Noob Needs Amp Design and Schematic Drawing Help

[MelodianChew]

Hello 18watters:

I am new to the community, I am a guitar player and a budding amateur electronics aficionado, but unfortunately have been relegated to having to teach myself electronics while having no mentor to turn to, So I have many gaps in my knowledge and I appreciate any help you guys could give me. I plan on building the 18watt Marshall style super lead at some point, but for now I am trying to understand amplifiers and their component networks. Most of my resources for instruction material have come from textbooks and technical forums and articles and websites, and of course experimentation and building of various circuits. The most hindering aspect of self-learning is not having someone more expert to ask questions.

To cut to the chase, right now, I am basically trying to teach myself about amp layout, design and about schematic reading and drawing, all of which I have packaged as a learning exercise into an attempt at trying to "redesign" the amplifier topology of a little PA head I have that I would like to repurpose for guitar some day.

The help I am looking for at this point is with critique about this schematic I have been working on. The amp I would like to use as guinea pig for all of my mistakes and errors before I attempt the 18 watter is a relatively obscure 60's 20w PA head, the McMartin MA-20 that I got for cheap. (I have included the schematic.) The schematic SEEMS like it maybe could have been drawn more clearly, but there again, I may be wrong. Am I correct about that?

Anyway, this seems like a good skill to have, so I decided to just jump into the water and try to create one.

On this attempt, to simplify things, I limited myself to being concerned only with learning about how the components are arranged and about the basic signal flow route on the AC portion of the circuit, and not worrying too much about the DC involvement, which I don't understand as well yet. No component values either. The schematic is drawn using LTSpice, and my rendition rather crude, as this was my first attempt drawing such a large schematic and first time using the software, so there will be MANY elementary errors and mistakes. Also, forgive me, but I couldn't figure out in the software how to choose the exact icon to represent one or two components that I have, so I just used the generic components that I could find that were most similar. In places, I couldn't find the same part at all, so, where the component is NOT of the same type, one is standing in for it, and I have labeled them as such.

So for this exercise I imagined as if I had stripped all the wires from the components in the MA-20, then what would be the best amp topology to try to emulate using the pieces that I have and how should I repurpose and then wire them, and how would I render that schematically.

The existing components (not circuit) seem like they could most closely support something like a Blackface deluxe, if with no reverb or tremolo. And I am basing this idea on what my guesses are about the capabilities of the PT and OT from looking at the schematic of the MA-20. This particular schematic doesn't make it obvious, I believe, but It looks like (estimations) the PT voltage pre-amp plate can furnish about 170-180V, power tube plate @ about 410V, current and heater voltages @ 6.3V., and center tap arrangement, and then the impedance available from the OT of about 8 Ohms. The tube set is 2x 6L6GC and 2x 12ax7 and 1x 6av6. Rectification is solid state. Critique of this evaluation would be helpful as well. This is why I chose to most closely emulate the Deluxe. Was this a good decision?

I know that my schematic is likely more than just wrong, but I would like to know if my first attempt is at least on the right track as concerns representing what I was trying to lay out with the circuit. Using the Deluxe and my MA-20 schematics for reference, along with others. I tried to rearrange the components into sections.

Here was my line of thinking:
First I copied the PT, the diode rectifier section and the heater wires from the MA-20. I erased the portion with the large filter caps to deal with them later. Then starting from a single input jack, I tried running through the signal flow by drawing the 6av6 triode as a first gain stage, after which I inserted a volume pot which went into one half of a 12ax7 for a second gain stage, then into the other half for a 3rd gain stage, into a "James" style TB tone stack, which then runs into a master volume, and then into the second 12ax7 which is repurposed as a Long tailed pair Phase inverter section before running into the power tubes then out to the OT and tapping off of the 8 Ohm output into an 8 Ohm speaker. I then made an attempt to insert a choke because the Deluxe layout that I referenced had one and then I tried to approximate where I guessed that the large filter caps would connect. One major part I had trouble figuring out was how the rectifier and PT section connected to everything. Was this a logical mode of thinking?

I know full well that I am putting the full extent of my ignorance on display here, but I feel like it may be worth it if I can learn a few things and speed up the impossibly difficult task of educating a slow person.

Thank you for any time that you can spare to help teach me the craft.

Scott

[JMPGuitars]

Hi Scott,

Welcome to the site. I don't have time to look through that thoroughly, so I'll just give you a couple quick pointers and ask a question.

Pointers:
1. Use KiCad for circuit design. It's free, and much more is available for it including tubes. Lots of addon content floating around the internet.
2. If asking about a circuit, you should have the component values listed on the schematic.

Question:
Are both sides of the choke supposed to be connected to each other? Follow-up question: what's that called?

Thanks,
Josh

[MelodianChew]

Thanks Josh,

Thanks for the info about the other schematic program, I'll check into it for sure. The reason I didn't have any component values listed is because at this point, being so new, I really just wanted an evaluation about the logic I used in setting the parameters into the schematic.

Like for instance, did I lay it out correctly? I was of course using other schematics as reference, but the original MA-20 schematic seemed like a poor layout to me. Like it could have just been done better. Didn't seem to flow to me. I'm certain I could be wrong, but I was curious if my instincts about that were at all correct. This was an attempt to try and clean it up, however futile that would be. But did my schematic seem more "logical" at all? Also, did I make a good choice for an amplifier topology considering the components I had to work with? Did I get close to laying THAT out correctly? Were the lines spaced and routed in a coherent fashion? Stuff like that. Next attempt at this I'm sure I will have a better handle on choosing the correct component values.

Great question about the choke. I will definitely do more research trying to find out more about what exactly it is they do and have an answer for ya. Remember, I'm doing this on my own with no one to ask questions from. But, thank you, those are exactly the kind of critiques I want because you saw something that just didn't make sense, now I happily have to go figure that out.

But, thank you for the looksee, and if you find the time to dive deeper into it, that would be great too.

Scott.

[JMPGuitars]

It looks like an amp circuit. Layout and design of the schematic diagram could be refined, but certainly not bad for an early attempt while learning. This is not a comment on the presence of errors or accuracy of the circuit.

Let us know what you learn about the choke being connected to itself like that.

Thanks,
Josh