Tube constuction

[Gus]

Questions

Does anyone have a good link(s) or book or know

What is changed in a the 12a_7 family of tubes to change the gain etc?

12au7,12at7,12ay7,12ax7......

Is it spacing between grid and cathode and plate?

Is it open area in the grid winding?

I wanted to ask before I break open some tubes

I know pitch lenght shape etc can make all kind of differences

I have not been able to find 12a_7 constuction differences information with search engines and the books I have

 

[Eric H]

I know, Gus, I've looked for tube construction info before. There is lots of stuff on how to use them, but almost nothing on how they're made (materials, dimensions, etc).

-Eric

 

[pucho812]

I know there are gain differences between the various 12_7 tubes .

 

[rodabod]

There is some good info. here:

http://www.john-a-harper.com/tubes201/

But it is not too specific.

 

[Gus]

I have books with the theory of what can effect different specs.

What I am looking for is what is changed in that tube family.

 

[rodabod]

Do you know historically which came first - the ECC81, ECC82, ECC83.... or the 12AT7, 12AU7, 12AX7...?

That might help your search a bit, as for a start, we call them valves or thermionic valves. There are some good Mullard books.

 

[NewYorkDave]

Got some pulls sitting around? You can break them open carefully and take a look inside... IF you're equipped to take appropriate protective measures, since there are some toxic substances inside (barium used for getters, etc.). Yes, I have broken tubes open--mostly unintentionally--and lived to tell the tale, but it would be irresponsible to not warn of the potential dangers.

 

[PRR]

> I wanted to ask before I break open some tubes

I don't want to stop you.

(DO mind NYDave's comments: people assembled tubes without protection, but they also put radium paint on watches and sprayed asbestos in ships without protection.)

I'll tell you that you may not be able to reconstruct the grid-cathode spacing after you bust a small tube open.




Oh, all right. To a first approximation, "Mu" is like a privacy fence. You can plant a few bushes to hide 10% of the view and negligible privacy, stagger boards to hide 50% of the view for slight privacy, use lattice to block 75% of the view, or slide those plastic strips in chain-link fence for 80% or 90% privacy. If the cathode can't see the plate, the tube has large Mu.

And, like a privacy fence, spacing matters. At a distance, you can't see details through lattice. If the peeper puts his eye to a hole, he sees all, no privacy. High perveyance requires small cathode-plate spacing, which forces small cathode-grid spacing. But when that space is similar to the smallest size of grid-wire you want to use, the electrons are hardly hindered.

> what is changed in that tube family.

I do not think the AU, AT, AX are a true Family. The AU has clear roots in older general purpose triodes, just packed in a smaller bottle. The AT is a VHF tuner tube, somewhat like Bell Labs' first miniature tube. The AX is in many ways (not just Mu) a better performer than either, and has no direct ancestor. I agree that these and 12AY7, 12AV7, 12AZ7 were deliberately named to suggest a family, but I think it is a put-together family and not a close kinship.

 

[burdij]

The 12AU7 appears to have come from the 6SN7 and the 12AX7 from a 6SL7 athough with somewhat higher gain and plate resistance.

 

[Gus]

Why I asked this question

After building/modding some tube microphones I have noted that the tube can have an effect on the sound.

Yeh I know big deal the sound changes but there is somethings different going on with tube microphones

Grid to ground R is way out of tube max spec.

Often under heated and run at lower plate voltages and plate currents

Underheating should change the virtual cathode size, Maybe grid current and grid noise

If you look at older tube microphone schematics often the tube is under 1ma plate current and the plate is at 36 to maybe 83 volts, grid is greater than -2Vdc

Lower plate voltage lower electron eV, less noise?

Now when I try a 6dj8 type in a standard tube circuit I seem to hear a change the sound is "smoother" for lack of better words If I change the tube to say a 12ay7 same capsule transformer etc maybe change the cathode R I "think" I hear more detail with voice using headphones.

I don't think you would hear this change in a mix It is something that puzzles me.

If I am understanding what I read the 6DJ8 and tubes like it are a frame grid with close grid to cathode spacing and at low current follows a 5/2 law not a 3/2 law

I am looking for a tube family that have a known change to construction to test and learn from.

There is more about what I am thinking about I built a simple test bed with fixed grid voltage and different heater voltage and check different tubes for Rp and maybe other things like rp I have it setup more like a tube microphone plate out circuit.

 

[ioaudio]

[quote author="PRR"]
Oh, all right. To a first approximation, "Mu" is like a privacy fence. You can plant a few bushes to hide 10% of the view and negligible privacy, stagger boards to hide 50% of the view for slight privacy, use lattice to block 75% of the view, or slide those plastic strips in chain-link fence for 80% or 90% privacy. If the cathode can't see the plate, the tube has large Mu.

And, like a privacy fence, spacing matters. At a distance, you can't see details through lattice. If the peeper puts his eye to a hole, he sees all, no privacy. High perveyance requires small cathode-plate spacing, which forces small cathode-grid spacing. But when that space is similar to the smallest size of grid-wire you want to use, the electrons are hardly hindered. [/quote]

excellent!