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How Tube Amplifiers Work

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Re: How Tube Amplifiers Work

#21

Post by creeble »

AdamH12113 wrote: post #19

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.

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.

Re: How Tube Amplifiers Work

#22

Post by JKCalhoun »

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).

Re: How Tube Amplifiers Work

#23

Post by kazinator »

smitty1110 wrote: post #17
a_t48 wrote: post #6

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 po…

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.

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.

Re: How Tube Amplifiers Work

#24

Post by fumeux_fume »

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.

Re: How Tube Amplifiers Work

#26

Post by bsder »

AdamH12113 wrote: post #19

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.

The Koren equations are the only thing I've found.

"Improved vacuum tube models for SPICE simulations" 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/

Re: How Tube Amplifiers Work

#27

Post by fsckboy »

kazinator wrote: post #5
fsckboy wrote: post #2

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…

> 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 tra…

>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

since everything that happens inside a transistor is exactly what is going on in a quantum sense, you've described "not knowing how it works". You cannot understand a bipolar transistor without quantum effects, it's the thing that creates the transistor effect.

the theory of amplifiers you go on to talk about was well developed at that time because it's the same theory for vacuum tubes.

Re: How Tube Amplifiers Work

#28

Post by metalman »

redm wrote: post #3

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.

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.

Re: How Tube Amplifiers Work

#29

Post by analog31 »

AdamH12113 wrote: post #19

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.

"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.

Re: How Tube Amplifiers Work

#30

Post by kazinator »

fsckboy wrote: post #27
kazinator wrote: post #5

Earlier quoted context omitted.

> 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 tra…

> 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 since everything that happens inside a transistor is exactly what is going on in a quantum sense, you've described "not knowing how it works". You cannot understand a bipolar transistor without quantum effects, it's the thing that crea…

You can empirically drive the equations that apparently govern the macroscopic behaviors, right down to details like temperature sensitivity, and the Early effect, without having a detailed model of what is going on at the atomic and subatomic level. Then what makes an amplifier work is explained by those equations. And for that not even the full detail of them is necessarily required, depending on what aspect of the amplifier we need to explain. Like basic operation versus concern for thermal runaway.

What makes the amplifier work and what makes the transistor work are separate concepts.

That's why understanding translates from tube circuits to transistors. A transistor circuit maybe an emitter follower, which has a counterpart in tube circuits known as the cathode follower. The cathode resistor creates local negative feedback similarly to an emitter resistor. Early op amps where tube circuits. They have the same differential input stage and the same basic theory of operation. You program their game the same way with resistors. The familiar Sallen-Key filter topology was first described with the help of tube circuits for reference, back in 1955. To undestand it, we don't even need the details like how amplifiers work at the component level except when we get into design parameters in which certain issues matter, like frequency-bandwidth product, or input offset current or whatever.

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