Note the DC Voltages in the ovals on the Green channel. Try to get the same Voltages on the Red channel. Chances are the Drain Voltage is way too low. Increase the resistance of the resistor between the Source and ground, R22 or R26 to try to get the Drain Voltage into the right range.

There is no protection for input JFETs Q1 and Q5. To add protection, connect a SIGNAL diode like a 1N914, 1N4148 or 1N4152 between the Gates of Q1 and Q5 and the -12V source point D. Connect the end with the band (Cathode) to the Gate and the Anode to -12V.

Note: It's a good practice to always plug the guitar cable into the guitar (or effects board) first, then plug the cable into the amp. If you hear that loud BUZZZZ! out of the amp, you're doing it wrong. Turning the amp off or the Volume down won't protect the JFETs you can kill them even when the power is off.

"Cleaning the signal" is not something you're going to find in a textbook on electronic technology. They are likely used as general circuit components throughout the design.

Ceramic capacitors are low cost capacitors. When compared to the ideal capacitor, they are inferior at audio frequencies because they change capacitance with Voltage and temperature. Film type capacitors perform better in this area. Ceramic's good points are low cost, small size and very low impedance at high frequencies.

All these characteristics are taken into account by the Electronic Engineer who is designing the product and must weigh cost against performance of the product. Often, it is the Bean Counters in the back room that exert an overriding influence on design decisions.

104Z is likely a 0.1uF cap. That's 10 followed by four zeros pico-Farads.

Is the amp being plugged into a working, properly grounded outlet ? Try a different outlet. If that doesn't work, take it back to where you purchased it and see if it works there.

From your description, I would say the amp is acting normally. What would you have it do differently ?

The only difference between the diodes is the breakdown Voltage. In this amp, the diodes are not being operated anywhere close to the maximum rating so substitution is OK.

It would be a cool place to try one (and only one) of those bi-color LEDs that are one color with one polarity and a different color if you reverse polarity.

Does your amp switch both sides of the line like the schematic you linked ? If so, did you install the network on both sides ?

Is your cap(s) X2 rated ?

This effect is not what I would call "Diode hash". It's not any fault with the diodes. The circuit is doing exactly what the math says it will do. To get rid of the "1+" effect, use the inverting form of feedback. See attached. The one disadvantage is the low input impedance, so you really need an input buffer like the TS pedal.
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Where can I find out more about the EU's Right to Repair regulations ?

Do you have an oscilloscope and some kind of signal generator ? Do you have a smartphone ? Build the first stage of a Tube Screamer and run a signal into it. Turn the pot all the way down. The gain will be one. Turn the pot up slowly and watch the output on the scope. When it starts to look like the simulation above, turn up the generator amplitude.

Don't have a scope ? Get one.

Quote from: phatt on July 14, 2022, 07:51:00 AMHi Loudthudman,
Just to clarify, D9&D10 are surely a Limiter?   
I can't see how they could work to impart any kind of Xover dist
on the signal. They would need to be back to back in series
with the signal to cause Xover. Maybe that is what was meant?

You know how the equation for gain of non-inverting opamp is 1+(Rf/Ri). Well that "1+" means the input signal is added to whatever distortion the diodes create. If you look at the output simulation @Carriage posted in reply #2, you see the top of the wave is just the input signal added to a more of less square wave, so it looks like soft clipping. If the input signal gets bigger, the top and bottom of the output just get bigger. This is not the kind of "dead zone" crossover distortion you are used to seeing, it's kind of the opposite where the area near zero crossing is expanded. Run a Clean Boost pedal into a TS and it just sounds louder and cleaner than it did without the boost, but with a gravely undertone. Not what I want my guitar to sound like.

The Tube Screamer doesn't make a good preamp because you almost can't turn the gain down far enough to get a clean signal. When you do, the gain is less than one and you have no headroom. Most of the time you are feeding a distorted signal through a tone stack. What's that supposed to sound like ?

The first thing you need to do is get away from the 9V power supply. Can you find any kind of wall wort AC transformer in your country ? Something like 16 or 24 VAC ?

Variable gain solves a lot of problems in the first stage. Look at the Marshall Lead 12.

The diodes D9 and D10 really don't clip the signal, they just add what looks like crossover distortion. The Tube Screamer really doesn't make a good preamp.

First of all, all the pots go to 11. Here's the lineup:

Gain: A500K
Bass: A1M
Middle: B25K
Treble: B250K
Presence: C25K
Master: B25K wiper goes straight to the Send jack.