Quote from: saturated on May 12, 2023, 09:45:08 AMback to the light switch question...they are all single pole switches right?

I don't want to give away the answer, and I applaud your determination to figure it out for yourself, so I'll give you this: At the Home Improvement store where they sell what you need to do the trick, they call the switches you need to do the common two switch circuit a "three way switch" and the one you need to add for the three switch circuit a "four way switch". Once installed and covered up with a face plate, the switches look and feel like the normal On-Off switches you see and use every day. (These names apply in the USA, not sure about other countries.)

I was thinking it's kind of hard to learn those circuits out of a textbook, I learned them from an instructor with a chalk and blackboard. I see an opportunity for a You-Tube series with an instructor, maybe even animated graphics.

One of the things you do in early electric/electronic class is learn how to analyze simple circuits usually with a battery, a switch or two and several resistors or light bulbs. You learn how to apply Ohm's Law to calculate Voltages, currents and equivalent resistances. If you skip this step or don't know how to do it, you can look pretty stupid on an internet forum.

Do you know how to hook up a light bulb so you can flip it on of off from two different switches no mater how the other switch is flipped ? How about three switches ?

If the amp has an Effects Loop, try plugging a short patch cord between Send and Return.

Is this one of those 'Guitar Amp Head' style PA's with a 260 series power amp across the back ?

What speakers do you usually use with the PA ? Does the back of the amp get warm ?

An intermittent type failure can make it difficult to track down the exact cause.

With the Sweep in NORMAL mode, there won't be a sweep unless you get a trigger somehow.

With EXT trigger source, you won't get a sweep unless you connect a signal to the External Trigger Input and adjust the trigger level control to get a trigger.

A drop or two of contact cleaner in each of those switches will free them up. Don't touch the end of the CRT tube shown in the picture.

The 7808 has an internal current limit. If it draws 3 Amps, that current limit isn't working and the chip is bad. To get to the current limit, you need a load resistor on the output, 8V/R = 3A. R needs to be 1.666 Ohms. Looking at the data sheet, With a 13V input the current limit at 25C is about 2.25A. 8V/2.25a = 3.555555 Ohms. Note that the current limit goes down as temperature increases. At 125C the current limit at 13V input is only 1.7 Amps.

Edit: I should mention that you need to use a heat sink when testing with any load current higher than about 100mA. The data sheet says the chip is protected, but they will fail if abused.

Edit2: You can use a small heatsink with a plastic "Vice Grip" ( made for holding wood as glue dries ) that has rubber jaws.

Quote from: FleshOnGear on April 07, 2023, 05:24:32 PMWhy does the preamp appear to be so mismatched to the sensitivity of the power amp? Why wasn't the amp designed with lower voltage rails, so it would clip before reaching that amount of power? Why is the current protection set to clamp so high? Is the amp actually capable of safely providing much more output than the rated 100W into 4 ohms? How do I figure out how hard I can drive the amp before it burns out? What am I missing that makes this design make sense?

Almost all guitar amps are designed that way. The one exception I can think of is the Sunn Coliseum 300 where the preamp creates slightly rounded clipping waveforms with CMOS and the power amp doesn't have enough gain to clip the waveform harder. The clipping of the CMOS is adjusted to track the sag of the power amp rails. Current limiting sounds incredibly bad when you are driving a speaker so it is set pretty high so it just protects against shorts.

The big problem with the Marshall MOSFET design is that it will burn up those Hitachi FETs with little provocation. It really needs two or three pairs to be reliable. I'm not sure what it looks like inside but adding more FETs to the output could be difficult. You may be able to find some heavy duty replacements, but make sure they have SOURCE tied to the case of the TO-3. Most big FETs are TO-247 these days so you may have to improvise a heatsink.

The first thing to do to add the mixed mode feedback is to isolate the ground of the speaker and the external speaker jack.

Your profile gives no clues what country you are from. I know that in some country's there is a Right To Repair Law, but I don't know the details. Even if your country doesn't have such a law, you might be able to obtain service info from someone in a country that does.

Maybe just a power amp small enough to fit on a pedal board would be a good project. There are lots of preamps build into pedals so you are not stuck with just one preamp. If you need more or less power, you can just get, or build another power amp.

The original question in this thread was never answered. What amp to build ? I don't know if there is a solid state amp that people want to build. Maybe the Randall RG100 comes as close as anything, but I don't know that anyone has done it. There are a couple of threads about the Marshall Lead 12. Even some PCBs offered for sale.

Link: https://www.ssguitar.com/index.php?topic=5164.0

What you really don't find is a good layout. I mean a layout so you can build say a power amp  or preamp on some kind of proto-board. I used to use a PCB program to do layouts, but that software died so now I just do a 2X pencil sketch on graph paper.  For help drawing circles, I use a circle template. Attached is a scan of a layout for a power amp. I use three sizes of eyelets. Sometimes I use turrets to mount power resistors up off of the board.

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OK, I'm glad to hear that you have experience on tube amps so you know how to be safe.

Do you want to build a tube amp, or will you be building a solid state amp ? With tube amps, you can use techniques of old Fender or Marshall amps with eyelet or turret boards and chassis mount tube sockets. For a solid state amp, options are different. PCBs simplify things if you can find one for the circuit you want to build. I recommend you have at least three boards. A preamp, a power amp and a power supply board.

For a preamp, you can use the technique used to build a pedal. I like to use eyelets for any wires that go off the board and a one pad per hole type board for the rest of the circuitry. I hate vero board. I use a similar board for the power amp but use more eyelets for power transistors and large resistors and caps. For the power supply I mount almost everything on eyelets on either blank FR4 or a board with big holes on a large pattern. For most amps, you need to be able to drill and tap holes on the heat sink.

Using an IC power amp is usually a challenge to hook up on a proto board. It would be wise to look for a PCB.

The most difficult thing to deal with is the potentially DEADLY Mains wiring. What country are you in ? It's best to have a mentor that can look over your shoulder and keep you safe while you "learn the ropes".

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.