should be across pins 4-9 and pins 5 - 9

From the info provided I would think a very likely cause of ther problem is the heaters.

[PRR]

> like pins 4 and 5 have 0v

Yes, I should have asked the once-obvious question: Does The Tube Light-UP?

However if the meter is on DC and he pokes the AC heater, "most" meters read zero.

Anyway the symptoms of a cold tube are simple (at least with unipolar supply; bipolar makes my brain work too hard) and those are not the numbers he has reported.

> if the schematic is to be trusted

Get the actual circuit like the schematic and it will work. Pin numbers are correct.

On third thought, it may be scaled for 12AU7 rather than 12AX7, but the reported voltages are wrong for any likely tube.

> You have a signal on pin 7 of the valve, but no signal on pin 1. Is this correct.

QUOTE: "signal should be input to pin 7 of the tube and output to pin 1, but I get no signal on either of those - just that clean signal on 2."

Which makes NO sense.

[kvandekrol]

The tube (12AX7) does light up, fortunately - and I tried a few others as well just to make sure the tube itself wasn't the problem.

Let me double-check the pin voltages again on the tube. I may have actually gotten the pin numbering reversed now that I think about it, especially since it's looking so abnormal. I looked at a tube pin diagram before I measured, but must have gotten it flipped around when looking at the bottom of the board. I'll also double-check the transistors since that is strange that they were showing different voltages.

Thanks for all the help so far.

[kvandekrol]

Hey guys - sorry for the misinformation earlier, I had indeed switched the tube pins around. Here are the correct measurements and I apologize for the goose chase!

Tube
1 -10.95v
2 -11.74v
3 -12.12v
4 -0.04v
5 -0.01v
6 -8.56v
7 -13.27v
8 -12.13v

Q1
3 -13.86v (when on)
Q2
3 -13.87v (when on)

Signal IC
1 0.37v
2 0.38v
3 0.34v
4 -13.29v
5 0v
6 0v
7 0v
8 12.95v

Power/Switching IC
1 -16.20v
2 14.78v
3 0v
4 -17.51v
5 0v
6 -0.61v
7 15.12v
8 17.22v

[Ice-9]

So you have a dist sound on pin 7 of the IC but no signal on pin 7 of the valve. In that case check how far the signal travels between these two points (pin 7 IC - 0.47 cap - 100k res - 10k res - pin7 valve)

I don't understand how you have a clean signal on pin 2 of the valve though, can you recheck that.

[kvandekrol]

Sorry, I forgot to mention that since I got the tube pins reversed, what I said was pin 2 was actually pin 7. So I get signal on pin 7 but none of the other pins.

[Ice-9]

Ok, so following the schematic, you have a signal on pin 7 of the 12ax7. There is no signal on pin 2 of the 12ax7 ?

Check the capacitor on the plate that goes the from pin 6 to pin 2 of the next grid and also the 220k resistor, one or the other may have a problem. The -17v on the grids looks good so the bias to them seems ok.

[kvandekrol]

I replaced the tube socket on this for good measure. Still didn't fix it, but I noticed something: when I play a low power chord (e.g. higher bass content), really hard, the signal comes through - but only barely, and only the lower frequencies. Does this offer any more clues to what might be going wrong? Is the tube getting enough power? It lights up, but not being too familiar with tubes, I'm not sure how bright it should be getting. All indicators point to the tube being the fault in the circuit, but as I said, I tried about 4 different ones and they all did the same thing, so I'm wondering if there's a problem with the heaters.

[kvandekrol]

Fixed it... The 0.047uF cap attached to pin 6 was bad, and was letting through negative voltage. I didn't realize that it was a bad thing to have negative voltage on all the pins of the tube, but I guess if half of the tube requires positive voltage then it wouldn't pass much of a signal if it was all negative.

After replacing that cap, everything works great!

[~arph]

Was tihs for a guy named Peter?