Drawing Schematics

[bradicusmaximus]

Some weeks really bring the challenges ...

A client brought me an amp that was custom built by a local builder that has since gotten out of the business. Based the best guess it was built around some form of 'wreck design. However, his layout is definitely non standard so I really don't know his thought process on the amp. Luckily it's well designed and clean and I'm just doing some tonal tweaks for the owner to tame some boomy bass.

But I digress ...

What I really need to do is create a schematic of this circuit and try to figure out what's going on. I've never really had to do this for something as large as a full amp before completely from scratch.

So, to you experienced amp wizards - do you have any tips or advice? I know I can muddle through it, but I'm sure there's going to be some trick that makes life easier.

Thanks!

[vibratoking]

I think the main thing is to have a good grasp of how most schematic blocks are typically drawn. Without that you can easily end up with parts and connections all over the page. These types of schematics are nearly impossible to read. Think of each stage and put the stages together logically. Inputs, gain stages, tonestacks, coupling circuits, PI, power amp section. PT, power supply string, bias. Take special note of the grounding.

Otherwise, multimeter to check continuity.

[bradicusmaximus]

I think the main thing is to have a good grasp of how most schematic blocks are typically drawn. Without that you can easily end up with parts and connections all over the page. These types of schematics are nearly impossible to read. Think of each stage and put the stages together logically. Inputs, gain stages, tonestacks, coupling circuits, PI, power amp section. PT, power supply string, bias. Take special note of the grounding.

Otherwise, multimeter to check continuity.

Thanks for the advice. I figured a good start would be something similar to a Fender style from the 60s. They seem to be broken out in logical sections - and I'm pretty used to their flow so it just "feels" correct.

I guess I'll dive in and see where it leads me.

[vibratoking]

Knowledge of Fenders won't hurt. There are very nicely done schematics for the wrecks here if that is your best guess. Even if the layout is very different, the schematic should be the same or similar. What tube complement are we talking about? Reverb? Tremolo? Effects loop? Switchable output taps? TMB tonestack? Presence/cut? Master? These things should lead you in a direction.

[Phil_S]

In addition to what VK recommends, I want to suggest a modular approach at the first stage of work. Sketch by hand on letter size paper each section or module. Tape them together. Use software to draw the final documentation.

The large size paper in comparison to the number of components will allow you to "think out loud" before you commit to a real drawing. All that space will simplify the task of making corrections.

I think I'd organize around each tube.

Good luck!

[John_P_WI]

Yep, the guys above are right on the money. Start on the drawing left (input) work towards the right. Use a DMM to verify connections with continuity, but be aware that using a DMM for "in circuit" resistance and capacitance readings will probably be wrong.

Download an app like ElctroDroid to help with resistor decoding or remember the clean version of "Bad boys rape our young girls but violet gives willingly"

black, brown, red, orange, yellow, green, blue, violet, gray, white

Find a capacitor chart to decode the cap values.

You can also make a quick sketch of the layout and "check off" the wires and components off as you add them on the schematic.

Finally, take lots of pictures, you can always refer to the pics at a later date if needed.

[bradicusmaximus]

Knowledge of Fenders won't hurt. There are very nicely done schematics for the wrecks here if that is your best guess. Even if the layout is very different, the schematic should be the same or similar. What tube complement are we talking about? Reverb? Tremolo? Effects loop? Switchable output taps? TMB tonestack? Presence/cut? Master? These things should lead you in a direction.

I'm not terribly familiar with wrecks in general, but I ran a few passes through the files and some of the pictures of the various circuits. Nothing immediately jumped out to me, which didn't really surprise me too much.

The amp initially struck me as a Marshall / Fender hybrid in a couple areas. It's powered by 2 EL34s with a solid state rectifier. There are two preamp tubes and a phase inverter - all 12AX7. The power transformer is a smaller letdown model. I haven't tracked the numbers yet. The output transformer is unmarked. There's no choke.

The power filtering is using 4 100uf caps arranged for 50 + 50 with the other stages at 20 uf.

The controls are pretty simple - two inputs - normal and high with a Volume, Treble, Mid, Bass, Presence, and a master volume.

All of the smaller caps are Yellow Mallorys and are clearly marked with values. Smaller caps are all silver mica. Most of the resistors are 1 watt metal film through the power resistors are metal oxide.

I know I'm not going to reference this properly, so please bear with me. His layout used for the main circuit is not strictly linear (i.e., like a marshall or fender board). He essentially laid out the components end to end with commons and ground in the center of the board. It's not something I'm used to seeing and I definitely don't do it that way myself; however, it is a nice and compact layout and uses space nicely.

All in all, I'm looking forward to when I have more of a chance to dive into this project more and begin identifying component values. It's kind of a cool amp to tweak for the customer. I only wish the guy that built it was still doing business to chat with him directly about it.

[bradicusmaximus]

I think I'd organize around each tube.

That was kind of what I was thinking myself. To me, it seems logical to center things around the main component and then link the "systems" together. The tubes seemed to be the most likely candidate since the tubes are really the set points for each section.

It's nice to think that I'm not completely off base in some of my ideas!

[bradicusmaximus]

Here's the interior shot of the amp. If anyone happens to recognize the circuit or layout, definitely let me know.

[tictac]

I dont know about recognition but it gives me the hebee-geebees to see B+ so close to grounded hardware.....

[Gaz]

I dont know about recognition but it gives me the hebee-geebees to see B+ so close to grounded hardware.....

Why?

[John_P_WI]

I dont know about recognition but it gives me the hebee-geebees to see B+ so close to grounded hardware.....

In particular both B+ wires from the PT feeding the diodes on the rt side of the board, seems awfully close to the mounting screws. That screen resistor is a little bothersome floating around too...

So much for keeping the transformer leads short and twisted... does a tie wrap ball count?

[Gaz]

I dont know about recognition but it gives me the hebee-geebees to see B+ so close to grounded hardware.....

In particular both B+ wires from the PT feeding the diodes on the rt side of the board, seems awfully close to the mounting screws. That screen resistor is a little bothersome floating around too...

I just don't see what you guys think is going to happen. The voltage is just gonna jump a 1/4" over to ground?

[John_P_WI]

I dont know about recognition but it gives me the hebee-geebees to see B+ so close to grounded hardware.....

In particular both B+ wires from the PT feeding the diodes on the rt side of the board, seems awfully close to the mounting screws. That screen resistor is a little bothersome floating around too...

I just don't see what you guys think is going to happen. The voltage is just gonna jump a 1/4" over to ground?

B+ bottom, right side of the board is less than 1/2 the screw head diameter away from the mounting screw - more like 1/8".

[Gaz]

Right, I was looking at upper right. Still (and sorry to derail the thread), I just don't think the voltage is just going to magically jump over to ground. Or are you mentioning it because it's one of those crappy fiber boards? Now that actually seems more reasonable.