8 ms·

How Tube Amplifiers Work

157 points by gokhan 10mo ago

fsckboy 10mo ago
it's a bit weird that I never thought about it before this, when I already had the facts in my head: the triode tube amplifier was invented by Lee de Forest, but he had no idea how it worked or even what it was capable of. then 45 years later, the solid state transistor amplifier was invented, and they had no idea how it worked either. for people who have not had much EE education, what is important about triodes and transistors is that they amplify. you can put a signal in (a signal like from a microphone responding to your voice), and put some power in (like from a battery) and these amplifiers can make an output "copy" of the signal which is more powerful/"louder" than the original. from this basic function, everything that we think of as "electronic" flows. we would still have electric things like light bulbs, heaters, spark plugs, electromagnets, but basically just electric steam punk frankenstein machines, and nothing subtle. Amplifiers are termed "active" electronics; without them, we'd simply have passive electricity. I didn't read this article because I already know how these things work, and the article looks extremely confusing, and I've already read my fill of explanations that don't explain anything and (not saying this is one of those) I don't want to even risk that again. it is very difficult to find explanations for how transistors work that make any sense at all.
- kazinator 10mo ago
  > he solid state transistor amplifier was invented, and they had no idea how it worked either. That cannot possibly be true. Not knowing what exactly is going on with the charge carriers at the subatomic and quantum levels is not the same as not knowing how the amplifier works: like if we fiddle with the voltage at the base, we can influence the collector current, and all the rest. What is true is that some early transistor designs of audio amps treated transistors like tubes: they featured a phase inverter transistor that fed two non-complementary push-pull stages whose output was combined by a center-tapped output transformer. The excuse that well-matched complementary PNP transistors were not readily available at that time rings hollow, because it's possible to create an push-pull output stage with just NPN transistors. This is called "quasi complementary" (lots of search results for this). Output transformers, if they have multiple taps in the secondary winding, do allow for different impedances. If the end users expect to be able to plug a 16 ohm speaker into a 16 ohm output jack and a 4 ohm into 4 ohm, then they will understand that kind of amp better.
  - kazinator 10mo ago
    Radio Shack sold PA amplifiers with an output transformer well past the age of the tube, like the MPA series, e.g. MPA-40, a 20 W mplifier. On that thing you can obtain the raw amplifier output using the "70V" terminal. Then it has a number of through-the-trafo outputs labelled with nominal ohmages of speakers. The Owner's manual extols the advantages of using transformers for speakers and describes how to use the 70V output in conjunction with external transformers. Quote: For complex multiple-speaker arrangements that require many speakers and long runs of connecting wire, we recommend you use a line transformer (not supplied), available at your local RadioShack store. [...] There are several advantages to using transformers. • You can connect speakers with different impedances without causing differences in output between the speakers. • You can add or remove a speaker without having to recalculate the entire system’s impedance. • You can reduce signal loss when you use speaker wire over 50 feet long. LOL!
    - quickthrowman 10mo ago
      Sound masking systems still use 70V audio output with output transformers at each speaker, voltage drop is rough when your signal is only a few volts and you’re using small conductors. Last time I sold a sound masking install we used 14/2 cable for the 70V audio signal. https://www.atlasied.com/speech-privacy-speakers?srsltid=AfmBOoqRvEImBr52QWhIt81FOIpvWaByhKti96msfyWgdkBl5ul_ghyV https://www.atlasied.com/speech-privacy-speakers?srsltid=Afm...
      - sethhochberg 10mo ago
        They're quite popular for distributed audio systems in general (of which sound masking is one type). "Constant voltage audio" comes in a few flavors and 70v is very common in the US, other parts of the world often use 100v. Background music systems in retail, voice paging systems, etc use constant voltage hardware because its much better technology for very long cable runs, daisy-chained speakers, and centrally located amplifiers. The cost is fidelity. Full-range audio transformers aren't cheap, so these systems usually make some compromises because your announcements or smooth jazz over the pasta aisle don't need to be true hi-fi. Its cool technology. Most of the speakers have variable power taps, so you can run a bunch of them in parallel on a single line and control the actual volume as-needed based on where the speaker is deployed by varying the transformer tap on each speaker.
redm 10mo ago
This is an interesting topic, but the ads overlaying the content make this very hard to read :( * Please don't suggest I install an Ad blocker.
- Night_Thastus 10mo ago
  Install an Ad blocker. I'd recommend uBlockOrigin on Firefox, or Firefox for mobile. I mean just in general it makes the web less awful. Webpages are so much easier on the eyes without all the crap they try to stuff in there. And it can prevent malware, especially for those less tech-inclined. And it means you use less data/bandwidth, since the blocker prevents the request from ever being made in the first place. If you want to support a site, just buy a subscription or donate to them or something.
- loloquwowndueo 10mo ago
  What ads? Install an ad blocker.
- 1970-01-01 10mo ago
  Sorry, but adblock is a genuine quality of life hack for everyone online. Since you don't want to install anything, how about working at the DNS level and/or hosts level? https://adguard-dns.io/en/welcome.html https://adguard-dns.io/en/welcome.html will change your mind. https://github.com/Ultimate-Hosts-Blacklist https://github.com/Ultimate-Hosts-Blacklist is another option for doing it in the hosts file.
- pestatije 10mo ago
  id never do it, but you could install an ad blocker
- metalman 10mo ago
  Ok, I suggest that you install the "Stoutner, privacy browser", which treats seeing text, as the default, with the ability to allow other content as optional, though certain sites that are still hand coded, show, as they always have....adds and all.
- kazinator 10mo ago
a_t48 10mo ago
I had a 1971 Marshall tube amp land in my lap, for free. I'm not a guitar player, but wanted to get it fixed it up before either selling it or learning guitar. There's a lot of "magic" there - the amp guy asked if I wanted to swap the tubes for some "more authentic" tubes that were used in England at the time. Pro tip - don't ask the internet for advice for making your tube amp sound nice, you'll get every opinion possible.
- cluckindan 10mo ago
  You don’t even need to ask! Generally speaking, you don’t want a guitar amp to sound nice, you want it to sound good, good being a function of many things. For clean sound, use compatible radio preamp tubes and bias the power tubes conservatively. For distorted sound, use the lowest overhead preamp tubes you can find, and bias the power tubes as hot as you dare without them breaking within the hour. You can always change them after a gig, or between sets. :-)
- smitty1110 10mo ago
  Guitar amps are all about getting the right kind of harmonic distortion, so of course the guy had opinions. But tube rolling is madness, avoid it at all costs.
  - kazinator 10mo ago
    There is a whole range of useable distortions, so a lot of the opinions mainly boils down to genre bias. E.g. to a metalhead, any tone that doesn't "chug" is useless, including something useable to a jazz fusion player.
  - a_t48 10mo ago
    In this case it was a very common swap, 6550->EL34 for Marshall JMP 50W. I ended up going for it (the existing tubes were damaged), it sounds great.
- kragen 10mo ago
  Goodness, I hope you weren't injured.
Animats 10mo ago
That's a cute little article. The key diagram is the one that shows the signal path through the amplifier. Input feeds grid, plate feeds next grid, final output is from plate. Everything else is supporting circuitry. Note that between each stage there's a capacitor in the signal path. That's to block DC. If you want an amp that amplifies DC, each stage has to run at a higher voltage than the previous stage. The plate must be above the grid in voltage. This was a huge headache in tube computers, both analog and digital. Transistor circuits don't have the increasing voltage problem. Outputs and inputs are in the same voltage range. That's because transistors are current gain devices, not voltage gain devices.
- monocasa 10mo ago
  > Note that between each stage there's a capacitor in the signal path. That's to block DC. If you want an amp that amplifies DC, each stage has to run at a higher voltage than the previous stage. The plate must be above the grid in voltage. This was a huge headache in tube computers, both analog and digital. You can also stick a voltage divider (and probably some diode clamping) in there to pull the signal off of the plate down to a grid compatible voltage for the next stage if you're just doing digital computing. That was the most common setup I've seen in tube based computing. They tended to play pretty nice with the resistors needed for the plate current anyway so it wasn't that much extra RC constant delay.
  - HPsquared 10mo ago
    That won't help with the power consumption though, I guess. (Or is that a rounding error compared to everything else?)
    - monocasa 10mo ago
      It's not exactly what I'd call a rounding error, but it's manageable. But yeah, tube computing in general is an exercise in building a really fancy space heater. I'm trying to keep my tube computer I'm building down to ~3KW, and that's probably the biggest actual constraint on design complexity.
AdamH12113 10mo ago
Tangential question: Does anyone know of a basic large-signal equation for a triode (or any other vacuum tube type) like the simplified Ebers-Moll equation for BJTs or the square law equations for the linear and saturation regions of a MOSFET? It would really help my understanding, but whenever I google it I only see academic papers, like it's a weird thing to search for.
- creeble 10mo ago
  If I understand them correctly, Ebers-Moll equations are based on the exponential relationship between voltage and current in a BJT. But tubes aren't current amplifiers, they're voltage amplifiers, like FETs. You can look at the "characteristics curves" of tubes (plate curves and transconductance curves), which tell the story of current against plate-to-cathode voltages for fixed grid voltages.
- bsder 10mo ago
  The Koren equations are the only thing I've found. "Improved vacuum tube models for SPICE simulations" https://normankoren.com/Audio/Tubemodspice_article.html https://normankoren.com/Audio/Tubemodspice_article.html The intractability of the Triode is part of the reason why the Pentode exists. And, you will note, the Pentode curves in certain modes looks a lot like your bog standard MOSFET. This also discusses how the "constants" ... well, aren't. https://www.john-a-harper.com/tubes201/ https://www.john-a-harper.com/tubes201/
  - AdamH12113 10mo ago
    That’s exactly what I’m looking for! Thank you very much!
- analog31 10mo ago
  "All models are wrong, but some are useful." -- George Box With that said, a N type JFET is not a bad start. The main rules of thumb work: The grid draws negligible current. The tube will pass enough current from plate to cathode, to maintain a roughly constant cathode voltage above the grid.
- 10mo ago
creeble 10mo ago
Rob Robinette is a great guitar-amp resource; knows just about everything about Fender amps in particular. He has many mods to many common/not-so-common Fenders. Just his list of 5E3 mods (Fender Deluxe) is awesome: https://robrobinette.com/5e3_Modifications.htm https://robrobinette.com/5e3_Modifications.htm
JKCalhoun 10mo ago
Echoes of vacuum tubes in my memories: seeing tube testers in drug stores as a child (thinking they looked like either scientific equipment or else science-fiction props—and accidentally left just feet from the penny-candy), as well as peering into the back of our small B&W TV growing up (and marveling at the "city of light" inside there: all the orange glowing filaments from the tubes…). And gone by the time I was old enough to be interested in electronics. Nonetheless, my curiosity about them remained and I did eventually seek out books to understand how they worked. I have since built perhaps a dozen hi-fi stereo and mono-block tube amplifiers—some from kits, some from scratch. I've built a handful of guitar amps as well (even sold some as kits for a bit). Point to point, tagboard, PCBs… Anyone that likes to tinker in electronics I recommend they try their hand at at least one tube project (probably an amp of some kind).
- leptons 10mo ago
  >Anyone that likes to tinker in electronics I recommend they try their hand at at least one tube project (probably an amp of some kind). Only if they are aware of the voltages and current often associated with tube setups. One bad move can be painful, or fatal in some cases. I used to work on guitar amplifiers, doing modifications on tube amps. Messing around with the internals demanded my focus, a level of attention most "tinkerers" aren't likely ready for. Not trying to gatekeep here, just suggesting it may not be something for "anyone that likes to tinker".
  - marpstar 10mo ago
    This is what has kept me from working on amps. I'm comfortable working inside of pedals (9v) or with USB-powered circuits, but anything with a large transformer just seems a lot scarier.
    - JKCalhoun 10mo ago
      It depends on the "power" of the project. I was building ½ Watt Darling hi-fi amps that never had more than about 225 VDC or so running through them. But by all means, you never touch it when plugged in, you use chopsticks if you need to poke/debug a live circuit (you keep one hand in your pocket, etc.).
fumeux_fume 10mo ago
Fun vacuum tube history fact: the humble vacuum tube actually traces its origins back to Edison’s incandescent light bulbs. Those early bulbs would mysteriously blacken over time, and for years nobody could figure out why. It wasn’t until 1904 that John Ambrose Fleming connected the dots — the darkening came from metal burned off the filament, and in studying it, he created the first true vacuum tube. So the vacuum tube, the heart of early electronics, was born from the same simple light bulb that first lit our homes.
WorkerBee28474 10mo ago
I'd like to plug the YouTube Video by Jim Lill - "Tested: Where Does The Tone Come From In A Guitar Amplifier?" https://www.youtube.com/watch?v=wcBEOcPtlYk https://www.youtube.com/watch?v=wcBEOcPtlYk
- throwaway17_17 10mo ago
  I will second your recommendation and also recommend all of his ‘Tested’ videos. The microphone ‘Tested’ video was also an absolute delight. The delivery style gets to some people (i.e. “I’m not ___ I just play guitar…”) but i find it absolutely fine. I moved on from tube amps about 15 years ago and now really enjoy a variety of different solid state amplification stages with varying EQ and ‘dirt’ options at various places. Turns out a lot like were Jim’s Video goes.
- Anthony-G 10mo ago
  Thanks for that. I hadn’t come across Jim Lill before. For someone who’s “just a performer”, he knows a lot (about circuits). I found the comparisons of different order of Equalisation and Distortion to be interesting and I loved his Tacklebox. I’ll definitely check some of his other videos.
dmitrygr 10mo ago
> WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. I promise you it does not contain AC when unplugged :)
- bogomog 10mo ago
  But capacitors may hold high voltage DC, if there's no path to discharge them.
- kragen 10mo ago
  Nope, probably not at all, and certainly not for very long.
shermantanktop 10mo ago
I was a tube amp tech for several years, have built multiple guitar amps from scratch, and still dabble in it. What may not be obvious is that modern tube amp designs are an evolutionary branch from 1930's technology, with only a little coming across from the transistor->digital tech tree. The amps of the 40s and 50s were pretty closely based on reference designs that came from RCA and other tube manufacturers. Modern passive components (resistors, diodes and caps) are made to a far higher tolerance and are better understood, but tubes and transformers are a mixed bag. The older designs were somewhat overbuilt and can be more reliable or have tonal characteristics that are not available in modern parts.
- mickael-kerjean 10mo ago
  I always found fascinating the power section of valve amp for guitar will always be made of a very basic rectifier circuit to convert AC to DC that requires a expensive transformer and produce power with a terrible efficiency compared to more modern SMPS. Why is it nobody interested in valve amp never go the SMPS path? Is it all because sag is a desirable sound distortion?
  - theamk 10mo ago
    You mean a modern construction with semiconductor-based SMPS, but tube for output? If so, plenty of those, here is hit #2 in my google search for "guitar tube amplifier": "Orange Micro Terror" [0]. This one take 15V DC input - no way you can get this to tubes without some sort of SMPS. Or do you mean why people who do period-authentic tube amps don't use SMPS? That's because tube-based SMPS is very complex, often as complex as amplifier itself, and needs unusual parts [1]. [0] https://www.sweetwater.com/store/detail/MicroTerror--orange-micro-terror-20-watt-head https://www.sweetwater.com/store/detail/MicroTerror--orange-... [1] https://www.righto.com/2018/09/glowing-mercury-thyratrons-inside-1940s.html https://www.righto.com/2018/09/glowing-mercury-thyratrons-in...
  - dreamcompiler 10mo ago
    I noticed the same thing. Like, why not at least use a fullwave rectifier with semiconductor diodes? Surely nobody believes that a tube diode in the power supply makes any audible difference.
TrackerFF 10mo ago
Excellent website, I'm an electrical engineer by trade, and play guitar. Back in college tube amps were long, long gone for anything other than microwave engineering. My first real amp was a JCM800 2203 (technically a JMP "Mk 2 master model", which is just a cascaded JMP/Plexi, which Marshall then later re-released as JCM800 when their export deal expired...but I digress), and when I got into modding this website was my first real encounter with easy explained guides of the circuits.
skopje 10mo ago
very good article! i am surprised how such simple circuit was used in the 5f1!
disruptthelaw 10mo ago
When a webpage loads that looks like it was made in the 90’s, no ads, and is too wide for mobile so I have to drag from side to side, I know the content is gonna be legit.
- grayson-st 10mo ago
  [dead]
- tomasphan 10mo ago
  I got two ads that took up 2/3rds of the screen :(
  - kjs3 10mo ago
    Yeah...and I like how if you hit the 'hide ad' arrow, it veeeery slowly scrolls out of the way. Keep it classy. And don't put a 'donate' button and then pack the page with so many ads I have to use an adblocker[1] to actually view the content in a non-rage-inducing way. [1] yes, yes...I know...always use an ad blocker.
yardshop 10mo ago
A complementary resource for learning about tube amps is the YouTube channel Fazio Electric. Colleen Fazio does a nice job of repairing old amps and explaining various aspects of their construction, history, and significance. Plus she has a very calming voice and is probably one of the loveliest amp repair technicians out there. https://www.youtube.com/@FazioElectric https://www.youtube.com/@FazioElectric
- santix 10mo ago
  Also https://www.youtube.com/@UncleDoug https://www.youtube.com/@UncleDoug
platevoltage 10mo ago
I definitely came across this website back when I was learning how to fix and build tube amps. Glad to see nothing has changed.
TomMasz 10mo ago
Tubes are definitely inferior to transistors in a lot of ways, but nothing sounds like them (FETs come close). Plus, there's the comforting glow of those little glass bottles that just seems right.
- bogomog 10mo ago
  My favorite, kind of ironic, advantage of tubes over transistors is that it's easier to source replacements.
krbaccord94f 10mo ago
Tube amplifiers function using compression test or where voltage alternates into a unitary current. A reversal, which occurs in the vaccum chamber compresses electrodes, tagging battery terminal from the +/- amplifier schema AC electricity is transformed.
baylessj 10mo ago
Neat to see a Rob Robinette article pop up here, his website is a fantastic resource for guitar amp work. His articles on Amp Startup and Troubleshooting were particularly helpful when I built a tube amp for the first time. I hadn't heard of using an incandescent bulb as a current limiter in startup before, I was glad to have additional options for trying to make the troubleshooting process a bit safer given the high voltage involved.

157 points by gokhan 10mo ago

it's a bit weird that I never thought about it before this, when I already had the facts in my head: the triode tube amplifier was invented by Lee de Forest, but he had no idea how it worked or even what it was capable of. then 45 years later, the solid state transistor amplifier was invented, and they had no idea how it worked either. for people who have not had much EE education, what is important about triodes and transistors is that they amplify. you can put a signal in (a signal like from a microphone responding to your voice), and put some power in (like from a battery) and these amplifiers can make an output "copy" of the signal which is more powerful/"louder" than the original. from this basic function, everything that we think of as "electronic" flows. we would still have electric things like light bulbs, heaters, spark plugs, electromagnets, but basically just electric steam punk frankenstein machines, and nothing subtle. Amplifiers are termed "active" electronics; without them, we'd simply have passive electricity. I didn't read this article because I already know how these things work, and the article looks extremely confusing, and I've already read my fill of explanations that don't explain anything and (not saying this is one of those) I don't want to even risk that again. it is very difficult to find explanations for how transistors work that make any sense at all.

kazinator 10mo ago
> he solid state transistor amplifier was invented, and they had no idea how it worked either. That cannot possibly be true. Not knowing what exactly is going on with the charge carriers at the subatomic and quantum levels is not the same as not knowing how the amplifier works: like if we fiddle with the voltage at the base, we can influence the collector current, and all the rest. What is true is that some early transistor designs of audio amps treated transistors like tubes: they featured a phase inverter transistor that fed two non-complementary push-pull stages whose output was combined by a center-tapped output transformer. The excuse that well-matched complementary PNP transistors were not readily available at that time rings hollow, because it's possible to create an push-pull output stage with just NPN transistors. This is called "quasi complementary" (lots of search results for this). Output transformers, if they have multiple taps in the secondary winding, do allow for different impedances. If the end users expect to be able to plug a 16 ohm speaker into a 16 ohm output jack and a 4 ohm into 4 ohm, then they will understand that kind of amp better.
- kazinator 10mo ago
  Radio Shack sold PA amplifiers with an output transformer well past the age of the tube, like the MPA series, e.g. MPA-40, a 20 W mplifier. On that thing you can obtain the raw amplifier output using the "70V" terminal. Then it has a number of through-the-trafo outputs labelled with nominal ohmages of speakers. The Owner's manual extols the advantages of using transformers for speakers and describes how to use the 70V output in conjunction with external transformers. Quote: For complex multiple-speaker arrangements that require many speakers and long runs of connecting wire, we recommend you use a line transformer (not supplied), available at your local RadioShack store. [...] There are several advantages to using transformers. • You can connect speakers with different impedances without causing differences in output between the speakers. • You can add or remove a speaker without having to recalculate the entire system’s impedance. • You can reduce signal loss when you use speaker wire over 50 feet long. LOL!
  - quickthrowman 10mo ago
    Sound masking systems still use 70V audio output with output transformers at each speaker, voltage drop is rough when your signal is only a few volts and you’re using small conductors. Last time I sold a sound masking install we used 14/2 cable for the 70V audio signal. https://www.atlasied.com/speech-privacy-speakers?srsltid=AfmBOoqRvEImBr52QWhIt81FOIpvWaByhKti96msfyWgdkBl5ul_ghyV https://www.atlasied.com/speech-privacy-speakers?srsltid=Afm...
    - sethhochberg 10mo ago
      They're quite popular for distributed audio systems in general (of which sound masking is one type). "Constant voltage audio" comes in a few flavors and 70v is very common in the US, other parts of the world often use 100v. Background music systems in retail, voice paging systems, etc use constant voltage hardware because its much better technology for very long cable runs, daisy-chained speakers, and centrally located amplifiers. The cost is fidelity. Full-range audio transformers aren't cheap, so these systems usually make some compromises because your announcements or smooth jazz over the pasta aisle don't need to be true hi-fi. Its cool technology. Most of the speakers have variable power taps, so you can run a bunch of them in parallel on a single line and control the actual volume as-needed based on where the speaker is deployed by varying the transformer tap on each speaker.

redm 10mo ago

This is an interesting topic, but the ads overlaying the content make this very hard to read :( * Please don't suggest I install an Ad blocker.

Night_Thastus 10mo ago
Install an Ad blocker. I'd recommend uBlockOrigin on Firefox, or Firefox for mobile. I mean just in general it makes the web less awful. Webpages are so much easier on the eyes without all the crap they try to stuff in there. And it can prevent malware, especially for those less tech-inclined. And it means you use less data/bandwidth, since the blocker prevents the request from ever being made in the first place. If you want to support a site, just buy a subscription or donate to them or something.
loloquwowndueo 10mo ago
What ads? Install an ad blocker.
1970-01-01 10mo ago
Sorry, but adblock is a genuine quality of life hack for everyone online. Since you don't want to install anything, how about working at the DNS level and/or hosts level? https://adguard-dns.io/en/welcome.html https://adguard-dns.io/en/welcome.html will change your mind. https://github.com/Ultimate-Hosts-Blacklist https://github.com/Ultimate-Hosts-Blacklist is another option for doing it in the hosts file.
pestatije 10mo ago
id never do it, but you could install an ad blocker
metalman 10mo ago
Ok, I suggest that you install the "Stoutner, privacy browser", which treats seeing text, as the default, with the ability to allow other content as optional, though certain sites that are still hand coded, show, as they always have....adds and all.
kazinator 10mo ago

a_t48 10mo ago

I had a 1971 Marshall tube amp land in my lap, for free. I'm not a guitar player, but wanted to get it fixed it up before either selling it or learning guitar. There's a lot of "magic" there - the amp guy asked if I wanted to swap the tubes for some "more authentic" tubes that were used in England at the time. Pro tip - don't ask the internet for advice for making your tube amp sound nice, you'll get every opinion possible.

cluckindan 10mo ago
You don’t even need to ask! Generally speaking, you don’t want a guitar amp to sound nice, you want it to sound good, good being a function of many things. For clean sound, use compatible radio preamp tubes and bias the power tubes conservatively. For distorted sound, use the lowest overhead preamp tubes you can find, and bias the power tubes as hot as you dare without them breaking within the hour. You can always change them after a gig, or between sets. :-)
smitty1110 10mo ago
Guitar amps are all about getting the right kind of harmonic distortion, so of course the guy had opinions. But tube rolling is madness, avoid it at all costs.
- kazinator 10mo ago
  There is a whole range of useable distortions, so a lot of the opinions mainly boils down to genre bias. E.g. to a metalhead, any tone that doesn't "chug" is useless, including something useable to a jazz fusion player.
- a_t48 10mo ago
  In this case it was a very common swap, 6550->EL34 for Marshall JMP 50W. I ended up going for it (the existing tubes were damaged), it sounds great.
kragen 10mo ago
Goodness, I hope you weren't injured.

Animats 10mo ago

That's a cute little article. The key diagram is the one that shows the signal path through the amplifier. Input feeds grid, plate feeds next grid, final output is from plate. Everything else is supporting circuitry. Note that between each stage there's a capacitor in the signal path. That's to block DC. If you want an amp that amplifies DC, each stage has to run at a higher voltage than the previous stage. The plate must be above the grid in voltage. This was a huge headache in tube computers, both analog and digital. Transistor circuits don't have the increasing voltage problem. Outputs and inputs are in the same voltage range. That's because transistors are current gain devices, not voltage gain devices.

monocasa 10mo ago
> Note that between each stage there's a capacitor in the signal path. That's to block DC. If you want an amp that amplifies DC, each stage has to run at a higher voltage than the previous stage. The plate must be above the grid in voltage. This was a huge headache in tube computers, both analog and digital. You can also stick a voltage divider (and probably some diode clamping) in there to pull the signal off of the plate down to a grid compatible voltage for the next stage if you're just doing digital computing. That was the most common setup I've seen in tube based computing. They tended to play pretty nice with the resistors needed for the plate current anyway so it wasn't that much extra RC constant delay.
- HPsquared 10mo ago
  That won't help with the power consumption though, I guess. (Or is that a rounding error compared to everything else?)
  - monocasa 10mo ago
    It's not exactly what I'd call a rounding error, but it's manageable. But yeah, tube computing in general is an exercise in building a really fancy space heater. I'm trying to keep my tube computer I'm building down to ~3KW, and that's probably the biggest actual constraint on design complexity.

AdamH12113 10mo ago

Tangential question: Does anyone know of a basic large-signal equation for a triode (or any other vacuum tube type) like the simplified Ebers-Moll equation for BJTs or the square law equations for the linear and saturation regions of a MOSFET? It would really help my understanding, but whenever I google it I only see academic papers, like it's a weird thing to search for.

creeble 10mo ago
If I understand them correctly, Ebers-Moll equations are based on the exponential relationship between voltage and current in a BJT. But tubes aren't current amplifiers, they're voltage amplifiers, like FETs. You can look at the "characteristics curves" of tubes (plate curves and transconductance curves), which tell the story of current against plate-to-cathode voltages for fixed grid voltages.
bsder 10mo ago
The Koren equations are the only thing I've found. "Improved vacuum tube models for SPICE simulations" https://normankoren.com/Audio/Tubemodspice_article.html https://normankoren.com/Audio/Tubemodspice_article.html The intractability of the Triode is part of the reason why the Pentode exists. And, you will note, the Pentode curves in certain modes looks a lot like your bog standard MOSFET. This also discusses how the "constants" ... well, aren't. https://www.john-a-harper.com/tubes201/ https://www.john-a-harper.com/tubes201/
- AdamH12113 10mo ago
  That’s exactly what I’m looking for! Thank you very much!
analog31 10mo ago
"All models are wrong, but some are useful." -- George Box With that said, a N type JFET is not a bad start. The main rules of thumb work: The grid draws negligible current. The tube will pass enough current from plate to cathode, to maintain a roughly constant cathode voltage above the grid.
10mo ago

creeble 10mo ago

Rob Robinette is a great guitar-amp resource; knows just about everything about Fender amps in particular. He has many mods to many common/not-so-common Fenders. Just his list of 5E3 mods (Fender Deluxe) is awesome: https://robrobinette.com/5e3_Modifications.htm https://robrobinette.com/5e3_Modifications.htm

JKCalhoun 10mo ago

Echoes of vacuum tubes in my memories: seeing tube testers in drug stores as a child (thinking they looked like either scientific equipment or else science-fiction props—and accidentally left just feet from the penny-candy), as well as peering into the back of our small B&W TV growing up (and marveling at the "city of light" inside there: all the orange glowing filaments from the tubes…). And gone by the time I was old enough to be interested in electronics. Nonetheless, my curiosity about them remained and I did eventually seek out books to understand how they worked. I have since built perhaps a dozen hi-fi stereo and mono-block tube amplifiers—some from kits, some from scratch. I've built a handful of guitar amps as well (even sold some as kits for a bit). Point to point, tagboard, PCBs… Anyone that likes to tinker in electronics I recommend they try their hand at at least one tube project (probably an amp of some kind).

leptons 10mo ago
>Anyone that likes to tinker in electronics I recommend they try their hand at at least one tube project (probably an amp of some kind). Only if they are aware of the voltages and current often associated with tube setups. One bad move can be painful, or fatal in some cases. I used to work on guitar amplifiers, doing modifications on tube amps. Messing around with the internals demanded my focus, a level of attention most "tinkerers" aren't likely ready for. Not trying to gatekeep here, just suggesting it may not be something for "anyone that likes to tinker".
- marpstar 10mo ago
  This is what has kept me from working on amps. I'm comfortable working inside of pedals (9v) or with USB-powered circuits, but anything with a large transformer just seems a lot scarier.
  - JKCalhoun 10mo ago
    It depends on the "power" of the project. I was building ½ Watt Darling hi-fi amps that never had more than about 225 VDC or so running through them. But by all means, you never touch it when plugged in, you use chopsticks if you need to poke/debug a live circuit (you keep one hand in your pocket, etc.).

fumeux_fume 10mo ago

Fun vacuum tube history fact: the humble vacuum tube actually traces its origins back to Edison’s incandescent light bulbs. Those early bulbs would mysteriously blacken over time, and for years nobody could figure out why. It wasn’t until 1904 that John Ambrose Fleming connected the dots — the darkening came from metal burned off the filament, and in studying it, he created the first true vacuum tube. So the vacuum tube, the heart of early electronics, was born from the same simple light bulb that first lit our homes.

WorkerBee28474 10mo ago

I'd like to plug the YouTube Video by Jim Lill - "Tested: Where Does The Tone Come From In A Guitar Amplifier?" https://www.youtube.com/watch?v=wcBEOcPtlYk https://www.youtube.com/watch?v=wcBEOcPtlYk

throwaway17_17 10mo ago
I will second your recommendation and also recommend all of his ‘Tested’ videos. The microphone ‘Tested’ video was also an absolute delight. The delivery style gets to some people (i.e. “I’m not ___ I just play guitar…”) but i find it absolutely fine. I moved on from tube amps about 15 years ago and now really enjoy a variety of different solid state amplification stages with varying EQ and ‘dirt’ options at various places. Turns out a lot like were Jim’s Video goes.
Anthony-G 10mo ago
Thanks for that. I hadn’t come across Jim Lill before. For someone who’s “just a performer”, he knows a lot (about circuits). I found the comparisons of different order of Equalisation and Distortion to be interesting and I loved his Tacklebox. I’ll definitely check some of his other videos.

dmitrygr 10mo ago

> WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. I promise you it does not contain AC when unplugged :)

bogomog 10mo ago
But capacitors may hold high voltage DC, if there's no path to discharge them.
kragen 10mo ago
Nope, probably not at all, and certainly not for very long.

shermantanktop 10mo ago

I was a tube amp tech for several years, have built multiple guitar amps from scratch, and still dabble in it. What may not be obvious is that modern tube amp designs are an evolutionary branch from 1930's technology, with only a little coming across from the transistor->digital tech tree. The amps of the 40s and 50s were pretty closely based on reference designs that came from RCA and other tube manufacturers. Modern passive components (resistors, diodes and caps) are made to a far higher tolerance and are better understood, but tubes and transformers are a mixed bag. The older designs were somewhat overbuilt and can be more reliable or have tonal characteristics that are not available in modern parts.

mickael-kerjean 10mo ago
I always found fascinating the power section of valve amp for guitar will always be made of a very basic rectifier circuit to convert AC to DC that requires a expensive transformer and produce power with a terrible efficiency compared to more modern SMPS. Why is it nobody interested in valve amp never go the SMPS path? Is it all because sag is a desirable sound distortion?
- theamk 10mo ago
  You mean a modern construction with semiconductor-based SMPS, but tube for output? If so, plenty of those, here is hit #2 in my google search for "guitar tube amplifier": "Orange Micro Terror" [0]. This one take 15V DC input - no way you can get this to tubes without some sort of SMPS. Or do you mean why people who do period-authentic tube amps don't use SMPS? That's because tube-based SMPS is very complex, often as complex as amplifier itself, and needs unusual parts [1]. [0] https://www.sweetwater.com/store/detail/MicroTerror--orange-micro-terror-20-watt-head https://www.sweetwater.com/store/detail/MicroTerror--orange-... [1] https://www.righto.com/2018/09/glowing-mercury-thyratrons-inside-1940s.html https://www.righto.com/2018/09/glowing-mercury-thyratrons-in...
- dreamcompiler 10mo ago
  I noticed the same thing. Like, why not at least use a fullwave rectifier with semiconductor diodes? Surely nobody believes that a tube diode in the power supply makes any audible difference.

TrackerFF 10mo ago

Excellent website, I'm an electrical engineer by trade, and play guitar. Back in college tube amps were long, long gone for anything other than microwave engineering. My first real amp was a JCM800 2203 (technically a JMP "Mk 2 master model", which is just a cascaded JMP/Plexi, which Marshall then later re-released as JCM800 when their export deal expired...but I digress), and when I got into modding this website was my first real encounter with easy explained guides of the circuits.

skopje 10mo ago

very good article! i am surprised how such simple circuit was used in the 5f1!

disruptthelaw 10mo ago

When a webpage loads that looks like it was made in the 90’s, no ads, and is too wide for mobile so I have to drag from side to side, I know the content is gonna be legit.

grayson-st 10mo ago
[dead]
tomasphan 10mo ago
I got two ads that took up 2/3rds of the screen :(
- kjs3 10mo ago
  Yeah...and I like how if you hit the 'hide ad' arrow, it veeeery slowly scrolls out of the way. Keep it classy. And don't put a 'donate' button and then pack the page with so many ads I have to use an adblocker[1] to actually view the content in a non-rage-inducing way. [1] yes, yes...I know...always use an ad blocker.

yardshop 10mo ago

A complementary resource for learning about tube amps is the YouTube channel Fazio Electric. Colleen Fazio does a nice job of repairing old amps and explaining various aspects of their construction, history, and significance. Plus she has a very calming voice and is probably one of the loveliest amp repair technicians out there. https://www.youtube.com/@FazioElectric https://www.youtube.com/@FazioElectric

santix 10mo ago
Also https://www.youtube.com/@UncleDoug https://www.youtube.com/@UncleDoug

platevoltage 10mo ago

I definitely came across this website back when I was learning how to fix and build tube amps. Glad to see nothing has changed.

TomMasz 10mo ago

Tubes are definitely inferior to transistors in a lot of ways, but nothing sounds like them (FETs come close). Plus, there's the comforting glow of those little glass bottles that just seems right.

bogomog 10mo ago
My favorite, kind of ironic, advantage of tubes over transistors is that it's easier to source replacements.

krbaccord94f 10mo ago

Tube amplifiers function using compression test or where voltage alternates into a unitary current. A reversal, which occurs in the vaccum chamber compresses electrodes, tagging battery terminal from the +/- amplifier schema AC electricity is transformed.

baylessj 10mo ago

Neat to see a Rob Robinette article pop up here, his website is a fantastic resource for guitar amp work. His articles on Amp Startup and Troubleshooting were particularly helpful when I built a tube amp for the first time. I hadn't heard of using an incandescent bulb as a current limiter in startup before, I was glad to have additional options for trying to make the troubleshooting process a bit safer given the high voltage involved.