Is it bad for the chip when only one channel is mostly used? So my idea would be to hook up one channel to an internal speaker and the other to an extension speaker jack.

Tested it, and I still got occasional pops on powerchords  .
Now I'm fed up with varying resistor values, I just built a diode limiter and put it at the input before the coupling cap. Now it is rock solid, finally I can windmill my Les Paul and get no unpleasant cracks or pops! I used a 22k pot to make clippig asymmetric on demand, but the effect is more subtle.
Main thing is, that this is now a very good portable pedalboard amp. With that para mids you can dial in any flavour of distortion with just one dirt box in front.

I was given a LA4282 from my fathers workshop (he repaired lots of TVs in the 90s).
To use this for my guitar projects, I would like to know if I can bridge this like a LA4440 for example, and what external circuitry is needed.
The LA4282 already has input and feedback resistors for a gain of 101 integrated.


Datasheets:

LA4282:
https://www.mouser.de/datasheet/2/308/1/LA4282_D-2314539.pdf

LA4440:
https://www.electroschematics.com/wp-content/uploads/2012/07/la4440-datasheet.pdf

Thanks for the suggestions, guys.
I don't think that clipping was considered by the designer, because it sounds like someone is lifting the needle off a vinyl record too roughly - like "pbbfffzzttt"  .
Last night I simulated the first stage in LtSpice, and the clipping behavior was exactly the same to my measurements.
I played with the values and tried to stick to the scheme in Figure 2 of this:
https://www.analog.com/en/resources/analog-dialogue/articles/avoiding-op-amp-instability-problems.html
But when I use 100k/100k bias resistors in reality, then my 12V ref descends to about 9V. So I got back to 10k/10k, that gave me 11.8V. I also lowered the 820R in the supply to 680R, so I have now real 24V instead of 22V. The gain was also lowered to 19; the screenshot shows the final values I soldered in.
On the o-scope it looks very good now, should be enough headroom. Tomorrow I will test it with guitar and speaker.

I guess what this modding is making so difficult is that the amp was designed for the CA741's and the change to NE5534 brought higher bias and supply currents.
Like someone already wrote, perhaps I should have chosen the TL071.

Hi g1,
I scoped the first 5 graphs with a signal generator on the normal input. The blue wave is from in front of the 68k input resistor, like directly at the hot tip of the plug (the voltage at pin 3 is then about half of that value, e.g 700mV in the first image). The yellow is at pin 6 (output) of the first opamp. In the bottom bar you see the Vpp values, at 16V it is beginning to flatten on the negative side.
The riff.jpg is the "smells like teen spirit" riff played with a Les Paul on the bridge Burstbucker 2. There you can see, that I get easily peak voltages that would cause clipping.

Ok so my options to avoid the (asymmetric) clipping in the input stage are:
1. lower the gain, e.g. with 5.6k in place of the 2.2k resistor
2. lower the series 820R resistor further to get over 24V for the IC supply (now 22V) so the 24V zener diode also finally gets involved.
3. Make the bias resistor pair uneven in value, so that I get a bit a higher than 12V, so that when it clips, it clips to the positive rail instead to cutoff?

If I understood the NE5534 datasheet correctly, the maximum output p-p voltage I can achieve is around 5V lower than the supply. And 0.7V input with a gain of 22.4 is already giving 15.4V just by calculation.

Anyway, so do I have to make the 100k resistor at the positive input of the first opamp against 12V smaller? Doesn't it have to be half the value of the 12V midpoint resistors? So in my case 5k Ohms?

Hi Phil,
here are the photos that helped me. The photo from the solder side is already mirrored to allign with the top side.
I also traced it node by node, so I can't imagine I got something wrong.
I think everything is recognizable on the pictures, the transistors with the flat side facing the output transistors are BC212, the ones facing to the rectifier BC182. Under the dual gang pot sits a 4k7 Ohm resistor (R34).

So, here's the final schematic.
Are there any obvious design flaws?

Tracing is in progress, see attached pdf. Took me 3 evenings to get this far. The preamp part is hard to understand for me, but it seems that the tone controls will turn out to be all active.
I also played a little with the values of R16 and R17, 100k made the input clipping worse, 22k better than the stock 33k and finally 10k gave me the most headroom. I still can get it to clip when I hit the strings real hard, but now that's unlikely in a normal play situation.

I think I am going to trace it this week, just need to make photos from both sides and then lay them over each other half transparent in some imaging software.
Just did some quick measurements with an o-scope and it showed at the first opamp output that the clipping is asymmetric and in the negative half wave. At the input I had 0.7Vpp and on the output around 14Vpp when clipping starts.
Seems that also the bias voltage isn't exactly Vs/2, I have Vs=22.2VDC but 10.4VDC at the input. The single supply bias resistors are 33kOhms.

What I also noticed is that I get an unpleasant clipping when the input gets too high, for example when I hit the strings a bit harder on a humbucker guitar. I tested with a sine wave generator, and the clipping begins at 0.5 volts, regardless of set amp volume. Is this normal for old solid state designs? And can I modify something to prevent input clipping?

Ok, thanks. Would I also hear that oscillations if they happen?
I hear some background fuzz underlying the notes.

The amp was working with the 2k2 resistor when the CA741 opamps were in there. When I changed them to NE5534 there was more current needed.
Min-Max Currents:
CA741: 1.7 - 2.4mA
NE5534: 4 - 8mA
So current draw is approx 4 times higher with the new opamps and the series resistor was limiting the current.

Oh man, what an odyssey I've been through...
I wanted to know what can I improve on this design and ordered new caps, opamps (NE5534) and power transistors.
The amp has a sweet sound, but all of the tone controls, the more they were turned up, added some background hiss on notes, reminiscent of a noisegate. I guess that's because of the CA741 opamps. And then there was also a 100Hz buzz (230V/50Hz wall supply) which increased exponentially when volume knob was above 50%.
So my first step was to renew all electrolytic caps. Noise problems were still there, but I got a noticeable increase in maximum bass and volume.
Then I swapped the opamps from CA741 to the NE5534s. On the first test the amp was dead quiet, no signal, only the mid shift/gain circuit could self oscillate with fart-like sounds. Then I checked Vcc on the opamps (the CA741's before had 22VDC), and I only measured 3VDC!
Then I analyzed the opamp power supply, and spotted a series 2.2k resistor in front. There is 40VDC coming from the rectifier, and 37VDC were falling on that resistor, leaving 3 volts for the opamps (there are also a 24V zener diode and a 220uF cap involved).
With crocodile clamps I tried different resistors in parallel, and with finally around 800Ohms series resistance I got 22 volts at the opamps again. So I soldered a 820Ohms 2W resistor in place of the 2.2k old one.
Now the amp worked and finally lost the background hiss but I still got that 100Hz buzz at higher volume. It was also there when nothing was plugged in.
I checked all supply circuits and there was no excessive ripple or something like that.
Desperate I also changed the output transistors (one of them had burn marks anyway), but this didn't solve it.
Long story short, in the end it were the input jacks, the switching contacts to ground on both were dirty and had high resistance...
Tomorrow I can finally do a meaningful test run on this little amp.