Thanks for all those comments guys, I was only thinking of replacing the Electro-caps and the pots if the school wants them.
I just wanted to make them last as long as possible for the kids.

Thanks Joe.
I was given this amp and a Marshall G15R-CD for free, apparently the pots are noisy.
I am thinking of giving them to the local primary schools music department if they want them.
I think young kids will like them, as long as the school has some guitars.
I think I will change all the Electrolytic caps and all the pots in both amps before I ask. The amps are 23 and 28 years old I think

Caps and Pots will cost up to $13 total for both amps from Tayda, and I am putting in an order soon anyway. That's if I use the best Electros Tayda has.
I might also mount some speaker cloth across the backs to keep the little fingers out of the speaker. Or maybe a metal mesh would be better.

G'day SemiConductive
I'm fairly new to audio circuits myself, but I have heaps of experience with industrial electronics.
The way I understand the active inverting low pass filter of IC1B is as follows:
The gain is set with R4 and R6(and R5), we don't want to mess with this, unless you feel there is too much gain with the pedal engaged. then increase R5 till you are happy with max gain. (Pot ?)

So R6 is 150k, or 46.78899k with R5 in the circuit.
My understanding is that you calculate this the same as a RC low pass filter.
Capacitance is 470pF
Look at this page, the same formula for normal RC Low Pass filter as an Active Inverting Low Pass Filter like yours. So we can use the top calculator for frequency drop off.
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer1

With R=150k and C=470pF you have a drop off frequency of  2.26 kHz
With R=46.78899k and C=470pF you have a drop off frequency of 7.24kHz
If you want 7.2kHz with the R=150k you have to change the capacitor to C=146pF (150pF)

You can easily put in a switch to change the frequency roll off capacitance to set it were you like it, rotary switch you get heaps of otions, but a bit overboard

But maybe I misunderstood the whole thing and got it wrong, If I did can someone please give me a hard smack over the ears please, That way I will remember next time.
Michael

Quick question for someone, when you change capacitors in an old amp,
do you just change the Electrolytics ? (Preventative Maintenance)

I have only changed the electrolytics in most amp I have come across.
Should I change greencaps and ceramics as well ?

In the 45 years I have been an electrician I never replaced capacitors just because they were old, and I mainly worked in maintenance, Industrial electronics mainly.
Lots of huge DC drives for DC motors, PLC's, big stuff mainly. I also did instrumentation  calibration and testing as well.
Just never any Audio stuff, until now.
Thanks

I was given a pair of small practice amps last week, one was a Samick SM-10.
I searched the Net for a schematic and some information,-----Nothing----.

I wanted to know what I had before I started re-assembling it, it was pulled apart.
This was the hardest circuit I have ever traced, and strangest for a budget amp.
I have traced it as well as I can for now, but I think I will do it again in the future just to see if I can find any faults in my tracing.

It has a strange setup with the Volume pot and Overdrive Pot.
I have no clue yet what the Overdrive Socket does, when I get it working I will try shorting it out to see if it kills the Overdrive I think.
Serial Number is 94060140, 1994 model ?

Here is the schematic I have traced at this moment:


 

The Schematic on https://www.schematicsunlimited.com/e/epiphone seems a little bit clearer.
Look for the "epiphone regent 2050r" file.

https://www.schematicsunlimited.com/e/epiphone/epiphone-regent-2050r-amplifier-schematic

This is a good place for finding schematics.

Ops...sorry if I raved on a bit too much in my last post about earthing.
I hope I didn't offend Enzo or anyone else here.
I guess I am a bit sensitive about safety when it comes to electricity.
I have had to revive work mates after getting zapped, not a nice thing.

Some quick reading, and playing with the calculator will help to understand maybe ?
https://www.learningaboutelectronics.com/Articles/Low-pass-filter-calculator.php#answer4
There are several calculators on this site for easy learning, just have to look hard for them.
Another good place to learn:
http://sim.okawa-denshi.jp/en/opampkeisan.htm
http://sim.okawa-denshi.jp/en/Fkeisan.htm

Quote from: Enzo on April 11, 2022, 12:54:41 PMIf double insulated, no need for earth.   Just my opinion.

I don't think I have ever seen a guitar amplifier that is in a plastic box, with plastic pot shafts and knobs.
I think any older amplifier, especially  from the last century, should be earthed if at all possible.
When an appliance is directly connected to your body (strings), there is a potential of electrocution.
Even if the appliance in question (amp) is correctly manufactured we don't know who has modified it, or used the correctly rated parts in any repairs made to it.
Not all power transformers have the right insulation between the primary windings and the secondary windings, when a transformer malfunctions it can short the mains voltage to the low voltage rails and then you end up doing a weird little dance if you are on the other end.
Thankfully it only hurts until you pass out.
I could rave on about the "handy man" installed power outlets I have come across in the last 45 years working as an electrician, how I have seen someone use a wire coat hanger as cable to connect a second power point beside the first one, or thin speaker wire,etc.
I must have found dozens of power outlets where the neutral and earth wires have been swapped, or active and neutral swapped.

Anyway, if it has a metal chassis then I earth it, if any metal you can touch is connected to the circuit (like guitars, microphones etc) then earth it.

If I was a performing artist I would carry a small power point tester with me, and I would most likely have an earth leakage protecting plug on the lead to my amp, and any other equipment that plugged into a power outlet.
At least in Australia you can get plugs that will protect against electrocution that you can fit on the end of a power lead.

Here is a crazy thought Jimi, what if you put the 5010 amp and the 5205 amp side by side, and wired the pre-amp of the 12 watt reverb to the power amp of the 30 watt ?
Go from pin 1 of IC2 of the 12 watt and go to C8 of the 30 watt.
Just to see how it would run with the 30 watt amp attached to the 12 watt pre-amp.
I think I will just make a picture up so it is easier to understand. =)

I would not be using the reverb tank you have, I think it should be a 8FB or 8EB tank, you have a working tank so measure the DC resistance of that one and work it out from there.
If you use a tank that has an input that is too low you could burn out the Op-Amp.

I personally would replace the 2 wire mains cord with a 3 wire earthed one.
You could get electrocuted if the mains transformer shorts out if there is no earth wire going to it. But that is just my feelings on the subject.

Eliott from ESP has a good article on "Care and Feeding of Spring Reverb Tanks",
https://sound-au.com/articles/reverb.htm
At the bottom of the page he has a list of part numbering of reverb tanks, Table 4.
In the table he lists the bobbin colour that match the impedance for that coil.
So if you have say a red bobbin its a F type coil with an impedance of 1475 Ohm.
I don't know if this is always the case, especially with older tanks but it might help.

This might help a little bit:

In my opinion, and it is only my opinion based on my understanding.
If you ignore the LED's,R13 and C9 would be a low pass filter, the LED's will clip the signal when it reaches the voltage to make the LED's conduct.
Look at the LED's while using the amp, full gain and the switch on overdrive and if the LED's light up its working as a clipper.
Q1 changes the gain of the op-amp, it puts R5 in parallel with R6.
If you want more clean head-room, you could add a germanium diode in series with each LED.
That would slightly increase the clipping voltage.
You could try different led's or different diodes in series with the led's to experiment until you are happy with the sound.
I think removing the LED's might not be a good idea, too much signal might get through.

Which amp model do you have ?
It appears that there might be 2 models using that circuit, the watts available depends on which power transformer is used.

This is circuit as I traced it from the pictures, very similar to the online circuit for the 5205 with a few small differences.

The C12 in parallel with R17 is actually C16

The tone controls are connected slightly different

The power rails on mine are 24 Volts, ignore this as its approx 21 Volts.
KiCad only has 15 Volt or 24 Volt symbols and I haven't made any 21 volt ones yet

Your Bridge rectifier is different number then Marshall drawing, but both are 1.5 Amp.

That's all the differences I found from the original schematic.

Michael