• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Recommend DIY build learning resources?


Can anyone point me to some reading material that will assist me in learning about how to DIY audio electronics?

Some background:
I've decided I'd like to build a tube preamp. Specifically this: https://www.cascadetubes.com/the-12au7-cathode-follower-color-preamp/
This will be my first attempt at a project like this. I'm an electrician, so I know my ohms law and electrical safety, and I can solder and read the circuit diagrams just fine. I could no doubt fumble through this and be A-OK.
I'd like a deeper understanding.
The language used online to describe these circuits is foreign to me. Presumably because the majority of you are American/European, and I'm from Australia, or maybe it's just because it's a different field, but B+ voltage for example is a term I'd not heard before looking into this. Does that just refer to the active (positive) side of the circuit or is there more to it?
Also, I look at that circuit linked above and ask myself 'OK but WHY does that capacitor connect to earth (ground), at that point in the circuit? What is the intended effect on the audio?'

I could continue detailing my ignorance, but essentially, I just want to know more on the subject.
Any resources you can point me to?

Thanks so much!
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I'm sure others here who are more technically oriented than I am, which is almost everyone else here, will point you to some excellent reading material.

For practical, and detailed introduction to DIY building for newbies I think @stephe 's (Skunkie Designs) YouTube channel is excellent. She also just did a build of that same preamp, which is documented in a series of videos.

Here's a link to her channel: https://www.youtube.com/@SkunkieDesignsElectronics/videos

And here's a link to the first video of the "Color Preamp" build series, which she posted about a month ago:

There are some recent threads here that she started about some of the technical details of the same preamp, just scroll down or search.

She posts a bit more often over at Audio Karma, such as this recent thread: https://www.audiokarma.org/forums/index.php?threads/potentiometer-placement-in-tube-gear.1031281/

For questions about why / how a specific part is used in a particular circuit you might want to start a separate thread and post the schematic.

The term B+ is pretty universal and it comes from the early days when batteries were used to power radios. The main battery was known as the B battery so B+ indicated the positive voltage output from that battery. Other batteries provided the filament voltage to heat the tubes and the bias voltage.
Oh wow! That's a lot of helpful information for me to work through. What a detailed and relevant response! Thanks so much for that. I'm really glad I signed up and asked the question. I'm going to really enjoy digging into that and educating myself.

Thanks for the info on B+ too. It raises some follow up questions for me but I'll follow etiquette and explore the info you've provided before asking more. No doubt I'll be back soon haha. Thanks again!
Yes. Safety first. Always.
Reading through that safety practices thread is a bit concerning actually. A lot of that advice is far too relaxed. One example: Somewhere near the top the advice is to make sure your device is switched off at the wall before working on it. NO! Physical isolation. Always. Switching a switch on and off while working on something is a really good way to get distracted by what you're working on and forget what position the switch is in. Sounds dumb, but we're human. It happens. Not to mention, do you really trust your life with that cheap switch? Because you shouldn't. They fail all the time. Physically isolate your device from supply and test with a multimeter before touching ANYTHING. Every time.

Back to topic though, those Morgan Jones's books do indeed look good! I'm going to order 'Building valve amplifiers' now. Thanks for that!
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If you are forgetful enough to not remember which position the switch is in, you may also forget to pull the power plug.
Even if you do pull the plug, you may still get shocked . . .
"Gee, I forgot to plug the tube in, so I'll pull the plug and put the tube in."
What? No B+ Bleeder Resistor, those B+ capacitors are still charged . . . Ouch!

Safety First!
Prevent the "Surviving Spouse Syndrome", be sure to install a B+ bleeder resistor, even though 1/2 of the schematics do not show a bleeder resistor.
I recommend - http://www.valvewizard.co.uk/ & http://education.lenardaudio.com/ & http://www.r-type.org/articles/art-010g.htm for a start. Some of those are slewed towards guitar amps but the same priciples apply. Also if you search for something like " How to build valve/tube audio amplifiers" or "Valve/tube audio design". Lastly there are numerous Youtube channels but this one - https://www.youtube.com/@UncleDoug is a good one for begginners, again guitar amps etc.

Finally you'll learn a lot more if you muck about with the actual valves on the bench whilst researching. start by building a simple common cathode triode gain stage as described in great detail on Merlins site.


PS B+ is what Americans call the positive rail of a power supply, we English call it HT, it's the same thing. In lower voltage circuits it could be labelled Vcc+
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The term B+ is "very" historical, it goes back to when radios were run on batteries. IIRC, the A battery was for the filament, the C battery was for the (negative) grid bias, and the B battery was or course for the plate supply voltage. It's similar to the more "modern" (but still 60+ years old) nomenclature of Vcc (collector supply voltage for a bipolar transistor circuit - Vcc is generally used to indicate the positive supply voltage for all solid state circuits, even PNP and all-FET circuits) and (the less common) Vee (emitter supply of same). But as they said in that movie, "but that's not important now."
Here, I found a description, apparently there's some really old codgers writing Wikipedia articles:
More in next post.
You're asking about "that capacitor," the 33uF from the first tube's cathode to ground. It's across that resistor, and the resistor is there raise the voltage (by having current go through it) of the cathode above ground, so the grid can be "negatively biased" but be referenced to ground and not need an actual negative supply voltage (see the C battery that we don't use anymore). The "problem" with this resistor is when we have an input signal that we want to amplify and show up on the load (plate) resistor, some of it ends up across this cathode resistor, and the cathode ends up moving in phase with the grid, lowering the grid-cathode AC voltage of the signal we want to amplify. This greatly lowers the gain of the stage. If we put a big capacitor across the cathode resistor, it keep the cathode at a steady voltage and lets the tube run at its full gain. The capacitor is thus named the "cathode bypass capacitor."
If you can get ahold of some old Heathkit assembly manuals to read, I think it would help a lot, I read every one that my father had when I was a teen (he showed my how to connect up a really old battery powered two-tube radio that he used when HE was a teen, that's why I know about the A, B and C batteries). The manuals always had a schematic and a well-written circuit description section. The older (say 1970s for before for tube gear) ARRL Radio Handbooks, practically any year, can be helpful too.
Here is a playlist of videos I have made trying to explain some basic tube stuff. Also Morgan Jones' book is a really good one!


This video is my attempt at explaining how to work and test tube gear safely. Blueglow Electronics YT channel also has a good video on safety. As others have said, this stuff can literally kill you if you aren't careful!

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I remember my first valve pre amp in 1980.
Simple inverter stage to use with a guitar.
Built it up and it didnt work.
So powered down and touched the circuit, got a huge shock and nearly fell off my chair.
My tutor said the cap was still charged hence the shock and I had to discharge the cap with a resistor.
So fixed a fault and powered it up again but still no sound.
So discharged cap with a resistor and touched circuit and got a huge shock again.
I had forgotten to turn it off !
Amazingly I am still alive 43 years on.
I tend to work much safer now and usually have an LED with high value resistor across smoothing cap so I know when its charged up.
I also have an isolation transformer to help with safety and allow connection of a scope.
The saying 300 volts will let you know you’re alive and 500 volts will let you know you’re dead! Putting a LED with a large resistor across the PS caps is a great suggestion. Read as many tutorials and ask questions. Better to ask than do something that will hurt you or the parts. Use good parts, really cheap off brands might work at first but not for long. Nothing is worse than working hard to build something you’re proud of to have it melt down because you saved a buck! If you have kids or pets keep an eye on them. I found out a 300 volt shock can lift a 20lb dog about 4 feet off the floor instantly! Simple stuff like make sure your wire is rated for high voltage, I use Teflon jacketed wire rated at 600v so there is no question. I use 1watt resistors for the same reason even if only a quarter watt is needed, cheap insurance. Most importantly, have fun and don’t over analyze to the point of paralysis.
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From someone pursuing this hobby for two years now and coming from a background of embedded systems and plant sensors and servos, I knew little about tubes other than I wanted to make tube amps in retirement. But knew some electronics. The hobby has many levels of fun from wood/metal work, to pcb design, to layout design of your project on the easier side, to kit or proven circuit making on the medium side, to designing from first principles and its math on the real hard side. I'm not at all to the third level, but amazed by folks here who are. My advice is to start collecting schematics, Google images makes that easier than ever, save them into category folders by circuit type or by output tube or by whatever. Study schematics, then come here to ask questions. Analyze 2 or 3 schematics a day. When ready breadboard some real simple circuits, measure voltages and currents, change part values, etc. or build a kit or proven circuit. This hobby is not one of destinations (finished projects), its a journey, especially if you have ADHD like me. Be safe, one hand in pocket, shoes on, etc.

When you do make your first Amp anything metal someone can touch must be attached to the house safety ground, thats code everywhere, period. Circuit "grounding" is a complicated subject, but knowing that one simple rule of the safety ground is your most important grounding rule. Without that your Amp can be an electrocution hazard if its enclosure isn't a physical part of your house ground. Would you touch your Amp while standing in bare feet on a damp concrete floor is a good question to ask.
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