Tube plate voltages

[gui_tarzan]

So what do you think prompted the original designers to use such high voltage in tubes? Could they not have figured out a way to make something like that with say 150v instead of 350+?

[Alan0354]

You talking about small tubes or power tubes?

I feel I can hear more transparency with preamp tubes run at higher voltage, I have them running at least 230V to 250V. For power tubes, if you lower the voltage, you have to up the current to get the same power output. So that's a trade off. I did experiment on my Pro Reverb where everything remains the same except I have a switch to lower the +B by 40V for the whole amp ( suppressor grid adjusted accordingly to get the same cathode current of cause). Again, I get slightly more transparent sound with higher voltage. I even posted a video demo and people agreed.

[J M Fahey]

So what do you think prompted the original designers to use such high voltage in tubes? Could they not have figured out a way to make something like that with say 150v instead of 350+?

Why would they?

Tubes work better with 350V than with 160V .

By the same token, the Digital Guys worry about our +/- 15V Op Amp supplies and wonder aloud: why didn't they use +5V , which are easily available everywhere?

A DIYAudio forum every week there is somebody trying to interface a guitar to some Rabsperry Pi (whatever that is) or to an USB powered converter to send it to a Notebook or similar and they are very pissed off at having to build a new power supply, which of course is larger and heavier than the rest of the project, including case.

So much so, that Maxim makes a chip which gets +/-15V out of a +5V line.

[gui_tarzan]

You both are talking about tubes as they came to be, I'm asking if anyone knows or has thought about why they were designed to use such high voltage. Was it a limitation of materials? It doesn't seem like it would take much to get electrons moving, with heaters as an example.

[Alan0354]

I am going to guess by my limited knowledge of physics in this. You need electric field to accelerate the electron after it is out of the cathode. Electric field is in volt per meter( well inches etc.) The stronger the field, the faster the electrons move and more current flows. Now back to electric field, again it is V/m, this mean if the cathode is closer to the plate, you need less voltage to create the same field intensity. The farther the plate to cathode, you need higher voltage to create the same field. So the voltage needed is governed by the distance of the plate to cathode.

Now this is my wild guess, people cannot make the plate so close to the cathode cheaply in the older days, so with the cost of production in mind, they designed the tube in the most cost effective way and they need higher voltage to generate that much current.

Ha ha, I should know more because I worked with mass spectrometer all these years!!! But I never have an interest in knowing more when I had the chance to ask the experts that design ion guns, electron guns that are all about emission, focus, acceleration, deflection of ion and electrons. these are all the bits and scraps I over heard over the years.

[Mick Bailey]

There were plenty of low voltage tubes at one time; Early radios used tubes running with 90v plates and 2v DC heaters. I still have plenty of them NOS, pentodes, triodes and unusually, triodes with two control grids. The problem is when you come to get any real power from them.

Moving to a high plate voltage means more headroom and higher voltage swing, and when transformed, more power. 90v tubes were OK for tabletop radios, but not much more. The other problem with low-voltage tubes is linearity and it's difficult to design amplifiers that don't have excessive distortion.

Car radios also used low-voltage preamp tubes. So did hearing aids. These are specially designed space charge tubes intended for battery voltages. Even so, to get the necessary output from a car radio the output tubes are run off HT generated form a vibrating reed and step-up transformer.

[Alan0354]

Yes, I am sure if you pay more, you can get lower voltage tubes in the older days. The spacing has to be smaller. Also, like you said, current is limited if the voltage is too low and then distortion and other things come into play. But, what do I know about the history. I can only comment on the physics part of it.

There are circuits that run 12AX7 at 9V, but you run into very low input impedance that you have to buffer the guitar with opamps before driving the tube!!! There was a thread about a 40V guitar amp here also. But why?

[g1]

You both are talking about tubes as they came to be, I'm asking if anyone knows or has thought about why they were designed to use such high voltage.

To get equal power, running at lower voltage requires higher current. I suppose it is much easier to build a high voltage tube than a high current one.

[Mick Bailey]

One of the limitations is the tube impedance, which means that pulling the higher current necessary to compensate for lower voltage drops the output voltage right down. There are some tubes designed for high-current operation and lower plate voltage, such as the 6AS7G (originally designed as a regulator). Output transformerless designs sometimes use these and similar tubes to drive an 8 Ohm load directly at relatively low plate voltages.

[überfuzz]

Every now and then I see articles about micro tube, I believe I've even seen nano tube, in the amplifying sense. It's hard to tell from a text aimed at selling the idea of these new thingys... The guitar amplifier situation is often mentioned in these articles, how this new tube will revolutionize audio amplification. One thing often mentioned is the voltages/ampere levels. In these micro tubes the gap that electrons have to travel is much smaller, i.e. the electrons don't have to be accelerated as much in 'our' of crude tubes. Hey Presto! Voltage levels can be keep much lower.

Two things, first of all most of us had our youth messed up by dreams of amplifiers with freaking big tubes. We all have our favourite circuit etc... Even if these micro tubes could be used in a cool sounding amplifier they have to go up against our stubbornness and unwillingness to give them a chance. Secondly, I didn't find any of these articles when I googled around, a link or so would have been apropos.

[Mick Bailey]

Do microtubes exist outside of a lab? The idea of a deposited vacuum tube laid down on a substrate seems to be academically-based, as does the application of field emission. I wonder what the advantage would be over semiconductors? I doubt that fab plants would be set up to make guitar tubes - there would have to be a worthwhile global application.

[J M Fahey]

It doesn't seem like it would take much to get electrons moving, with heaters as an example.

There you have a conductive path .
To make it easier: metallic.

Now think that between plate and cathode you have vacuum, the most perfect insulator known to Man.

It's just our luck that:

a) when cathode is very hot, electrons leave the surface and form a cloud around it.

b) such electrons can be pulled from that cloud by an external electrostatic field and carry current themselves across vacuum.

For me, that it happens at all, looks like some kind of miracle.

[Alan0354]

Do microtubes exist outside of a lab? The idea of a deposited vacuum tube laid down on a substrate seems to be academically-based, as does the application of field emission. I wonder what the advantage would be over semiconductors? I doubt that fab plants would be set up to make guitar tubes - there would have to be a worthwhile global application.

It is very expensive to develop a process to make tube on a chip. I worked in Exar Corp designing analog IC, you need very expensive equipment to fab the IC. You have to have a big market before you can get you return of investment. Problem is tube application is almost non existing outside of guitar amps and high end audiophile amps. Hell, even transistors are going out of style. Even 9 years ago when I left the field, RF transistor are being replaced by IC, even the IC has only one transistor inside, they put in the bias and impedance matching network inside for ease of application. People avoid transistor at all cost already.

You go ask the engineers in the industry, I bet 99.9% don't know tubes. Here, we champion tube distortion for the organic sound. In real world, people want gain, linearity and efficiency. Tubes have lower gain, are quite lousy in linearity and you don't talk efficiency with tubes.

[soundmasterg]

Do microtubes exist outside of a lab? The idea of a deposited vacuum tube laid down on a substrate seems to be academically-based, as does the application of field emission. I wonder what the advantage would be over semiconductors? I doubt that fab plants would be set up to make guitar tubes - there would have to be a worthwhile global application.

The advantages are that they operate in a vacuum and electrons move quicker in a vacuum, so they have the potential for more gain than transistors. The other is they are much more immune to radiation so they would be helpful in space for NASA among others. The key is to make it to use existing production processes to keep the product cheap. Here are a couple articles talking about one potential solution.

Introducing the Vacuum Transistor: A Device Made of Nothing - IEEE Spectrum

The vacuum tube strikes back: NASA?s tiny 460GHz vacuum transistor that could one day replace silicon FETs | ExtremeTech

I don't know that they would be very helpful to guitar amps or have the sound we are looking for but they may end up being the next thing as MOSFET's don't have much left to shrink and still work and they are what has been driving all of our technological growth over the last 30 years....
Greg

[Mick Bailey]

Now think that between plate and cathode you have vacuum, the most perfect insulator known to Man.

The dielectric strength is on the low side compared to many other materials - PTFE, polyethylene and many others.