Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: meestromusic on June 26, 2026, 09:46:15 am
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Thanks to the organizers of this board for populating it with such great information. I've posted in the greeting thread, and reviewed the pinned threads to see if there was any m.o. for posting requests like this. Understanding the DIY nature of this forum, I also know my own limitations, hence the following request. If, indeed, there are threads about this type of request, please direct me and feel free to delete, I will not take offense.
In getting back to the guitar playing, and missing my old 30w 6SL7 v1, KT66 Siegmund Midnight Blues quite a bit, I'm looking to put together a sort of 'tribute' amp that carries some of its characteristics. I've hobbled together a handful of characteristic and design concepts, and have what I think may be enough to go on for a testable schematic & layout, but I am extremely deficient when it comes to taking such things from the listed idea realm to the design app (which may seem strange considering my graphic design background). I even have conceptual head cabinet and chassis front design mockups, but it's the electronics hump that I'm needing to conquer, and haven't been able to quite yet. I'm hoping assistance in that aspect of the design realm will give me a leg up in continuing the project and seeing it through.
Any tips or recommendations for folks to assist with the schematic and layout if I were to provide the notes? Happy to compensate. Thank you.
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As far as schematics go, there may be something in the library under the Misc section?
https://el34world.com/charts/TubeAmpSchematics.htm
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Download this schematic drawing tool.
https://www.expresspcb.com/ (https://www.expresspcb.com/)
Tutorial
.
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I appreciate it, I have some of these tools, but my hand-eye coordination of getting concept into mechanical drawings is lacking. I would love someone to help me get through this portion of the process so I can get on to using the testing software and then build.
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@meestromusic,
Welcome.
This is definitely a "you learn by doing it yourself" sort of forum. I know it definitely can be an uphill climb to teach oneself tube electronics this way, (as opposed to being classroom-trained,) but anytime you read something you don't understand, come here and ask a question. And heed all the safety instructions you read. They aren't kidding.
Books:
Search here on "what books should I read?" IMO the gold standard for preamp design and theory is Merlin Blencowe's "Designing Valve Guitar Amplifiers." Robert Megantz wrote a good book on amp-design and building. Richard Kuehnel has written several on various aspects of Fender's 60s amp design. If you can only afford one of them, get Blencowe's.
On eBay, try to find "The Radiotron Designers Handbook 3rd Ed." (RCA). Try to get one dated 1945 or later. Much useful stuff there.
On YouTube, check out Uncle Doug. His instructional videos are the best I know of for beginners. Many basic videos to explain the concepts and subcircuits of tube amps. A great resource for the noob.
As JPK said, ExpressPCB helps you draw your schematics. DIYLC is another free app which can be used to draw both schems and layouts. Many people here use it. Some folks use Visio (yeah, not free but a great tool if you have it). Others use JSchem to draw schematics (.SCH files). Download that to read .SCH files. There is a good schematics library here, and also a good one at SchematicHeaven.com. Many schematics also come with a pre-drawn layout. Look at Hoffman's schematics for any of his Fender AB763 boards for ideas.
TDSL is a site where you can download tube datasheets.
Instead of the KT66 Siegmund for a first amp, may I suggest you consider starting with a simpler push-pull amp like an early Fender Deluxe, just to get your bearings? (I say that just because my first two or three amps were garbage as I learned the skills.)
A lot to throw at you here, but I hope some of it helps.
Good luck!
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I truly appreciate it, but I'm here interested in building one specific amp, not getting into the business of building multiple amps or projects, if that makes sense. I am a great fan and supporter of builders, designers, etc. and I helped w/ the cabinet design of the Siegmund amps going from a full width to a shorter head cab, etc. I've even spoken to a few folks who built the cabs for him. If he was still alive, I would be working w/ him on this, of course, but he's not, so I am trying to find assistance in working with someone on the project. I'm fairly certain that my descriptive layout is enough for someone to draw up a schematic w/ values, I just need that initial leg up, and then I can take it from there and test, try other values, etc.
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Understood. Somewhere on this site there is a list of technicians who build and work on tube amps. Maybe there is one on the list near you? Just a thought.
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The front panel looks very similar to a JCM 800.
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_2204_lead_series.pdf
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without a schematic, building anything will be a crap-shoot
how many guitarist with tube amps do you know?
guitar center type store near by;
hit them up, play as many different amps as possible to find the one that gets you closest to "that sound"
then you have a starting point, otherwise it'll just become another "folly of man" sorta thing:)
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without a schematic, building anything will be a crap-shoot
how many guitarist with tube amps do you know?
guitar center type store near by;
hit them up, play as many different amps as possible to find the one that gets you closest to "that sound"
then you have a starting point, otherwise it'll just become another "folly of man" sorta thing:)
I'm not sure I'm conveying myself properly. I know what 'that sound' is, and I have a full build spec for it, I just need a schematic and tube layout drawn to the spec sheet that I've already prepared.
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This is the spec sheet—perhaps this will help get the ball rolling a little better. Thanks guys!
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This is the spec sheet—perhaps this will help get the ball rolling a little better. Thanks guys!
Finally! if you expect to catch a big fish you must put some bait on your hook.
I stopped reading at the "Preamp – V1 (6SL7 octal)" section. Your header wants a 6SL7 but the paragraph specifies pin numbers for a 12AX7. Might want to fix that.
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Have you validated that your spec sheet results in a functional amp that sounds like the one you remember, or do you imagine this to be a collaborative project to iron out the details if any critical issues present themselves during the schematic phase? This sounds like a project that isn't straight-forward and will have the potential for at least a couple "lost in translation" errors that will require someone guessing at what it is you want to resolve them.
With all due respect, this seems like a paid job for someone you feel comfortable trusting in a contractual project scope, who is also themselves comfortable with the amount of work required for you to have an end result that you're happy with.
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Have you validated that your spec sheet results in a functional amp that sounds like the one you remember, or do you imagine this to be a collaborative project to iron out the details if any critical issues present themselves during the schematic phase? This sounds like a project that isn't straight-forward and will have the potential for at least a couple "lost in translation" errors that will require someone guessing at what it is you want to resolve them.
With all due respect, this seems like a paid job for someone you feel comfortable trusting in a contractual project scope, who is also themselves comfortable with the amount of work required for you to have an end result that you're happy with.
Yes, as stated in the beginning, this is a hoped-for collaborative project with willingness to compensate.
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This is the spec sheet—perhaps this will help get the ball rolling a little better. Thanks guys!
I stopped reading at the "Preamp – V1 (6SL7 octal)" section. Your header wants a 6SL7 but the paragraph specifies pin numbers for a 12AX7. Might want to fix that.
Where are you seeing pin numbers for a 12AX7 specified under V1 preamp?
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Where are you seeing pin numbers for a 12AX7 specified under V1 preamp?
In the first three sentences of that section. Also, the resistor values you have for V1 are great for 12AX7s but not so great for 6SL7s.
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Where are you seeing pin numbers for a 12AX7 specified under V1 preamp?
In the first three sentences of that section. Also, the resistor values you have for V1 are great for 12AX7s but not so great for 6SL7s.
I'm not seeing the discrepancy other than running it a little hotter as a 6SL7. Perhaps a 2.2 kΩ rather than 1.5 kΩ would run smoother.
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Did AI build the spec sheet?
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Did AI build the spec sheet?
It filled in a heavily populated prompt. Happy to be flagged in areas worth taking note.
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Most guitar tube amp circuits fall into the following categories:
1) single ended amps with very simple circuits e.g.; a pre-amp tube and an output tube and maybe a rectifier tube - with a volume control and some sort of tone control(s) (think tweed Champ or tweed Princeton - or Blackface Vibro champ or Vox AC4 for something a bit fancier with tremolo)
2) push pull amps that are in the 10-20W power bracket - with two or three preamp tubes and two output tubes (6V6s or EL84s) and maybe a rectifier tube and volume and tone controls (think tweed deluxe, brown deluxe, or Blues Junior, Marshall 18Wer - or for something a bit more complicated - Vox AC15) Most of these have a cathodyne phase inverter (although the brown deluxe as a Long Tail Pair inverter) - some earlier tweed amps and supro amps have a paraphase inverter
3) more powerful push pull amps in the 30-50W bracket - as in 2 above except with 6L6 or EL34 output tubes (instead of 6V6s or EL84s) and maybe a rectifier tube think tweed bassman, marshal JTM45, or Vox AC30 (albeit that has 4 x EL84 output tubes). These usually have a long tail pair phase inverter. Dumble amps like Overdrive Special fall into this category (albeit they have slightly fancier pre-amp wiring). These can extend into 80-100W circuits with 4 output tubes (Tweed Twin, Blackface twin or Showman, Marshall 100W Plexi, etc) or 100W but with higher gain preamps (more preamp tubes) - like Soldano SLO100, PV 6506/EVH5150 etc
Your amp looks like it could be in the 3rd category. This might give you a clue about what sort of circuits to study before you rip into reverse engineering the schematic. Good luck
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Yes, it's a bit of a take on a JTM bluesbreaker w/ octal V1.
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DIYLC can draw schematics and layouts and it is free and works on PC and Apple
plus you can have custom boards made
All that info is on the forum here
https://el34world.com/Forum/index.php?board=26.0
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I don’t think you need 4 triodes for an effects loop
You keep saying Bluesbreaker but I see no mention of the cathode follower or dual input stages
No one’s going to hold your hand here, you’ll have to do the work. And with a new design the process will be iterative.
I suggest starting with a known good Bluesbreaker layout and tweaking for the changes you want.
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I don’t think you need 4 triodes for an effects loop
You keep saying Bluesbreaker but I see no mention of the cathode follower or dual input stages
No one’s going to hold your hand here, you’ll have to do the work. And with a new design the process will be iterative.
I suggest starting with a known good Bluesbreaker layout and tweaking for the changes you want.
I'm not sure where the miscommunication is. The original design from Siegmund was based around a bluesbreaker (the iron was Mercury Magnetics from the JTM45 line of Axiom). I'm not asking someone to 'hold my hand' I'm asking to commission someone to draw up the schematic and tube layout so I can begin the next stages of the process of testing, iterating, etc.
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I'm asking to commission someone to draw up the schematic and tube layout so I can begin the next stages of the process of testing, iterating, etc.
Good luck with that! There are several people on this forum who would be up to the task. I think they would have spoken up by now if they were interested. This project will require a lot of collaboration with you because your text file is too vague in some areas and leaves a lot of unanswered questions about the circuit. That will take a lot of time just to arrive at a workable schematic that pleases you. You don't seem to have any electronics knowledge so I'm unsure that you would recognize a workable schematic.
You may have better luck by getting the schematic of the original design from Siegmund to use as a basis to draw a logical layout. Or possibly take your text file and feed it into AI and ask it to create a schematic and layout. Keep in mind that AI is just a baby and still makes lots of mistakes.
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Here's a schematic I found for the Bluesbreaker.
https://www.audioservicemanuals.com/m/marshall/Marshall-1962/Marshall-1962-Bluesbreaker-Reissue-Schematic-1.jpg
From what I can see, it's basically a plexi with trem.
If you want no trem, and the same gain structure as a Bluesbreaker with a single input, I would consider something like this.
https://el34world.com/charts/Schematics/files/Sluckey/November_01.pdf
Check it out. Compare your notes with the schematic. I looked into the midnight blues, and it is possibly similar to this design except the midnight blues has reverb. Also, the November amp is built with EL84's, so if you were to build it with KT66's the power section and attenuation would be different.
Note there's a layout included.
If you do some of your own legwork, there's lots of people here who can help with making it work, maybe even with no commission involved.
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My opinion:
An amp built to those specs will not work properly. I do not want to volunteer many hours of design time on a project that is doomed from the outset.
An alternative path: Tell AI, or this board, or both (cool experiment!) what qualities you want in an amp and ask AI or us what proven amp design provides those qualities. Modify that design with octal preamp tubes or a properly designed effects loop if necessary. Make sure the power transformer has enough current capability to work with the design. 100ma for two big power tubes is a huge red flag to me that the designer of the spec sheet is not to be trusted.
Go forward and enjoy the process.
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Just for fun, I asked ChatGPT to draw a Fender Champ after giving it links to a fender .pdf file in the schematics library.
It drew the image below
No doubt it's first try may not be 100%
I am trying to get it to understand how .diy files are created.
I showed it a .diy file example
I am working on educating it to see if it can do that
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I think you are better off creating schematics and layouts in DIYLC rather than in PCExpress or Jschem
Plus you can have custom boards made from your .diy file
For example, here is a .diy Princeton Reverb board with the eyelets and components
Rob Robinette has some complete .diy files with all components, wires etc
Here is a link to Rob's champ
https://el34world.com/charts/DiyLayoutCreator/files/Rob_Robinette/5F1_Champ_Layout.diy (https://el34world.com/charts/DiyLayoutCreator/files/Rob_Robinette/5F1_Champ_Layout.diy)
And here is the .diy library
https://el34world.com/charts/DiyLayoutCreatorFiles.htm (https://el34world.com/charts/DiyLayoutCreatorFiles.htm)
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Just for fun, I asked ChatGPT to draw a Fender Champ after giving it links to a fender .pdf file in the schematics library.
It drew the image below
No doubt it's first try may not be 100%
Not shabby for first try. Baby Chat has a lot to learn. We need some more data banks. Keep those RAM prices up! :laugh:
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Keep those RAM prices up!
:laugh:
own stock in Micron???
I bought into autonomous drones, currently loosing my shirt cuz the hardware costs went nuts feeding the BigBrain!!!!
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Micron is going to be -75% down the road.
AI bubble is a reality
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I bought a Crucial (Micron) 32GB RAM kit last September for $85. Today that same kit is $450! Thank you, AI. Micron no longer caters to the individual computer nerds. AI demands all the RAM that Micron (and others) can produce. If the AI bubble ever bursts, Micron will probably just shut the doors and die with a smile on their faces.
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There are already AI firms that are offering way cheaper rates than the big names and investors know this
This chart shows a topping and the red lines are where it will go back down and test support levels
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Just for fun, I asked ChatGPT to draw a Fender Champ after giving it links to a fender .pdf file in the schematics library.
It drew the image below
No doubt it's first try may not be 100%
Did you give it more instructions and directions that than? I tried to recreate what you did and it went like this:
draw me a fender champ. Here's a schematic https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5c1.pdf
Sure. If by "draw me a Fender Champ" you mean a simplified layout/block diagram of the classic 5C1 Champ based on that schematic, here's an ASCII version showing the signal flow and power supply. (I'm not reproducing the copyrighted schematic itself.)
And the result was a shabby block diagram.
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This is exactly what I typed into ChatGPT
can you draw a simple amplifier schematic for a Fender Champ?
I have an example here that has the schematic and the layout.
Can you read this and draw your own?
https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf)
Then it came back with this
Yes. If you provide the schematic (or I can use the PDF you linked), I can redraw it into a much cleaner, easier-to-read version.
I can produce several styles:
Classic electronic schematic
Standard resistor/capacitor symbols
Tube pin numbers
B+ voltage labels
Similar to old Fender drawings, but cleaner.
Modern simplified schematic (my recommendation)
Signal flows left to right
Power supply across the top
Ground bus across the bottom
Components aligned neatly
Much easier to follow when troubleshooting.
Block diagram
Guitar Input
│
12AX7 Preamp
│
Volume Control
│
6V6 Power Amp
│
Output Transformer
│
Speaker
Layout drawing
Eyelet board
Tube sockets
Transformers
Wiring paths
Similar to the original Fender layout.
I can also make it look like a professionally drafted CAD drawing instead of the hand-drawn 1950s style.
The PDF you linked is the original Fender 5E1 Champ schematic and layout.
One note: because that schematic is copyrighted, I can't simply trace or reproduce it exactly. What I can do is create a new, original schematic that represents the same 5E1 circuit in a cleaner style, with my own layout and drafting. That usually ends up being easier to read anyway.
If your goal is building or servicing the amp, I can also add:
component values
expected DC voltages
tube pin numbers
color-coded signal path
color-coded power supply
a matching wiring layout that corresponds exactly to the schematic.
Then I said this and it produced the image you saw
Yes, please draw a schematic using this list of ideas above
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AI slop is real. Someday AI will be useful, but not for me right now.
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I use AI for programming in vb.net, php, Home Assistant yaml and a couple others
It is an incredible tool
As far as AI screwing up consumer PC parts, that sucksBut These companies will have their day when the bubble pops
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This is exactly what I typed into ChatGPT
can you draw a simple amplifier schematic for a Fender Champ?
I have an example here that has the schematic and the layout.
Can you read this and draw your own?
https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_champ_5e1.pdf)
Then it came back with this
Yes. If you provide the schematic (or I can use the PDF you linked), I can redraw it into a much cleaner, easier-to-read version.
Then I said this and it produced the image you saw
Yes, please draw a schematic using this list of ideas above
I tried entering your quotes into ChatGPT and still got the same type of response w/o any type of schematic. So, I decided a Login may produce something different and it did. Entering your quote again
ChatGPT - I can also redraw the entire 5E1 schematic as a clean SVG-quality drawing, suitable for printing on a single page. That version is much easier to read than the original Fender drawing while remaining electrically identical.
Yes, please draw a schematic using this list of ideas above
And I got: ChatGPT 5E1-1.png
ChatGPT - I can also include a matching turret/eyelet board layout that corresponds directly to the schematic, making it much easier to build than the original Fender drawing.
The finished drawing would be a completely new schematic based on the original circuit, not a traced copy, while preserving the original electrical connections.
I can even make it look like something drawn in KiCad or CAD software, with perfectly straight wiring and consistent spacing, suitable for printing on a single page.
yes please give a board layout and schematic making it look like something drawn in kicad or cad software
And I got: ChatGPT 5E1-2.png
I then entered:
yes please give a board layout and schematic making it look like something drawn in kicad or cad software. include all power supply bus voltages
And I got: ChatGPT 5E1-3.png
No need to nitpick all the issues, it was an interesting exercise and ChatGPT has a lot to learn. But hey, it just might
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Nice
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ChatGPT has a lot to learn. But hey, it just might
pretty soon you'll be able to tell the bot to solder up the board n ship it, might even get to the point where it tells you you're wasting valuable resources on antiquated things n subtract $100 from your bank:)
be careful what you hope for, you just might get it :icon_biggrin:
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I think it will learn, though to be honest I think there will be tools that do really well with one thing -- for example, designing electronics -- and not so well with other tasks. I dunno.
My recent experience trying to learn 20th century SS electronics has been interesting. I was repairing an amp from the 70's which had more than several blown transistors. I'd ask for advice regarding modern transistor replacements -- would get one answer for today, a different answer the next day (same question and same LLM), and some answers that were obviously wrong. When these obviously wrong answers were challenged, the bot would come back and apologize and then give an entirely different answer. Net effect -- almost useless information, not authoritative, and I didn't trust it.
When I asked for references, the answers were better AND it was easy to check the reference to see if it made sense (it was a 50/50 proposition).
What I did find very useful was asking it to compare datasheets from a group of transistors (or other components), lining up the key parameters in a table format. It was really good at that and I found no instances where it would hallucinate.
Using different chatbots gives similar but different flavors of answer. Can't say I have a favorite, but Google Gemini is sure easy to access and seems as good as any.
Mild apologies for being so off topic but it seems this thread has morphed . . .
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I think it will learn, though to be honest I think there will be tools that do really well with one thing -- for example, designing electronics -- and not so well with other tasks. I dunno.
that should be the norm, I agree; want legal ease, ask an AI legal agent, want SS design.....
sadly most want you staying engaged, since the more you're connected, the bigger the Boss's bank account, we are the commodity of the 21st century, no longer the producer....