Why Gold Grids?
Sometimes we see valves with gold grids. I am thinking special low noise tubes.
I am thinking the gold grids are to limit secondary emissions.
See the data sheets for the 6AK5 for example.
Any Thoughts
Sometimes we see valves with gold grids. I am thinking special low noise tubes.
I am thinking the gold grids are to limit secondary emissions.
See the data sheets for the 6AK5 for example.
Any Thoughts
This is correct. Some old transmitting tubes had platinum grids. A few years ago when platinum was around $1700 an ounce those grids were worth $150 each. They don't use it any more since advanced metal processing techniques were developed.
Gold is a poor electron emitter when it gets hot? Hence particularly useful in valves where the grid is very near the cathode - typically high gain RF valves.
"A few years ago when platinum was around $1700 an ounce those grids were worth $150 each."
I noticed that a 6HB6 I dissected recently had a gold plated g1 grid. And at 20K gm, its got 125 closely spaced g1 wires like a frame grid. With the price of gold lately, maybe this is why some Chinese buyer bought up all 30,000 6HB6 tubes in this country? The tubes probably got crushed and de-plated.
I noticed that a 6HB6 I dissected recently had a gold plated g1 grid. And at 20K gm, its got 125 closely spaced g1 wires like a frame grid. With the price of gold lately, maybe this is why some Chinese buyer bought up all 30,000 6HB6 tubes in this country? The tubes probably got crushed and de-plated.
Sounds like an urban myth.
Do the math, folks.
A typical RF receiving tube has grid wire of the order 30 um radius, wound on a mandrel 5 mm diameter, around 30 turns. That's a wire 470 mm long having a surface area 89 x 10^-6 square meters.
A typical gold plating thickness is 10 um. So the volume on the wire is 89 x 10^-10 cubic meters. That will weigh 17 milligrams (density of gold 19300 kg/m^3) and at the current price of US$37/gram (source: http://goldprice.org/gold-price-per-gram.html), be worth 63 cents.
The going price of RF receiving tubes (NOS) on ebay is around $4 to $20 depending on type and condition.
Incidentally, the going rate for recovered platinum today is about the same as gold.
So anyone buying up old receiving tubes to get the grid wire gold would have to be a fool.
Kilowatt-size transmitting tubes are another story.
Do the math, folks.
A typical RF receiving tube has grid wire of the order 30 um radius, wound on a mandrel 5 mm diameter, around 30 turns. That's a wire 470 mm long having a surface area 89 x 10^-6 square meters.
A typical gold plating thickness is 10 um. So the volume on the wire is 89 x 10^-10 cubic meters. That will weigh 17 milligrams (density of gold 19300 kg/m^3) and at the current price of US$37/gram (source: http://goldprice.org/gold-price-per-gram.html), be worth 63 cents.
The going price of RF receiving tubes (NOS) on ebay is around $4 to $20 depending on type and condition.
Incidentally, the going rate for recovered platinum today is about the same as gold.
So anyone buying up old receiving tubes to get the grid wire gold would have to be a fool.
Kilowatt-size transmitting tubes are another story.
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Using your figures, but with 125 turns of wire (instead of 30) for the 6HB6 gives
$2.625 worth of gold in a 6HB6. Tubelab said the buyer paid $0.30 each for 30,000 tubes. 30,000 x (2.625 -0.30) => $ 69,750 profit. Those tubes got crushed.
I can think of several more tube types that will soon disappear.
$2.625 worth of gold in a 6HB6. Tubelab said the buyer paid $0.30 each for 30,000 tubes. 30,000 x (2.625 -0.30) => $ 69,750 profit. Those tubes got crushed.
I can think of several more tube types that will soon disappear.
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I still think its a myth.
Someone just might has sold 30,000 tubes to a buyer for $9000. The seller probably didn't want to muck around selling on eBay etc in dribs and drabs. But the buyer, instead of crushing and getting $70 K profit, could release them to the market in batches. Probably several hundred could be sold on eBay and elsewhere without collapsing the price. Each batch would return roughly $1000.
If someone had 30,000 tubes of one type sitting in a warehouse, there's bound to be small quantities sitting around elsewhere. So availability on eBay probably won't change much - currently about $4 each NOS 6BH6.
I've been buying up tubes over the last few years too. At a rate limitted by the budget I've set. And noticed certain types disappear from the market. But they were pretty rare types. Certain types favoured by audiophiles now attract truely rediculous prices of course. I used to be able to buy EL34's for about $8 thru trade channels, when they were $35 retail in music shops. Can't do that now though - every seller has woken up. The cheapest trade outlet is about $70.
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I had a back-and-forth via email with one of the tube engineers at New Sensor and he said that gold plating grids was done to reduce emission of the grid and further reduce grid current at negative voltages.
I was trying to get an idea of how much a KT88 grid could dissipate without damage in an AB2 design.
I think that the close spacing of grid to cathode in high gm power tubes like KT88 and EL34 need extra care in an AB2 design. I was told that the gold can vaporize if it gets too hot and stick to the cathode, which damages the cathode's ability to emit electrons. The grid in high gm tubes already has a tendency to run hot due to the close spacing to the hot cathode.
When tube designers aren't so concerned with minimizing a grid's emission and are more concerned with power dissipation (like with a screen grid) they will often be plated with a black coating to make them more efficient radiators.
I was trying to get an idea of how much a KT88 grid could dissipate without damage in an AB2 design.
I think that the close spacing of grid to cathode in high gm power tubes like KT88 and EL34 need extra care in an AB2 design. I was told that the gold can vaporize if it gets too hot and stick to the cathode, which damages the cathode's ability to emit electrons. The grid in high gm tubes already has a tendency to run hot due to the close spacing to the hot cathode.
When tube designers aren't so concerned with minimizing a grid's emission and are more concerned with power dissipation (like with a screen grid) they will often be plated with a black coating to make them more efficient radiators.
" The seller probably didn't want to muck around selling on eBay etc in dribs and drabs. But the buyer, instead of crushing and getting $70 K profit, could release them to the market in batches. Probably several hundred could be sold on eBay and elsewhere without collapsing the price. Each batch would return roughly $1000."
Only problem with that scenario is that there are maybe one dozen 6HB6 tube amps on the planet so far. If George (Tubelab) had managed to produce a DIY project using them, as he had planned, 6HB6s might have sold like hotcakes.
The Pete Millett/DCPP amp project, with George's power mods, did indeed manage to deplete the 6JN6, 6GV5, and 6HJ5/6HD5 tube supplies.
Only problem with that scenario is that there are maybe one dozen 6HB6 tube amps on the planet so far. If George (Tubelab) had managed to produce a DIY project using them, as he had planned, 6HB6s might have sold like hotcakes.
The Pete Millett/DCPP amp project, with George's power mods, did indeed manage to deplete the 6JN6, 6GV5, and 6HJ5/6HD5 tube supplies.
Hi,
And at positive grid voltages just the same.
It's still done nowadays especially with high gm RF valves, small signal 9 pinners or big emitters just the same.
Anything with "gold" in its nomen seems to empty pockets quite well it would seem......
Cheers, 😉
And at positive grid voltages just the same.
It's still done nowadays especially with high gm RF valves, small signal 9 pinners or big emitters just the same.
Anything with "gold" in its nomen seems to empty pockets quite well it would seem......
Cheers, 😉
That is in accordance with textbooks on tube design, eg the old RCA books downloadable on the tubebooks.org website.
Correct.
Don't where you got that from. A black coating on a screen won't help much as it "sees" the hot cathode, which is always the hottest part in a tube (apart from teh heater which is only verry slightly hotter). Black surfaces are not just good heat emitters, there are good heat absorbers.
In power tubes suchs as EL34, 6CM5, 6DQ6 etc, the heat caused by the screen grid current is conducted up the grid side support legs, which are made quite substantial, and radiated of attached sheet metal flags above the top mica. These flags are isolated from the ctahode radiation and "see" the glass, which runs cool, 150 C or less.
(Don't make the mistake in thinking all heat can radiate out through the glass. The glass used to make vacuum tube envelopes absorbs all but the shortest infrared wavelengths and the visible spectrum. 90% or more heat is abosorbed by the glass, which then gets rid of it by conduction into the surrounding air.)
Perhaps. Perhaps not.
The market for tubes for home built tube amps is very small. The market for tubes for restoration of old equipment is far larger.
Not only is there quite a bit of resortation of old radio and stereo equipment going on around the world, there's other things. I have some association with a group that maintains a museum of old TV station equipment - their consumption of tubes in a wide range of types is dozens per week.
There's another factor in this too. Smoking Amp said a 6BH6 (a sharp cutoff RF receiving pentode) has 125 turns of grid wire. Such a tube is likley to have grid wire much thinner than the typical example I used in calculation. Thus the value of gold will be much less than he calculated. More than the 63 cents I estimated for generic RF tubes, but not as high as SM's $2.65.
Are we all dyslexic?
Hello
Are we all dyslexic? Or is it just me?
Have you heard about the dyslexic devil worshipper? He sold his soul to Santa!
We started this discussion speaking about the gold grid in the 6BH6.
Now we are talking about a supply and demand model of the 6HB6.
I am confused!
DT
Hello
Are we all dyslexic? Or is it just me?
Have you heard about the dyslexic devil worshipper? He sold his soul to Santa!
We started this discussion speaking about the gold grid in the 6BH6.
Now we are talking about a supply and demand model of the 6HB6.
I am confused!
DT
My mistake.
Ok, 6HB6. Which is an American TV vertical output tube. That will have a lot more gold than an RF pentode (if indeed it has a gold plated control grid) but its market value on eBay is also much higher - about $10 instead of about $4.
So it still seems unlikely that anone would buy them just to crush them and get the gold. As a source of gold, maybe worth $2. As a NOS tube, worth $10.
What is the source of this rumour about a Chinese buyer anyway?
Ok, 6HB6. Which is an American TV vertical output tube. That will have a lot more gold than an RF pentode (if indeed it has a gold plated control grid) but its market value on eBay is also much higher - about $10 instead of about $4.
So it still seems unlikely that anone would buy them just to crush them and get the gold. As a source of gold, maybe worth $2. As a NOS tube, worth $10.
What is the source of this rumour about a Chinese buyer anyway?
"He sold his 6HB6 guts to Santa?"
So THAT's what I got in my Christmas stocking!
"I am confused!"
Some small talk can take on a life of its own.
George "Tubelab" mentioned a short while back that he had tried to buy some 6HB6s from the two large stock tube sellers in Florida, and they told him some Chinese buyers had bought up the entire supply, something like 30,000 pieces.
Then we determined they had big gold grids. From there on, pure speculation as to what happened. But likely driven by economics in some way. The tubes could be going into the next generation of cheap Chinese tube amps, or they could be scrap yard fill.
So THAT's what I got in my Christmas stocking!
"I am confused!"
Some small talk can take on a life of its own.
George "Tubelab" mentioned a short while back that he had tried to buy some 6HB6s from the two large stock tube sellers in Florida, and they told him some Chinese buyers had bought up the entire supply, something like 30,000 pieces.
Then we determined they had big gold grids. From there on, pure speculation as to what happened. But likely driven by economics in some way. The tubes could be going into the next generation of cheap Chinese tube amps, or they could be scrap yard fill.
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That was the first idea crossing my mind also, but work function of gold, about 5.1 eV is similar to many other metals, except perhaps for platinum, 6.35 eV
Work Functions for Photoelectric Effect
The grid absorbs a high proportion of the heat radiated by the cathode.
It also intercepts the electron beam, converting part of its kinetic energy into heat.
To prevent grid emission must be ensured high electron affinity.
Here gold run with advantage, about 2.3 eV vs about 2.1 eV for platinum.
In a nutshell, gold tends to absorb less heat from electron beam.
Another advantage is that gold (coated) grids are less prone to contamination in valves with oxide cathodes.
Keit, you are wrong. I just looked at KT88s, KT66s, EL34s, 6L6s, and not a single tube had radiating fins on the screen grid rods. They only had them on the control grid. I have dissected many beam tubes and it is common for them to have blackened screen grids, although there are plenty that do not.
The screen radiates heat to the plate when it gets hotter than the plate. That is the only place for the heat to go except for the small amount that conducts out of the pin through the lead which is often, even in a power tube, not any more substantial than any of the other leads.
You are right that it picks up more heat from the cathode if blackened, but it still has to radiate the heat to the plate to get rid of it (there is nowhere else for it to go) so a blackened coating will make that more efficient.
I'm not saying all screen grids are blackened, but you will notice if you dissect tubes that some of them are, and they are typically in places where you would expect high dissipations in normal use of that electrode.
The tube engineer indicated that a tube designed for positive voltage operation of the control grid might not be gold plated because the gold has a tendency to vaporize and poison the cathode if the control grid is over-dissipated. I'm not saying that a tube designed for class *2 operation necessarily would *never* have a gold plated grid, but that if you were designing a tube for positive grid operation, there are legitimate reasons for considering some other finish than gold.
ETA: And if you were designing a tube for *2 operation, you might assume that the grid circuit has a low DC resistance to the bias voltage anyway, so it might not be as critical to get grid current as close to zero as possible.
Second Edit: I just looked inside an 841 (designed for positive grid voltages and grid current) and the grid wires are plainly visible from outside the glass and are a very silver color so are obviously not gold plated.
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You are right in saying that the screen can only radiate heat to the anode. The trouble is, in a power tube, the anode is not all that cool. Whereas the cathode runs at 1050 K, the anode may only be a little below red heat - around 750 K. With an anode at 750 K and a nearby cathode at 1050 K, the grids must, even with zero dissipation, be well above 750 K.
I've rechecked the power tubes I have in my stock - yep, they have flags on the screengrid legs. And the legs are substantial - about 1.5 mm diameter.
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