NOS Metal Can 6V6 Tube Quesiton

[LJay942]

So I picked up some NOS 6V6 metal can tubes for my 5F1 (RCA and Tung Sol) from a swap meet recently.

Plugged them in checked the voltages which all checked out.

They sound really good

My question is how hot should they get? They get significantly hotter than any glass 6V6 I've ever used. After playing for about 15-20 minutes you could smell them. (That familiar warm electronics smell)

After I began to smell this I turned the amp around and placed my hand at the back could feel the heat so to speak which was significantly warmer than a glass tube.

I then performed the "wet finger test" in which I licked my finger and touched the tube and could hear the saliva sizzle.

From this rough description does this sound OK. They are mighty warm.

Thanks,

[Chuck H]

The only way to know if they're too hot is to check how much power they're dissapating and compare that to this particular tubes specifications. Metal is a much better conductor than glass. I might expect more heat transfer to your finger because of that. But I've never used metal tubes so I can't say if a sizzle is normal. I wouldn't think so. It's possible that the old metal tubes are more stringent to the actual specifications than the glass tubes. In this case it's entirely possible that the tube is overdissapating. Of course, you don't have the ability to see the tell tale red glow on the plates. So... Checking the actual dissapation, and knowing the actual specs of that tube, is the only way to know if the tube is over dissapating. All this assumes your amp allows for at least typical vent and air flow around the tube.

[eschertron]

My 2CW:
Since metal is opaque, all the heat transfer is by conduction through the metal can, there is no path for radiant IR. It has to all be converted at the can boundary. How much radiant IR escapes the common glass envelope? I don't know, but my guess is more than zero... Sorry, that's no definitive answer, but It does help explain the temperature difference.

[boots]

I think it is normal for the metal tubes to run hotter than their glass counterparts. You can actually smell the paint getting hot when they are new-ish, but that smell goes away after a while. I have been running a pair of metal RCA 6v6's in a 5E3 clone for a number of months, and to my ears, they sound every bit as good as any other RCA 6v6GT. Like Chuck H said, check to make sure they are running within spec, and make sure you have good ventilation around the tubes. Then crank 'em hard and enjoy them!

[Jazz P Bass]

Filament Temperature

Since the initial post, I keep coming back to why the OP would grab hold of a tube.
And then compare the sizzle ability of spit. (100 degrees C)
Here is a snippet I made off of Google books.http://books.google.com/books?id=FdS...erature&f=true

[deci belle]

I just started using a pair of 6V6y (metal jacket) last week in a 5E3-thing and that paint smoking really got my attention. They sound very good to my ears though.

I measured the temperature (forgot what the reading was), but it wasn't that hot. I have to t admit that initially, I was kinda freaked about how hot they seemed (and smelled). I believe the steel cans aren't so good at dissipating as much as radiating heat.

Also, since I have my Deluxe's pwr-tube sockets wired like a bigger blackface (with the resistor between pins 1+5), and the metal-clad 6v6's pin 1 is wired to the steel jacket, I snipped off the tube's pin 1. Does anyone else bother to do that?

[boots]

Hi Deci,
The smell should go away after a while. Yes, you sure don't want any voltage on the metal shell. Seems a shame to snip a pin off the tube, but at least that way you can use them safely. I actually switched back to 6V6GT's in my 5E3 thing, mostly cuz they look cooler, but I think the metal can tubes sounded every bit as good.

[deci belle]

Ya, the smell went away, boots!

[Mick Bailey]

The internal structures of the tube emit a fair amount of infra-red. With a glass tube this mainly passes through the envelope to heat the surrounding air and nearby components, though an amount is absorbed by the glass and where the gettering coats the surface. Conduction is mainly via the terminals passing through the glass seals in the tube base. However, conduction into the glass envelope here is minimal and localised due to the poor thermal conductivity of glass.

A metal tube absorbs nearly all of the internally radiated infra-red energy, and therefore becomes much hotter than an equivalent glass tube.

[Steve Conner]

Most glass tubes were specified for a maximum envelope temperature of 250C. (About 400F?) If we assume that the metal ones get about as hot, that's plenty enough to sizzle spit.

If Mick's argument is correct, the metal tubes would get hotter for the same rated plate dissipation, so it's no wonder they smell of burning paint for a while.

[Chuck H]

It may have even said, on the original packaging, something about the tubes smelling funny for awhile as they burn in. With no agency in place at that time to police toxic fumes Young, industrial nations do A LOT of that sort of thing.

[Mick Bailey]

Metal tubes are generally rated lower than their glass counterparts, possibly due to temperature limits imposed on the metal shell. They also have to maintain the integrity of the glass seals in the base, which have more metal/glass area of contact than a glass tube.

The really interesting tubes are ones where the anode is formed by the metal tube itself. Some were enclosed in a perforated and insulated sleeve, but others were just exposed.

[deci belle]

It may have even said, on the original packaging, something about the tubes smelling funny for awhile as they burn in. With no agency in place at that time to police toxic fumes Young, industrial nations do A LOT of that sort of thing.

Weeeeeeeeeee!!!

[loudthud]

Also, since I have my Deluxe's pwr-tube sockets wired like a bigger blackface (with the resistor between pins 1+5), and the metal-clad 6v6's pin 1 is wired to the steel jacket, I snipped off the tube's pin 1. Does anyone else bother to do that?

This is a serious safety issue having the grid circuit connected to the outside of the tube. It also applies to metal 6L6 tubes. This is why you will see many of the old Fender layouts connect pin 1 to ground. If you have grounded tube retainers or the bear trap type claws, they could short the grid circuit to ground. Not a problem in cathode biased amps but with fixed bias it will smoke or blow a fuse.

[Mick Bailey]

The tube is designed to have the metal shell at 0v - the original use for pin 1 on an Octal socket, and the reason it got freed up when tubes went to glass envelopes. This was both a screening and a safety precaution.

Edit; Should clarify - the connection to the shell was both a screening and safety precaution.