Mystery semiconductor substitute

[Randall]

I am working on a 2001 Eden Navigator WP-100 rack preamp that has it's 12AX7a tube not working. Both grids and both plates are tied together. The filaments are fed with -15vdc to pin 5, so when the tube is in circuit I measure 5.8vdc on pin 4 and -5.8vdc on pin 5. PLates have 112v. Signal is getting to the grids, but not to the plates. I do not have a schematic.

Here is where it gets strange. The grids and cathodes start out at power up at 2.5vdc and slowly count up to around 24v with my probe on the grids. If I touch the cathodes it counts up to around twice that. Both stay at about the same potential, but touching the grids makes it count down in voltage while touching the cathodes makes it count back up.

Not having the luxury of a schematic makes this tough, but the customer really wants me to try to fix it nevertheless. So I pulled the board and went hunting. I tested all the transistors and caps in the socket area, and came up with one transistor that says it is bad, but I don't know what it is or if I am indeed testing it correctly. Or what it's purpose is for that matter. It tests for two good junctions with my fluke, but beeps and shows 32 ohms either way on the third junction. I checked it again on an old B&K 510 Transistor Tester, and shows good in two positions, meaning it can't identify the C or E.

Can someone take a look at the photo and tell me what it is and where to get one or a sub? I looks to me to be 2SC030AA, but I'm not finding it.

[MarkusBass]

It seems to me that this is JFET J112 - switching transistor. Most probably there is nothing wrong with it. I would continue checking cathode voltage. Isn't there just ground missing? I'm not sure why you measure voltage on the grid. Just find the cathode resistor and measure voltage on the resistor.

Mark

[Enzo]

Yep, says J112 right on it. That is a JFET, so it is normal for source to drain resistance to be low, with the gate lead acting as a diode to the others.

[J M Fahey]

I am quite certain that the Eden Rack preamp must be the same as another Eden head preamp, just mounted on a rack cabinet.

So search which Eden has a 12AX7 tube in the preamp and you can´t be far.

I suspect the tube is a gimmick , just a do-nothing cathode follower, and in that case normal is NOT having any plate signal.
You may have an open grid resistor, so voltage there drifts up with no reference to ground.

Touching the cathode shows that, does not make it rise; whle grounding the grid through the multimeter 10M input resistance restores things to normal.

Do not pull and measure parts at random trying to find something unusual, waste of time.

What is the actual problem/symptom/complaint?

[g1]

Grids can be tough to read, especially when there are high cathode voltages like you mentioned in the other thread.
Forget the ground reference for a bit, what is the heater voltage with one probe on pin4 and the other probe at pin5 ?
How about the voltage from grid to cathode, one probe on each.

[Randall]

The J112 was a red herring. I found another one in the circuit and it measures the same, so that was a waste of time.

Man, has this thrown me for a loop! I drew out a schematic with my current voltages. I do not understand the 50 volts on the grids and cathodes at all. I mean, think I see where it is coming from, the 33K resistor between the plates and Q2, I think. But why? I have measured almost everything in this circuit, and found nothing.

The compliant is the tube channel does not work, it make no change.

[Enzo]

Did you notice in post #4 that Juan said it is probably a do nothing cathode follower stage? That is what the similar circuit in the WT300 is, and that explains the high voltage on the cathode. Is your signal on the cathode? Your empty connector was probably for a direct out.

[Randall]

Yes there is signal on the cathode, should have mentioned that.

I did catch that it is a cathode follower stage, but what is the point of this "do nothing" stage? Are you saying that it is working as expected? Did I spend half a day looking for a problem that doesn't exist?

"Tube Character This is a unique feature that allows the Artist to set up the exact amount ofedge or old tube style distortion that is needed for a particular sound or musical style. The effect produces a subtle soft asymmetric (single sided)distortion. The Tube Character operates like a vacuum tube in a light overload condition. It produces a warm soft distortion character. The overall effect is much more pronounced at higher input levels. With the control at the far left position (sine wave) there is no effect on the signal. With the control turned up toward the right side (square wave) the effect becomes more noticeable."

I didn't think I heard any change with this knob, so I went looking to makes that plate make a signal. Now I wonder if there is something wrong somewhere else, or if this is just a "do nothing" feature as Juan has said?

[eschertron]

Looking at the drawing you made, I can't see where the signal comes out 'on the other side' of the stage. What's next in the preamp? can you trace the signal to that point (knowing now that it is a CF and no signal on the plate)?

edit: If I'm looking at the drawing correctly, and the drawing is accurate, then I see the output of Q2 go to the "empty connector". This looks like the place I'd expect the output to be. is this an on-board header? something needs to be there to get the signal.

[nosaj]

Yes there is signal on the cathode, should have mentioned that.

I did catch that it is a cathode follower stage, but what is the point of this "do nothing" stage? Are you saying that it is working as expected? Did I spend half a day looking for a problem that doesn't exist?

"Tube Character This is a unique feature that allows the Artist to set up the exact amount ofedge or old tube style distortion that is needed for a particular sound or musical style. The effect produces a subtle soft asymmetric (single sided)distortion. The Tube Character operates like a vacuum tube in a light overload condition. It produces a warm soft distortion character. The overall effect is much more pronounced at higher input levels. With the control at the far left position (sine wave) there is no effect on the signal. With the control turned up toward the right side (square wave) the effect becomes more noticeable."

I didn't think I heard any change with this knob, so I went looking to makes that plate make a signal. Now I wonder if there is something wrong somewhere else, or if this is just a "do nothing" feature as Juan has said?

On the EDEN forums there is a lot of what you just described. Lot of market hype with a barely audible difference on heard by those with Golden Ears.
Some changed tubes to Ruby's or JJ's 7025's little differences.

What did customer say the issue was ? I haven't seen it anywhere.

nosaj

[nosaj]

The J112 was a red herring. I found another one in the circuit and it measures the same, so that was a waste of time.

Man, has this thrown me for a loop! I drew out a schematic with my current voltages. I do not understand the 50 volts on the grids and cathodes at all. I mean, think I see where it is coming from, the 33K resistor between the plates and Q2, I think. But why? I have measured almost everything in this circuit, and found nothing.

The compliant is the tube channel does not work, it make no change.
[ATTACH=CONFIG]43602[/ATTACH]

Maybe scoping will show something you can't hear. Was it working before and now it's not? or always been this way? Need more data.

nosaj

[Randall]

It came in for a cleaning and a ground hum (ground wire wasn't connected). While playing thru it for my own amusement, I noticed the tube knob wasn't doing much if anything, so I set about to fix it.

[nosaj]

It came in for a cleaning and a ground hum (ground wire wasn't connected). While playing thru it for my own amusement, I noticed the tube knob wasn't doing much if anything, so I set about to fix it.

Gotta be careful with those pitfalls it makes your labor rate go out the door when you can't bill for it.

nosaj

[The Dude]

Per the manual:

Tube Character
This is a unique feature that allows the Artist to set up the exact amount of
edge or old tube style distortion that is needed for a particular sound or
musical style. The effect produces a subtle soft asymmetric (single sided)
distortion. The Tube Character operates like a vacuum tube in a light
overload condition. It produces a warm soft distortion character. The overall
effect is much more pronounced at higher input levels. With the control at
the far left position (sine wave) there is no effect on the signal. With the
control turned up toward the right side (square wave) the effect becomes
more noticeable.

It seems it might be a very subtle thing. It also seems you might have to hit the input hard to notice it. Maybe it's working and you just haven't hit it hard enough to hear the effect well?

[Enzo]

Look up the WT300 or 400 schematic, the tube stage is VERY similar to this, and is drawn in maybe a more readable format.

Personally, I think its main strength is that it lets them claim it has a tube in it.