Tube terminology - please explain

[birddog01]

I have been watching for a while for a tube recommended to put in my arsenal for a while. An RCA nos black plate 5751, some of these can be pretty pricey and may be out of reality. Some may not however I see things like 3 mica and 2 mica, other things like D Getter, O Getter and Square Getter.

Can someone in the know explain these things in simple to understand terms?
And also what would be the minimum tested rating to buy with confidence.

Thanks

[muchxs]

Originally Posted by birddog01

I have been watching for a while for a tube recommended to put in my arsenal for a while. An RCA nos black plate 5751, some of these can be pretty pricey and may be out of reality. Some may not however I see things like 3 mica and 2 mica, other things like D Getter, O Getter and Square Getter.

Can someone in the know explain these things in simple to understand terms?
And also what would be the minimum tested rating to buy with confidence.

Thanks

The fact that a 5751 has a numerical type delineation rather than alpha-numeric indicates it's a military / industrial tube.



Let's look at a consumer type 12AX7

"12" is the filament voltage. But we run 'em on 6.3 volts! That's because the filament is center tapped. Cool!

Obviously there were a bunch of tube types with 12A nomenclature. 12AU7, 12AT7, 12AV7, 12AY7, 12AX7, 12AZ7. Same pinout and very roughly functionally similar.

"7" means there are two triodes in the tube plus a filament. Seven elements.


Military tubes got numerical designations. Maybe to prevent the evil Russkies from bootlegging our technology, who knows? Turns out they still make tubes and we don't. Go figure!

Anyway, the "getter" is a halo, disc or "D" shaped element that high voltage is applied to during the manufacturing process. It sacrifices a bit of itself and electro-plates any gas left in the envelope to the shell of the tube as a metal oxide. Remove the vacuum from the tube by cracking the shell, the metal oxide turns white as metal oxide tends to do when exposed to the atmosphere.

Anyway, once the getter does its job it's just along for the ride and to entertain audiophools who obsess over such things.

Mica: Tubes have die cut mica discs to seperate and support the elements. Look up mica, it's one of those ancient materials that's mined in sheets and has been used for all sorts of things over the years. Anyway, tubes and rocket science go hand it hand. Tubes were and likely are used in missile guidance systems. Tubes are better suited to function in a nuclear war zone than solid state devices but we won't go into that. Add extra mica discs, a tube is better able to cope with the massive forces of an ICBM launch. Again, it's a feature that provides endless hours of lively discussion and fascination for audiophools. All you need to know is that any old 5751 will work just fine in your guitar amp unless you drop it down a flight of stairs. If you're clumsy and accident prone (which begs the question how do you fumble through playin' your guitar? ) I can build you an amp you can drop down a flight of stairs. "Obsolete" missile guidanace tubes are the key ingredient.

Upscale tubes designed for ICBM guidance substitute burly ceramic discs for mica. Elements in such tubes are generally rugged-ized and stout, for instance the glass shell becomes more like a baby food jar. You can give 'em the ol' Three Foot Drop Test. They just bounce.

Support rods: Don't forget the support rods! Tube elements are self supporting and therefore somewhat fragile in consumer tubes. The assumption is you won't whack it hard enough to break the glass. The military likes things you can whack and that will still function when you whack them. "Grace under fire" is more about gettin' back to your own lines in one piece rather than audiophool treatment of your gear. You should keep yer M4 "audiophile" clean but I digress...

Support rods are little pieces of wire that pass through the mica and lend additional support to the internal tube elements. Thus, "support rods".

[celeste]

Muchxs, you forgot the use of borosilicate glass in place of soda lime glass.

Birddog, a tubes designation is about how it preforms, not how it is constructed, so over the years the same manufacturer may change the guts of a tube for supply or production reasons. In hyper critical situations, like HiFi, those differences can be heard by some people. If you are recording solo or with a small group, those differences may make a difference to you as well, but live or processed and the differences become inconsequential

[Keyser Soze]

For the purposes of tube instrument amplifiers those characteristics - type of getter, number and type of spacers, etc. - serve as visible methods of distinguishing tubes that are otherwise of the same type.

Whether those tubes actually perform differently (which may or may not be reality, and may or may not be dependent on the given application) has little if anything to do with those actual characteristics, but may instead depend upon other (non, or less-visible) construction characteristics.

[firemedic]

I'll go ahead & say it: spend $10 and get a Sovtek or whatever 5751. Unless you have won the lottery and can't think of what to buy after getting a solid gold rocket car.

I have lots & lots of NOS preamp tubes and switch them out indiscriminately with new production: the tone is not affected at all. Except: 12AX7EHs are a little brighter than I like.

[Hiker]

If you're ready to buy, contact Doug at Doug's Tubes. His prices are very competitive. I'd hate for anyone to waste his time.

If you'd like to contact him, let his know the amp you have, and the music that you prefer to play through it. He's my tube guy!

[hackworth1]

Found on internet:

This is the "milspec" version of the 12AX7, but is also slightly different. The 5751 has the same plate resistance as the 12AX7, but the gain factor is only 70 compared to 100 for the 12AX7. The 12AT7 also has a gain factor of 70, but since the plate resistance is different, the 5751 is closer to the 12AX7. The older "real" 5751's have extra thick mica spacers and extra support rods to minimize microphonic response. 5751's also have matched triode units. Late versions produced by Philips/ECG don't have the extra support and don't perform as well as the older 5751's.

[muchxs]

Originally Posted by firemedic

I'll go ahead & say it: spend $10 and get a Sovtek or whatever 5751. Unless you have won the lottery and can't think of what to buy after getting a solid gold rocket car.

I have lots & lots of NOS preamp tubes and switch them out indiscriminately with new production: the tone is not affected at all. Except: 12AX7EHs are a little brighter than I like.

Advertising hype notwithstanding Sovtek can't and won't make tubes like NOS. In critical applications like "tube" mics they're too noisy.

It's a different industrial culture today than it was 50 years ago. The plan now is to make it "good enough". If customers don't perceive quality beyond "good enough" then better quality simply adds extra cost.



Today's Moment of Zen: One might think that tone is a primary criteria and all else follows. The actual criteria:

Cost

Appearance

Availability

Weight

Nonsense like color

Resale value

(pick a few more)

eventually we get to

Tone

[birddog01]

Thanks Muchxs, nicley done the explaination that is and with some humor.

I will still continue to build up some "spares" and variety of the 12ax7 family and decide in the end what my ears tell me.
I suppose its much like swapping out and going through some speakers to come up with our opinion of what we like best.

Hackworth the info you found I find interesting, there may be something to this for me. This is for "the" preamp tube for my 5f2 Harp amp, I would surmise that "clean" is not the direction I want to go for a "Dirty Chicago blues harp tone.
Weber kits specify a 12ax7B, I suppose the B means maybe more gain.
Another Blues harper said that a Black plate RCA Nos 5751 is his favorite preamp tube for this amp, speaking from expierience.
Like any reviews I take them with a grain of salt sometimes but hey I don't have one to try out and may be missing something.

[muchxs]

Originally Posted by birddog01

Thanks Muchxs, nicley done the explaination that is and with some humor.

I will still continue to build up some "spares" and variety of the 12ax7 family and decide in the end what my ears tell me.
I suppose its much like swapping out and going through some speakers to come up with our opinion of what we like best.

Hackworth the info you found I find interesting, there may be something to this for me. This is for "the" preamp tube for my 5f2 Harp amp, I would surmise that "clean" is not the direction I want to go for a "Dirty Chicago blues harp tone.
Weber kits specify a 12ax7B, I suppose the B means maybe more gain.
Another Blues harper said that a Black plate RCA Nos 5751 is his favorite preamp tube for this amp, speaking from expierience.
Like any reviews I take them with a grain of salt sometimes but hey I don't have one to try out and may be missing something.

Thanks!

You can do a few tweaks beyond tubeswapping. I'm primarily a guitar player (well... primarily a technician now but I still got calluses on my fingertips) I'm a technician but I work with a skilled harp player. My harp player buddy in turn works with one of my amp builder buddies... when you get out in the boonies everyone knows everyone else. Anyway, first thing my harp player buddy says every time I show up with an amp is, "Can I blow through it?"

Last 5751 I scrounged came out of a Sanborn Labs preamp. Not audiophool stuff, this is a genuine precision piece of lab equipment from back in the day. I've had this thing on the shelf for 20 years. One socket is labeled "12AX7", I pulled the shield off and there's a Sanborn Labs gold pin 5751 in there. Cool!

[tubeswell]

Originally Posted by hackworth1

The 12AT7 also has a gain factor of 70...

Uh, that would be 60

[hackworth1]

Originally Posted by tubeswell

Uh, that would be 60

Your depth of knowlege is known and respected. You know that off the top of your head. I didn't check the tube data.

I cut and pasted that information from a website which sells vintage tubes. You'd think that vendor would have been careful enough to post the correct information.

Thanks for the correction. It is important in this tube discussion.

[celeste]

Actually, a tube does not have gain, a circuit has gain. A tube has Mu, and that is the theoretical maximum gain you can get out of the tube in a perfect circuit and load.

[printer2]

Originally Posted by celeste

Actually, a tube does not have gain, a circuit has gain. A tube has Mu, and that is the theoretical maximum gain you can get out of the tube in a perfect circuit and load.

Why do I feel we got our knuckles rapped.....