Cooltron - low voltage tube circuits

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AMZ-FX

Well-known member
Joined
Jun 3, 2004
Messages
304
Location
south Louisiana
True tube performance at low voltage and current? Hype or innovative?

Let me quote from the Vox site:

"Here?s what makes the COOLTRON system tick. The COOLTRON circuit basically operates by using two very special signal paths. The first is the ?servo circuit,? which provides the right conditions between the plate of the tube and the grid. This achieves controllable, stable parameters for the tube and provides the correct operating conditions for the tube to function as it would if run at a higher voltage. The second circuit is a patented power supply that provides a low voltage, low current supply to the heater elements in the tube. Since the tube is now running at such a reduced supply level, the anode current is much smaller than normal. This means that the amount of heat required at the cathode to achieve sufficient cathode current emission is much smaller ? hence the ability to run the heaters at a lower level and for COOLTRON pedals to run for 20 hours on 4 x AA batteries!"

http://www.voxamps.co.uk/products/pedals/cooltron.htm

I think I'll hunt for the patent...

regards, Jack
 
No impossible magic here.

There were battery-radio tubes that ran very well. Even though they were actually designed by battery companies to sell more batteries! 20 hours life on four AA cells is not odd.

Tubes will work at very very low voltage. They pass very very very low current, but we have chips to bring that back up to interface levels.

What you really lose is dynamic range, but they could be getting 60dB which is enough for a guitar effect.

Not sure what the "servo" is. But tubes may be hard to bias, in production, at superlow voltages where bandgap voltages are non-negligible.

As they imply: the specified heater power covers the maximum current the tube can pass, like 20mA for 12AU7. Here they may be running more like 20 microAmps, and don't need full heat. So heater power can be reduced. (The ideal thing would be to design a smaller more efficient heater-cathode system, but nobody is designing new tubes today.)
 
[quote author="PRR"]nobody is designing new tubes today.)[/quote]
What are you looking ?
Tubes are still developed, and literature about it printed
(what do you mean about MSDC klystrode with cool cathode ?,
I read some book about it from 2002)

In prague, there is one factory which develops tubes:

http://www.electron-tubes.cz/

And what about Varian Brothers in america? Are they still living?

xvlk
 
PRR must have meant receiving types.

Of course, high-power, high-frequency transmitting types are still being developed. One recent development in TV transmitting tubes is the MSDC-IOT (multiple-stage depressed-collector inductive output tube). This was all the rage a few years ago when I was still semi-involved with transmitters and still paid attention to this stuff. Nowadays, thank God, I don't have to.

IOT amplifiers

depressed-collector IOT
 
What you really lose is dynamic range, but they could be getting 60dB which is enough for a guitar effect.
Also when you get down in the low voltage range it is not very linear, but that's not always a bad thing with guitar effects :grin:

-Jack
 
[quote author="NewYorkDave"]PRR must have meant receiving types.
[/quote]
CRT is dead...
Is ITRON, indicating tube, used in every HiFi CD player or radio.
"receiving" tube? It must be receiving, it receives informations from RDS.

But what are really plasma displays??? I not believe, that it is
plasma-arc-tube. Is it somewhat like itron, where gas is used to
enlarges "emission"??? That contrasts can not be archieved by arc itself???
:::And they are cheep, big and nice.
Have anybody info about it. Supermarkets are full of it and I know nothing about it :-( , :-(, :-(

xvlk
 
From what I understand, the Cooltron is a 12AX7 run on 15vdc. Nothing fancy or esoteric--no 6GM8 et al. I don't know this first hand--it was told to me by an electronics technician friend.
 
The vox ad has a 12au7 listed . What I don't understand is the use of low voltage/current on the fil often tubes get real noisy when they are underheated. I would guess it is a constant current supply to the fil. Maybe they run the tube fil hot to form the oxide and then install it in the effect I wonder if the effect has noise problem with time.

Underheating a tube is not new the u47 did it.
 
What I don't understand is the use of low voltage/current on the fil often tubes get real noisy when they are underheated.
Oddly, the same person who told me that the tube is run on 15vdc also told me that the Vox Cooltron amp is "noisy as shit."
 
In the patent that I listed, the input goes to the plate and the output is taken from the grid. It has 2.5v on the heater and quite low voltages on the other elements... the patent claims a gain of 3.5x, which means there must be some other gain stage driving the tubes in the Cooltrons.

-Jack
 
> Could this be the Cooltron patent?

I have no idea if it relates to VOX Cooltron.

But it is interesting in its own right. And you can NOT read it by drawing on conventional tube work.

20040257158.gif


The "Plate" electrode is the input, is held at a small positive DC bias, seems to draw no current.

The "Grid" electrode is the output, draws about 23 microAmps from a +5VDC source through a 100K resistor.

Inverted (plate in grid out) is well-known, but this is not classic inverted-mode. IM with normal bias and heat voltages gives a gain of like 1/Mu, or say 1/20 for 12AU7. The patent text claims gain of 3.5. Also, classic IM is inverting: patent text says non-inverting.

Note that this is NOT an amplifier. Power gain is less than one. Signal output power is less than signal input power. Input is 0.07VRMS in 2K2 or 0.002mW or -26dBm; output is 0.25VRMS in 100K which is 0.000625mW or -32dBm. Power "gain" is a 6dB loss: a 100K resistor does the same thing, better, cheaper; a wire jumper works even "better". It is a flavor tool, not an amplifier. (That's cool: a TL071 gives power-gain of +40dB for a few pennies: power-gain is no longer the holy-grail of electronics.)

I would expect 12AU7, at normal heat, to flow Grid-Cathode about like a 1K to 5K resistor. Here it is clearly flowing like a little more than 100K. That makes sense: as the heater power and cathode temperature is reduced, the cathode-grid space works like a "worse conductor", obviously going to a non-conductor at low temperatures (like room temp).

A 12AX7, at normal heater voltage, will flow cathode current at zero plate-cathode current. You can also see this in the plots for small vacuum diodes. The electrons, at normal heat, boil off the cathode with sufficient energy to give a few electron-volts. But that is after they have passed through a voltage field drop at the cathode surface: a part of an electron-volt is needed for an electron to leave the cathode.

At 300V, at 50V, even at 15V, these about-one-Volt contact potentials are nearly negligible. But with 0.78V and 2.65V on the electrodes, they are not negligible. And everything we know about these effects is from "normal" heat, where electrons are hopping off the cathode. We have some experience running gear at 80% of nominal heater voltage; here the heat is less than 50% of normal. Emission is a power-function of temperature, so half the temperature is very-very-very low emission.

Here's what I think. The cathode is so cold that electrons don't really want to leave. The +0.78V and +2.65V combined is just enough to make a few hop off and flow to the grid. Raising the 0.78V plate voltage a bit makes more electrons flow to the plate, fewer to the grid, giving non-inverting operation.

> "20 hours on 4 x AA batteries!"

IF they only wire one of the two heaters in a 12AU7 (or use a single-triode), that claim is not preposterous, assuming they have a high-efficiency way to drop 6V-4V to a solid 2.5V.
 
The cathode is so cold that electrons don't really want to leave. The +0.78V and +2.65V combined is just enough to make a few hop off and flow to the grid.

So much for tube "warmth," eh? :wink:
 
> So much for tube "warmth," eh?

I really-really-really hate to admit this, but if the patent text is true, they have found Something New. Pretty nearly the first really-new thing in small amplifiers (or nearly-amplifiers) since the transistor. Or the tunnel diode.

Part of the karma of the thermionic amplifier, as normally used in audio, is the hot rich electron cloud. At these heater voltages, operation must be very different. In fact not too unlike semiconductor junctions, though at a very different scale. The apparent current-steering mode is also novel among audio amps.

Off-hand, I expect it may act like "something else", not like a 12AU7 or any other tube working in "normal" circuits.

The tube will have a little warmth to the touch, but you may have to turn out all the lights to see the heater glow.

My rough estimate of dynamic range from the cited signal output 0.25V 100K is about 80dB. It would be optimistic to hope for 10dB higher output; the text suggests there is more but "rounded". The noise level could be 80dB down from 0.25V, or a little better, or a lot worse. The electron cloud action plays a part in tube shot noise; lack of a rich cloud might increase the noise. Of course 80dB is more than a guitar should need; but as a general-purpose color-tool it may be noisy. On paper, paralleling a bunch of hyper-starved tubes reduces noise, but with this wacky mode the simple assumptions may turn complex and bite us. (Anyway another 20dB of noise needs 100 tubes.....)

If you build it: use their resistors and the +5V grid supply, take the plate a little positive like they did, and turn-down the heat so the grid sits near half the supply voltage. Pass audio, remembering that it will overload at disgustingly low voltage, and remembering that in is out and out is in. You may have to iteratively trim the heater and plate voltages for "best" linearity.
 
[quote author="AMZ-FX"]Could this be the Cooltron patent?

http://tinyurl.com/bzr95

-Jack[/quote]

Sorry, that link doesn't work here, I get something like this:
Optical disk and optical disk recording and reproducing device
What could be wrong ? As it seems others were able to read the correct article.

Thanks,

Peter
 
[quote author="clintrubber"]
Sorry, that link doesn't work here, I get something like this:
Optical disk and optical disk recording and reproducing device [/quote]
It worked when I tested it... try this one:

http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=8&p=1&f=G&l=50&d=PG01&S1=mieda&OS=mieda&RS=mieda

EDIT: wait a minute.... now that one is not working either! I'll get the printed copy that I have at the office and work out a new link for it and post it later today.

-Jack
 
[quote author="PRR"]
20040257158.gif

[/quote]

There are non standard modes of electron tubes.
One of them is electrometric tetrode.
Have EF80, heat it to +3 ... +5 V.
connect +12V to G1 via some 1k resistor,
signal to G2 (grounded via 100k)
and connect some 15k between A+G3 and +12 V.
and you will have relative good amp.

Not everywhere G1 must be on minus potencial and anode on
+ 100 volts.

xvlk
 
[quote author="AMZ-FX"][quote author="clintrubber"]
Sorry, that link doesn't work here, I get something like this:
Optical disk and optical disk recording and reproducing device [/quote]
It worked when I tested it... try this one:

http://appft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&u=%2Fnetahtml%2FPTO%2Fsearch-adv.html&r=8&p=1&f=G&l=50&d=PG01&S1=mieda&OS=mieda&RS=mieda

EDIT: wait a minute.... now that one is not working either! I'll get the printed copy that I have at the office and work out a new link for it and post it later today.

-Jack[/quote]

Just post the patent number, pls.
 

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