High Voltage BoARD

[Fretless]

Asked Google’s Bard the following prompt:
You are an expert in electronics, specifically high voltage circuits built using 4 layer PCBs for guitar pedals which use tubes running high plate voltages. How would you go about building the high voltage circuit and PCB layout? Also, what would be the overall guidelines or rules of thumb you would use when building the pedals e.g. how would you keep the high voltages from grounding out to the metal enclosure?

… it came back with three possible answers, and this seemed to possibly be the best one:

/*
Building high-voltage guitar pedals with tube circuits on 4-layer PCBs requires expertise and careful attention to safety and isolation. Here's how I would approach it:
High voltage circuit and PCB layout:

1. Schematic design: Start with a well-designed schematic that accurately reflects the desired tube circuit topology and operating voltages. Ensure proper biasing and safety measures like bleeder resistors across high-voltage capacitors.
2. PCB layout:
   • Ground planes: Utilize the 4 layers effectively. Dedicate one layer for a continuous ground plane under the low-voltage circuitry and another for a high-voltage ground plane under the tube circuitry. This creates a physical barrier between the two voltages.
   • Component placement: Keep high-voltage components close to the tube and minimize trace runs, especially for high-impedance sections. Use thick traces and wide spacing between high-voltage traces.
   • Isolation: Use creepage and clearance distances based on the voltage ratings to prevent arcing between high-voltage components and other traces or pads.
   • Bypass capacitors: Place bypass capacitors close to the power pins of ICs and tubes to decouple high-frequency noise.

Overall guidelines and rules of thumb:

• Safety first: Always prioritize safety when working with high voltages. Use proper safety equipment and work in a controlled environment.
• Component selection: Choose components rated for the high voltages and currents involved. Capacitors should have voltage ratings at least twice the operating voltage.
• Isolation: Maintain strict isolation between high and low-voltage sections of the circuit. Use isolation transformers for power supplies, optocouplers for signal paths, and double-insulated jacks and switches.
• Grounding: Ground the chassis to the AC ground pin of the power supply. Ensure proper grounding throughout the circuit, including the high-voltage section. Avoid ground loops by using star grounding.
• Shielding: Shield high-voltage sections with metal enclosures to prevent noise emission and accidental contact.
• Testing and commissioning: Use a variac to gradually power up the circuit and carefully monitor voltages and currents. Double-check all connections and component values before applying full voltage.

Preventing grounding to the metal enclosure:

• Enclosure isolation: Use an insulated enclosure or line the inside with a non-conductive material like Kapton tape.
• Standoffs and insulation: Mount components on insulated standoffs and use insulating washers for hardware.
• Component selection: Choose components with insulated cases or built-in isolation transformers.
• Leakage paths: Identify and eliminate potential leakage paths, such as through component leads or solder joints.

Building high-voltage guitar pedals is a complex task, and this information provides a general overview. It's crucial to consult relevant safety regulations and best practices, and seek guidance from experienced builders if needed. Remember, safety is paramount when working with high voltages.
*/




Is anyone else using these chat bots to work through this kind of thing, or circuits in general? It seems pretty useful, tho I can’t say I’ve done enough of it to have double checked its answers. The answers it gave to this question seem pretty good! The recommendation to use a variac when first powering up was something I hadn’t thought of before for this kind of thing.

Btw, Bard can analyze circuit schematics, or at least sounds to me like it can haha, tho don’t count me responsible for any hallucinations it might have

 

[Fretless]

Mentioning the use of an SMPS changed things a bit; I’m mainly concerned with how to ground these tube pedals. Anyhow, followed it up with the following, and it’s answer is below that:

”Would you change any of this advice if using a SMPS with a 9v DC adapter, rather than something which plugs directly into 120v AC?”

/*
If you're using a SMPS with a 9V DC adapter instead of a 120V AC power supply, my advice would change in a few key areas:
High voltage generation:

• DC-DC conversion: Instead of directly stepping down from AC, the SMPS will generate a higher DC voltage (e.g., 24V) and then convert it to the required high voltage (e.g., 150V) using a DC-DC converter. Choose a DC-DC converter with high isolation between input and output.
• Safety considerations: While the risk of electric shock is lower with DC, safety precautions are still crucial. Ensure the DC-DC converter has over-voltage and short-circuit protection.

PCB layout:

• Isolation: Maintaining isolation between high and low voltage sections remains important. Use isolated DC-DC converters and separate ground planes for high and low voltage sections.

Overall guidelines and rules of thumb:

• Component selection: Choose components rated for the specific DC voltages involved. Pay close attention to isolation ratings for DC-DC converters and transformers.
• Grounding: Grounding remains crucial. Connect the SMPS ground to the chassis and maintain a star grounding scheme within the circuit.
• Shielding: Consider shielding the high-voltage section to minimize noise and prevent interference with other circuits.

Additional points to consider:

• SMPS noise: Some SMPS can introduce unwanted noise into the circuit. Choose a low-noise SMPS or implement additional filtering if needed.
• Power limitations: SMPSs often have limited power output. Ensure the chosen SMPS can provide enough current for the tube circuit.

While the 9V DC input simplifies some aspects of safety compared to 120V AC, it's important to remember that high voltages are still present within the pedal and proper isolation, grounding, and component selection are essential. Always prioritize safety and seek guidance from experienced builders if needed.
*/




..this part seemed to make a lot of sense to me, not sure if this is common practice or not, but I’m guessing it is

• Use isolated DC-DC converters and separate ground planes for high and low voltage sections.

 

[zgrav]

interesting Q&As with the Bard. Thanks for sharing them.

 

[Fretless]

You’re welcome, zgrav

ChatGPT’s response to a similar question around building tube pedals (answer abbreviated to what seemed most relevant)

/*
To prevent high voltages from grounding out to the metal enclosure:

1. Isolation: Ensure proper isolation between high-voltage nodes and the enclosure. Use insulating materials like plastic standoffs, grommets, or mounting techniques to prevent direct contact between the circuit and the metal enclosure.
2. Shielding: Employ shielded enclosures or add shielding layers within the pedal to isolate the high-voltage components from the rest of the circuitry. This shields against electromagnetic interference and minimizes the risk of unintentional contact.
3. Grounding: Implement a dedicated ground plane on the PCB layout to separate the high-voltage sections from the low-voltage areas. Proper grounding techniques, like star grounding, can help maintain a clean ground reference and minimize noise.
4. Physical Layout: Strategically plan the physical layout of components on the PCB to keep high-voltage traces away from the enclosure and other low-voltage parts. Create a layout that minimizes the risk of accidental short circuits.
5. Insulation: Insulate high-voltage traces using appropriate materials and distances. Consider conformal coatings or insulating tapes where needed to prevent unintended contact.
*/

There just seems to be several approaches to this stuff, and I want to at least understand the reasoning behind the various strategies. Nathan’s designs seem sound enough to me, hopefully he doesn’t mind a bit of curiosity around this. Some builders use fuses, rubber grommets, etc.

 

[Fretless]

This was the prompt used for ChatGPT, for those interested:

”You are an expert in electronics, specifically high voltage circuits built using 4-layer PCBs for guitar pedals which use tubes running high plate voltages. How would you go about building the pedals? What would be the overall guidelines or rules of thumb you would use when building e.g. how would you keep the high voltages from grounding out to the metal enclosure?”

perhaps I’m overthinking this..

 

[vigilante398]

One day I dream of becoming an expert in something.

None of that is particularly bad advice, but I do have some notes:

• "Dedicate one layer for a continuous ground plane under the low-voltage circuitry and another for a high-voltage ground plane under the tube circuitry." - not a bad suggestion, but complete overkill in a simple circuit like a tube preamp. These use a common ground, no real reason to split them up. I find it to be more effective to use a common (but still continuous) ground plane for one of the internal layers and a split power plane for the other. This keeps the majority of the high-voltage access to the internal layer, which helps prevent arcing. Also the only low voltage circuitry in my pedals (aside from Lab Rat) is the input side of the SMPS itself.
• "Capacitors should have voltage ratings at least twice the operating voltage." - In circuits where you're running an AC input this is good advice, you want to be able to accommodate for input voltage spikes that could exceed the cap rating. But when you're using a regulated SMPS the chance of voltage spikes is near zero (the SMPS is more likely to fail than to exceed the set output) so I ignore this one. I typically use 250V rated caps no 235V circuits and 400V rated caps on 350V circuits.
• "Preventing grounding to the metal enclosure:" - we actually ground to the metal enclosure because we want the noise benefit of a shielded chassis. Audio circuits are a little different in that we have to make consideration for things like that.

On the next section:

• "Use isolated DC-DC converters and separate ground planes for high and low voltage sections." - As noted above this is overkill for simple circuits like what I deal with. Continuous common ground plane throughout is great for power stability and providing a continuous DC reference for AC signals to keep noise down.

Otherwise pretty solid advice on that one.

I don't consider myself a tube or high-voltage expert by any stretch of the imagination. I have a degree in EE and just over 6 years experience designing circuits in industry, but I'm still learning. Thanks for posting this, it was a fun read

 

[Fretless]

Haha yes, I was genuflecting somewhat to the AI gods by using the word ‘expert’, I agree. lol
Hey, one never knows when that singularity is gonna happen.

Thanks for the detailed replies!

I suppose “shorting to the enclosure” is more what I meant to say.

 

[Fretless]

Couldn’t resist poking the Bard, haha:

Ok, so let’s say the board is grounded to the enclosure in order to shield against electro magnetic interference. In terms of the physical build of the guitar pedal, how would you prevent or build to protect against an accidental high voltage short to the enclosure, say if a stray wire came loose or something? Would there be any point in using a fuse in the power supply section to help protect anyone handling the device in such a scenario, or would the capacitors involved discharge too quickly for a fuse to prevent a possible shock?

..and here was the response, tho it says to put a fuse *after* the DC-DC conversion “on the low voltage side” .. wait, what?? Lol
That feeling when you realize an LLM is really just guessing at and not reasoning what the next words should be



/*
Grounding the PCB to the enclosure for EMI shielding presents additional safety challenges when dealing with high voltage, but there are steps you can take to prevent or protect against accidental shorts and shocks:
Preventing shorts:

• Double checking connections: Carefully inspect and double-check all solder joints and component connections before powering up the pedal. Securely tie down any loose wires and use strain relief where appropriate.
• Component selection: Utilize components with insulated leads and cases whenever possible. Consider using isolated DC-DC converters for powering low-voltage sections from the high-voltage supply.
• PCB design: Design the PCB layout with generous creepage and clearance distances between high-voltage traces and other components, including the enclosure mounting points.
• Mechanical protection: Use potting compound or conformal coatings to protect the PCB from contact with the enclosure and accidental spills.

Protecting against shocks:

• Ground fault circuit interrupter (GFCI): If using an adapter-based power supply, consider using a GFCI outlet or power strip. This device detects imbalances in current flow and can quickly cut power to prevent electric shock if a fault occurs.
• Fusing: While fusing the high-voltage section might not be 100% effective due to fast capacitor discharge, it can provide some level of protection. Choose fast-blow fuses with appropriate current ratings for the circuit. Place the fuse on the low-voltage side of the power supply (after any DC-DC conversion) to avoid high-voltage fuse blowouts.
• Safety isolation: Implement additional isolation measures like optocouplers between the high-voltage section and the user interfaces (switches, jacks, etc.) to minimize the risk of shock through control paths.
• Enclosure selection: Use a metal enclosure with a solid, continuous construction. Ensure all openings and access points are properly insulated and shielded.
• User education: Clearly label the pedal as high voltage and include warnings about potential risks in the user manual. Advise users to avoid opening the enclosure or modifying the circuit unless qualified to handle high-voltage electronics.

It's important to remember that no safeguard is foolproof, and accidents can still occur even with careful design and construction. It's crucial to prioritize safety in every step of the build process and use professional help if needed.Additionally, consider implementing redundant safety measures for more comprehensive protection.
Remember, working with high-voltage circuits requires expertise and a strong understanding of the risks involved.Always prioritize safety and follow best practices to protect yourself and others from potential hazards.
*/



I’m thinking back to those gutshots of the Kingsley pedal, where there’s a fuse used in the SMPS; not sure if he still uses those .. I’ve seen many other Kingsley pedal gutshots and some have fuses, some don’t, so who knows?

Anything Kingsley

Simon's pedals are killer and so pricey. I think most of them (maybe all) have a preamp tube, so maybe enclosures are more an issue. Anyway, having a clone of one would be AMAZING! http://kingsleyamplifiers.com/products/pedals/20/

forum.pedalpcb.com

It seems weird tho that some big names like Soldano and EHX no longer sell tube pedals, especially with EHX having a tube side hustle. Did they get sued and quietly discontinue them? Or are they just too much of a hassle to be bothered selling them?

The sound is so good with these circuits, tho. Damn. Maybe putting them inside an insulating outer second enclosure made of wood or some kind of plastic that won’t melt would quiet any doubts around all of this.

Sorry, @vigilante398, hopefully this hasn’t turned into a roasting of all you‘ve done in making these circuits available for everyone to enjoy! The seriousness and effort you put into helping people learn and understand these circuits is commendable, as is your knowledge and qualifications around all of it. I really just want to understand what and where the risks are with all of this high voltage stuff, and find a way to mitigate or even eliminate them where possible.

 

[vigilante398]

I’m thinking back to those gutshots of the Kingsley pedal, where there’s a fuse used in the SMPS; not sure if he still uses those .. I’ve seen many other Kingsley pedal gutshots and some have fuses, some don’t, so who knows?

Anything Kingsley

Simon's pedals are killer and so pricey. I think most of them (maybe all) have a preamp tube, so maybe enclosures are more an issue. Anyway, having a clone of one would be AMAZING! http://kingsleyamplifiers.com/products/pedals/20/

forum.pedalpcb.com

It seems weird tho that some big names like Soldano and EHX no longer sell tube pedals, especially with EHX having a tube side hustle. Did they get sued and quietly discontinue them? Or are they just too much of a hassle to be bothered selling them?

The sound is so good with these circuits, tho. Damn. Maybe putting them inside an insulating outer second enclosure made of wood or some kind of plastic that won’t melt would quiet any doubts around all of this.

I don't have any answers there I'm afraid. I've never felt the need to put fuses in my pedals, but that's probably more because of how they're designed compared to Kingsley. Where he has a separate PCB generating the high-voltage, that means he has a wire floating around the pedal carrying high-voltage, so if that wire bounces around a bunch and the connection fails then you have 225V or so (if I remember right, been a while since I traced the Harlot) flying around the inside of a metal box, which is obviously dangerous. In contrast my pedals are on a single PCB, and there isn't as risky of a failure point like there is on the Kingsleys. I don't have any wires carrying high voltage, it's all internal to the power plane of the PCB. The only way something in one of my designs is going to carry high voltage and snap off and hit ground is if there's either 1) intentional damage such that components are being partially removed and allowed to float around, or 2) a DIY builder using one of my PCBs but doing a very sloppy job and not following my build instructions.

I don't know for sure why Soldano or EHX stopped selling the tube pedals, but some things I can think of that both had in common was taking AC power, being in a huge enclosure to accommodate a power transformer, and both having a reputation of sounding good but being very noisy, in some cases noisy to the point of being unusable. I have an original Soldano GTO because it's my favorite drive circuit ever and I wanted it as a collector, it is indeed much noisier than my Black Eye builds. I also have an old EHX Black Finger compressor, and the workmanship on that isn't great, and the pedal is also incredibly noisy, in addition to using what I consider an unsafe AC power supply connection.

I am of course incredibly biased as I sell tube pedals so you can keep that in mind when you read my stuff, but that's my two cents' worth.

 

[Fretless]

Thank you. That makes me feel much better about all of it!