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

I bought a couple of the Weber 50W (component level) auto-transformers for use on the output of a 50W @ 4 Ohm power amp. Worked OK didn't seem to change the sound much when driving an 8 or 16 Ohm load. Last I checked, the 50W was discontinued leaving only the 100W version. I ordered a pair of those, they weren't much bigger than the 50W units. Fit the same chassis holes.

I'm not sure the Wal-Mart units do the same thing, they look more like constant Voltage distribution transformers with big L-pads.

If you want organic and touch sensitive, you don't want opamps and regulated power supplies. Those things are mutually exclusive.  Even a tube preamp can be cold and lifeless with a regulated or isolated power supply. To keep the hum out of a preamp, start with a well laid out ground and adequate filtering.

If the amp has an effects loop, connect a cable from Send to Return and see if that corrects the problem. Other than that, a good guess would be a fractured solder connection between pots, jacks and switches on the front panel and the PCB that holds them.

How old are the batteries ? Are they Ni-Cad or some other chemistry ?

Some rechargeables can be problematic. They might not charge up to rated Voltage or hold rated maH. Many times one cell will be weak and may take on a reverse charge as the battery discharges.

Programmable pots open up a lot of possibilities. You could even make some of your stompboxes programmable. Beware that many programmable pots don't like signals that go beyond the rails so you need +/- supplies when the pot has an AC signal on it.

The 166M72 does have a center tap according to the data sheet. You'll need that to make +/- 50V or so rails. At 216VA it's a little oversized but it will run cool even at 50Hz. Could be used for roughly 200W @ 4 Ohms.

The 2N3904 is not a good choice for a low level preamp. If you look around at available schematics, you probably won't find a preamp or a distortion pedal made with discrete bipolar transistors since the 1970s. Don't try to reinvent the wheel, use opamps. FET input opamps like TL072 can easily achieve 1 Megohm or more.

First thing, C2 polarity is wrong, minus side should go to input (generator). Second thing, input impedance is quite low. To start with, increase R1, R2, R6 and R8 to at least ten times the value shown on the schematic. Lastly, install a resistor (something like 220 or 470 Ohms) between the +24V source and circuit. This will attenuate any hum on the +24V source.

Probably an issue with leakage (on the PCB) or partial static damage on the input of IC103. IC106 will be difficult to find and expensive. Find a flux removal spray (or use isopropyl alcohol) and cleanup any flux around IC103. Best to install a high quality socket, then you can shotgun chips until you find one that will work. If you are having trouble with static discharge, use something to raise the humidity, or work naked.

A workaround if you can't find IC106. You will need to join the forum to see the attachments: https://forum.sunnstillshines.online/index.php?topic=6864.msg29374#msg29374

The M80 is a complex circuit. There is no tone stack with a slope resistor like many tube amps. The Bass and Treble controls look to be of the Baxandall (google it) variety and the Mid control is some second cousin to a graphic equalizer with synthetic inductors and a dual pot. In addition, there is a bridged T network between the first and second stages.

Modifying this would be a nightmare or perhaps a train wreck. The most sane way to do it would be to use a modeling program, model the entire preamp and try to mold the EQ to something you like. Change components, test and try again.

Slightly lower tech approach would be to plot the EQ curve, change one component and plot again. Repeat as necessary.

Still lower tech, install some kind of graphic EQ in the effects loop and see if you can fix it that way. Or just build your own preamp and jam it into the effects return. You can't bypass the "Delta Comp" which seems to be some kind of compressor. It might have some influence on the tone you don't like.

FYI: The MN3007 and MN3101 chips are impossible to find replacements for. You might be able to remove them and sell for big bucks.

What kind of solder are you using? No-Lead solder usually has a high melting point. The lowest melting point is 63% tin, 37% lead if you can get it. Soldering irons can vary in temperature. I use a Variac to vary the temperature on mine, but you can use a low cost light dimmer.

Using chokes is no guarantee that your amp will be hum free. A bad grounding scheme can introduce hum that you can hear when there is no signal. In general you need to know how much current will pass through each choke. Once you get  up to the several Amp territory, you are looking at a 5 pound (2 kilogram) chunk of iron and for a dual rail amp, you will need two. That is the case when you pass all the current for the power supply through the chokes.

Link to data sheet: https://www.mouser.com/datasheet/2/389/tda7293-957143.pdf

The TDA7293 is a 15 pin chipamp so it may be possible to only pass the current for the driver stage through the chokes. (This allows the use of much smaller chokes.) This is uncharted territory, you'll need to experiment and you may blow up some chips before you get it right. A simple RC decoupling network might work just as well.

When a solid state power amp clips to the rails, any ripple on the rail is transferred to the speaker. Tube amps do the same thing. It's hard to hear this hum because the amp is as loud as it will go at that point. If you want to eliminate this hum, you need to lower the Voltage to the driver stage so the it "runs out of gas" before the output transistors clip to the rails. This lowers the power output by perhaps 10%.

Another way to eliminate this "slamming the rails" hum is to not let the power amp clip to the rails. You can do this with a Master Volume control, but this only works if the power amp is acting as a Voltage amplifier with no mixed mode feedback. Mixed mode feedback means current flowing in the speaker determines if the power amp needs to "Go to the rail" to make current in the speaker follow the input. (Flame On Non-believers)