Quote from: axepilot on January 31, 2020, 10:53:57 AM
Black probe to ground I get the same voltage noted on pin 1 of the IC,  moving from + .02v to -.02v.
That's TR1.

On TR2, the voltage is ever so slightly higher, like going to +/- .028v sometimes hitting .03v

Close enough for Rock and Roll. A Voltage of 10 to 15 Volts would indicate a problem that could cause the symptoms you describe.

Check that pins 1 and 7 of the IC are near zero Volts. If not, that indicates a bad IC, component or broken connection problem.

I have two of these things. The combo version (just opened it up today) has the headphone jack on the front and the speaker jack on the bottom. The head version has a line out on the front, two speaker jacks and a headphone jack on the back panel. They have different PCBs.

Does your amp have a Line Out on the front panel ? Does that work or have the same problem as the speaker out ?

First thing, try the amp with a known good speaker.

Next, if you don't have an oscilloscope, connect your DVM across the speaker and set to DC Volts to verify there is no DC across the speaker. Next set the meter to AC volts. Is the reading almost zero ? You are looking for an ultrasonic oscillation that may read several volts depending on your meter.

Unsnap one side of the 9V battery and rotate the connector so that the unsnapped side isn't touching the battery terminal. Now, set your multimeter to measure current (sometimes you have to move the red test probe lead to a different jack on the meter) and touch one lead to the unconnected side of the battery and the other lead to unconnected side of the battery connector. You are now measuring the current flowing out of the battery. You may have to change the range setting on the meter, expect a current between 1mA and 10mA.

BE SURE TO MOVE THE RED LEAD BACK TO THE vOLT-OHM JACK ON THE METER OR THE NEXT TIME YOU TRY TO MEASURE VOLTAGE, YOU WILL BLOW A FUSE INSIDE THE METER.

The NuTube can definitely add some even harmonic sweetness that tubes are known for. There are other ways to do the same that don't cost $50 bucks USD. (One cost estimate I heard.)  Although the gain is low, so is the B+ limit. So headroom refereed to the input is about the same.

Can anyone verify the price ?

They make the NuTube seem like the greatest thing since sliced bread, but take a look at this: https://www.diyaudio.com/forums/pass-labs/313612-b1-korg-triode.html (you may have to join the forum to see the attachments).

All the crap it takes to make the thing work, and some of them are microphonic. Since there is no cathode, they're also a PITA to work with.

Have you checked the low value resistors R55, R56, R57 and R58 ?

On the schematic you posted, what is the meaning of the note #7 ? Is that leg of the resistor grounded ? Look at the Bases of Q8 and Q9. The signals should look very similar. That tells us that the amp is basically working, but there is some problem in the feedback network or the amp just isn't getting enough signal to begin with. Is the signal at Q8's Base nearly the same as it is at the Effects Send jack ?

Quote from: vance on December 30, 2019, 06:08:15 PM
The two center output transistors are getting slightly warm to touch but outer two are cold

This is normal for this circuit.

Link: https://el34world.com/charts/Schematics/files/Randall/Randall_rg100.PNG

There is a 0.18 Ohm 10W resistor (R63) between the low side of the speaker and ground. Have you checked this for continuity ?

Have you tried bypassing the Effects loop by connecting a patch cord from Effects Send to Effects Return ?

The J109 would make on OK low level amplifier, but is a challenge to bias and has almost zero chance of working in any J201 or MPF102 circuit without substantial changes. Notice that there is no maximum given for Idss. The parts will melt or explode if you try to measure Idss with DC. This is typical on switching type JFETs. They depend more on R(on), channel resistance.

The J175-77 are P channel. I've used them where I needed a P channel, but they are weird.

The first thing you need to know about JFETs is that the specs can vary quite allot. So much so, that it is hard to design a circuit that will operate for any JFET that meets the specs you find on a data sheet. The usual fix for this is one or two trim pots.

On the vast majority of JFETs, you can reverse connections to the Drain and Source with no change in performance.

The first thing to look at is Gate breakdown Voltage. BV(GSS) is a term you might see, it means Breakdown Voltage, Gate to Source with Drain shorted to Source. For a J201, the number is 40V. For the MPF102 the number is 25V. Most circuits don't provide any input protection and a big negative input Voltage will damage the JFET. Sorry, that's the way it is.

The next thing to look at is called I(DSS). It means Drain Current with Gate and Source shorted (to ground). It's usually specified at some Drain Voltage like 5 or 10 Volts. For the J201 the spec is 0.2 to 1.0 mA. For the MPF102 the spec is 2.0 to 20 mA. You have to look at the circuit the part will operate in, but generally try to keep the minimum I(DSS) of the replacement at or above the minimum of the part to be replaced. Try to keep the max I(DSS) less than twice the max value of the part to be replaced.

The orientation of the pins may be different for the replacement part. You will have to check this. J201 and MPF102 have the Gate at one end of the package. Same place the Collector would be on a 2N3904. Most Euro parts have the Gate in the middle. Keep in mind that you can swap Drain and Source. This can be a real PITA if the layout uses inline leads instead of a triangular pattern.

There are lots of other specs like gain and leakage, don't lose any sleep over them, just get some parts to try and be done with it. Beware of fake parts from China, but they might actually work.

Quote from: teemuk on October 15, 2019, 03:43:04 AMWith a push-pull circuit the same triode produces almost no even order harmonic distortion at all, as usual.

That is true, until you start drawing grid current on the input side like when you overdrive a power amp. Depending on the source impedance, odd harmonics will drop out.

See experiment with TDA2040 here:
https://music-electronics-forum.com/showthread.php?t=48223