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Messages - Tassieviking

Amplifier Discussion / Re: op amp voltage and tone
March 21, 2023, 11:35:36 AM
I do not think it would matter, I think most signals going through the Op-Amps are in the Milli Volt range up to a few volts maximum.

There are some circuits that rely on the Op-Amp reaching the voltage supplied to start breaking upthe signal and making distortion however, but this is on purpose to give the tone some growl.

There are exceptions though and I hope someone who knows better will jump in and give their opinion.
I recently bought a Signal Tracer/Injector Pen V6.3 from
I haven't tried it yet but I believe it is a signal injector and an audio amp all in one. It injects a signal on one probe and then picks it up and makes it audible on the other probe, should be easy to see where you loose the signal.
Depending on where you are you can also buy one from TH Customs who makes them.
There is a Utube video of how to assemble and use it
Or just make something similar yourself.

Have you made sure that you have power going to the PCB in the amp ?
Do you have -9V on pin 4 and +9v on pin 8 of the 4558 OpAmps ?
Measure each one to ground, it should be 19v between pin 4 and pin 8.
Please check that you have the correct voltages on the PCB board.
Do any resistors look burnt ? , any capacitors swollen up ?
Remove the PCB and look at the copper tracks underneath, check all solder joints with a magnifying glass to see if they look bad, dull, etc.

I would start with a quick check of voltages followed by poking the components with a plastic pen or chopstick while strumming the strings, this might find a bad connection or solder joint even if you can't see it with the naked eye.
Remember to use something plastic to tap the components with and be very careful with any wires connected to the mains voltage.Fuse, main switch and transformer are deadly to touch, but like I was told when I was an electrical apprentice "It Only Hurts Until You Pass Out Or Die".
Do you have another amp that does have send/return jacks?
If so, run a cable from the Peavey Preamp Out to the return jack on the other amp.
That will check if the preamps work in the peavey.
This might be the schematic for it, but I think it might be for a head unit and not a combo as it has speaker jacks and no speakers on the schematic.
Is it the small combo with 2 x 6-1/2 inch speakers you have ?
I think it has 2 x 10watt output so 20watt total ?
I cheated when I made a power amp in a 125B pedal for the bench.
I got a 3116D2 mono amp module ($8) from E-bay and stuck it in a 125B box.
I got a 5v fan from Tayda and a 7805 voltage regulator to drive it and 2x6.35mm sockets as well.
It can run at 4.5v to 26v and can put out 100 Watt of power at 24v.
I can run it on 9v on the bench, 12v from the car, it's very versatile with a pre-amp pedal in front of it.
I run mine on an old Dell laptop 19.5v power supply.
If I make another one I will add a 7809 regulator and a 2.1mm socket so it can power the pre-amp pedal as well.
If you are up to it the Lab series of amps like the L5 is supposed to be a nice amp that's all solid state, you can buy the PCB for a stomp box build for a dual channel one or each single channel separately.
Just look up Preamps and the schematic is usually in the build documents.

There are a number of Marshall solid state amps you could try, but the mosfet ones you would most likely have to make a different power amplifier as the 2SJ49 & 2SK134 mosfets are very hard to get.
Orange Crush 20 looks like its fairly simple.
Roland Cube 20 might be ok.

If you want to learn to make your own PCB's I would suggest KiCad software, it is free and there are no restrictions on what you can make with it.
Are you going to build it with vero board or are you after a PCB to build the amp ?
Are you able to make your own PCB ?
Are you going to want reverb ?
Are you after clean tones or overdriven like Marshall etc?
It all depends on what sort of music you like to play.

There are some pre-amplifier pedal kits around if you want to go that way.
You would need a separate power amp pedal to go with the pre amp pedal.

You will need to provide a little bit more information so we know what you will like to build.

And lastly, welcome to the forum. :)

A lot of older amps have all transistor power amps in them, like the Marshall 12watt amps, Marshall 20watt and 30watt amps. They used Darlington power transistors in some of them.

Randal RG100 has all transistor pre-amps I think.
Yamaha G50 is all transistor apart from the reverb tank.
Roland Blues Cubes used to be all transistors except for one IC for the reverb tank.

Lets not forget the fantastic Albert Kreuzer 2002 BASS JFET Preamp, it might be re-voiced for guitar but might be too clean sounding.

Hopefully someone more experienced can make some suggestions of all transistor pre and power amps.

Just search around for some old schematics and borrow parts of them and get the breadboard out and experiment.

Also if you have some 4069 hex inverters or similar they can be used as Op-Amps.
Sunn Beta used the 4069 chip, and I know there are others that used them as well.

Good luck, I will be interested to see what you do.

Is the 24v 40VA transformer centre tapped, or is it just one output of 24v?
If you have 12v + 12v it would make it easier I think.
Do you have any Op-Amps or will it be all transistors?

There are lots of old transistor amp schematics around that you could borrow sections from, maybe a proven pre-amp that sounds good already.

It all depends on what transistors you have, I would like to see just the part numbers, I don't need to see the specs for each one.
I keep thinking/hoping that Phil plays Bass guitar with a moniker like PHATT, Phatt just makes me think of a deep fat bass riff being played.

I know its made up from parts of his name, but still, what a cool name for a bass player. :)
I totally agree with Phil, somewhere between the volume pot and IC102.
It would be best before or after R131, if the picture I found on the net is correct then there are 2 PCB's in that amp.
I would find the correct wire between the pre-amp PCB and the power-amp/ power supply PCB and cut that wire to install the 2 Jacks.
I think I spy a shorted track ?You cannot view this attachment.
The 2N3819 is a JFET transistor, you are looking at an old style TO-9 one in your amp, the new ones look different in the TO-92 casing.
You can still buy the old style one if you shop around, but I would not.
New ones are more likely better quality.
Amplifier Discussion / Re: 12w Marshall
January 24, 2023, 12:17:06 PM
Guzzies3, have you considered trying to build some of BajaMans Pre-Amp pedals from ?
Some of those can work well in front of an amp I believe.
He has made Amp sims for several different amps as well as speaker sims etc.
They all have vero board layouts for them.
I made up some of them as PCB's but I havent had time to make any yet.

As far as hijacking a thread goes, I don't think Joecool85 minds it happening, at least I hope he doesn't.
As long as it is in a friendly manner and it keeps this site as a friendly forum where like-minded people can have a chat and share ideas and stuff.