Anyone know what the real- world differences are ? The RCA rcving man. says the 12AZ7 is identical to the AT except for heater- cathode ratings, and interelectrode capacitances. The heater-cathode voltage ratings are higher on the AZ.
What I'm thinking of doing is replacing the chinese 12AT7 in this MXL 960 (chinese tube mic) (that doesn't sound half bad) with one of these 2 nice early 60's RCA 12AZ7s that I have laying around. So -- does anyone know what effect the differences in the interelectrode capacitances will have - if any??

Here's a link to a "free" computer-based tube reference that I use quite often:

http://www.duncanamps.com/tdslpe/

It's very good - I use it for substitutions and such. So far, it's been 100%!

Mark

Any 12Axx tube is made as a 12AX7, then it is tested and rated as to how much gain it creates. A 12AX7 gives 100%, the xxAU7 is 60% and so forth. the manufacture of the tubes in question my have a differant arangement inside the tube but the ratings are the important part, THEN the innards.

I would try the GTubes smooth plate or the 'M' series, but I would keep the numbers the same unless it is to the europian version (12AX7/ ECC92).

A great tube will make a HUGE differance in the mic performance.

yng-- did you even read my post??? The one that your supposedly replying to?? Anyway I pulled out some dusty books - did some research and figured it out myself. Thanks...

But just out of curiosity where in my original post do you see anything about 12AX7s or my asking for advice as to who's tubes to buy?
P.S. I ordered a N.O.S early 60's seiman 12AT7 that should sound very nice. Even nicer than the mullard 12AT7 that I found in the reverb driver stage of my Pro reverb-- the one with no reverb tank in it!!

Yng- I reread your post this morning and it's even more absurd than I initially thought! Are you actually suggesting that all 12a-7 series tubes are manufactured as 12ax7s,then tested, and the ones with lower amplification factor are then labeled Au or AT etc? This is completely nuts! Buy yourself a copy of the RCA receiving tube manual(Op Amp Technical Books in Hollywood and numerous other places on the web) and read some of it before you offer any further advice to anyone concerning vacuum tubes. While a 12AT7,12AU7,12AV7,12AX7,12AY7and 12AZ7 are all dual triodes, they are all uniquely different designs with different Mu, plate resistances , aplification factors etc. If your theory were true how would you explain a 12DW77?(do some research if you don't know what it is). Nothing personal, but lets not spread complete disinformation on this forum just for the purpose of impressing people wiht your knowledge about a subject that you aren't that knowledgeable about.

Sorry if I got a little carried away this morning -Having a bad day.

Ian Talcroft

I was gonna say, "what'd this cat do to you, Man?".

I would like to know the source of that - all 12A*7 are from the same die/run theory though.

Yng never did anything to me, I was having a bad day , then I started reading the Drumcore thread and realized what an ******* truly was and that I was headed in that direction myself!!
That is an interesting theory on tube design though, I just hate to see stuff like that perpetuated.

If the cat boot fits wear it.

Ian,

We all have bad days, and yes, I did read your thread AND answered. I will add this though, tube life is directly related to the voltage supplied to the tube. In the case of the 12Axx type the tube life at 10 volts is very low where at 6 it is way, way longer, maunfactures suggest 5.5 to 5.8, most are set to 5 volts. I like my PSU's for that tube type to be at 6, a little better S/N and a little clearer top end.

I'm a little confused about the DrumCore coment thing though, I'll re-read it.

Tim, just little bits of usless information picked up along the way, I guess... But lets look at it in a manufacturing standpoint. How many 12Axx versions are there? they all are made the same way, by that I mean they are all sealed in a vacume and can not be adjusted after manufacture. What do you do with the rejects of lets say a 12AX7? And what are the reasons to reject it? First you can't do anything with rejects except throw them away? Right? Or figure out a use for them in a another spec.. The 12AX7 delivers 100% volume where the 12AU7 only gives like 60% and on down the 12Axx line to the lowest amplifacations. Telefunken came up with a smooth heater plate that sounded really nice in an audio circit, Phillops - Japan came up with a 'halo' that cut down on the microphonics, but that is just how they came up with an electronic design to do a specific function. The manufacturing is done the same way everywhere, they make a pile of tubes and see where the dust settles on the specs.

I'm no expert on tubes in general, though I do have a lot of tube gear and am expanding that knoledge base every day. Power tubes are the same way, a KT88 looks just like a KT66, just not as much power and works its way down to the 6V6, which I believe is the smallest power tube. The 6L6 and 5881 are the same just differant specs for military needs like longer tube life and reliability. Now days the tubes even look the same in sizes because it is easier to manufacture, BUT a complete bitch if you need a small 6072/12AU7 for your AKG C 28 A/B, because the 'new tubes' won't fit in the space disigened for it in either length or width, YIKES!

Hope that helps even if I can't recall who exactly told me that information, I know it was a repair type person. I'm sure I could drop a name that would be recognizable but I'm not sure who said it because I go directly to the horses mouth for the info, I deal with crazy Germans and over weight gtr amp desigeners who use Radio Shack parts, it is part of the deal, over time you get a lot of contacts and a pile of info that is not exactly easy to quote.