If it is true, I'd guess that it's more so now than back in the old days.
I remember different 12A** models from the same manufacturer being physically different. My Genelec KT88s are larger than the Genelec KT66s of the same vintage that I have. A 7591 was different that 6V6 back then, but now they change the pinout of a 6V6 to make a 7591.....same with 7027 and 6L6 I think.

I'm sorry but I just can't buy this. If you compare a 12AX7 and a 12AU7 at a plate voltage of 250 v the rp of the AX is supposed to be 62500 ohms whereas the AU is supposed to be 7700 ohms at the same plate voltage. Then there's the tranconductance issue; the AX is High MU- the AU is medium MU. While we know that you can throw a 12AX7 into any position on most guitar amps in place of ATs, AUs etc - I don't beleive that modern tube manufacturing has fallen to the state you suggest.I could be wrong though, but I hope not. Now I'm going to get to the bottom of this. I will post my findings.
Yng - the Drumcore reference had nothing to do with you- I was comparing my un-called for rudeness to the even worse state of affairs on that post at the time.

Ian,

No worries...

I don't care if I'm right or wrong here, just passing on information I have been told in the past. Tubes are a very subjective thing, kinda like wine tasting, for lack of a better comparison. The more you get into them, the more you realize how much you don't really know about them. I'm still learning about plates, halios, 2nd & 3rd harmonics and voltages. And all I wanted was to be able to record a good sound

Yng, I know, but you've got me really curious as to what the situation is now. While I know for a fact that what I have been saying WAS true when all the U.S., Brittish, German etc...original manufacturers were still in production, the truth today may be somewhere closer to your theory. I am going to find out though.
I swamped with work right now but in the next few weeks I am going to research this.

Hey all I know is that I have a SE 5600 that I got very cheap and wanted to play with the tubes and interchange them to see how the mike would sound. .Went online to the tubestore.com did some reading on labels and markings and origins of different tubes. Found a RCA Black Label 12AV7 on ebay for 10.00 bucks and swapped it with the chinese 12AX7..... the top end was killing me. The mike sounds very very good to me now. I even have a sample of before and after. I believe if you don't mind playing around with an inexspensive microphone have at it and have fun. How else are you gonna know what sounds like what if you don't hear it for yourself.


Russell Taylor
Tuphunkefuya Productions/Recording
Tuphunkefuya@houston.rr.com

Ian,

I got that info before they stopped making tubes and now I'm curious too. And like thephunkeman, I will change out a tube to see how it sounds in the about 20 old tube mics I have. Most of the tubes are basicly the same except the AC701's which are usually specked out from 4.6 to 5.2 volts, where as the others are usually about 6, though the new vision (nuvista) tube gets specked out from like 4.5 to 6, depending on mic manufacture and model. Most PSU's that I get are atleast a volt down from manufacture specks so be sure to check that before you do any 'real' changes, the cord lengths will effect the plate voltage also.

All of the mics, that I know of, that use a 12Axx tube are only using half of the tube except the stereo AKG C24 and Ela M270 (which used both AC701's or one 12AU7). Not sure how the Neumann Gefell UM 69 did it, but the mono version UM 57 used a single sided EC92, I think.

My greatest trimph was upgrading my very old Presto comp from a single tube for the input and single tube for output to just putting one 12AX7 in the place of the two VF14 tubes, wow what a differance stabelizing the input/output sag did. Just made a bridge with the 12AX7 in the middle with VF14 sockets on both sides. I think I'll put a smooth plate or 6072 (M) in there to see 'that' differance for my next session.

On my original MD1's from GT use a 12AU7, but can use an AT7 if the AU7 isn't killing ya, it just looses a little volume and Greg, Austin's assistant/mic guy, said to just up the plate voltage for the AT7 tube

yng i don't care who told you this. it is utterly false.

the internal geometries and chemistries are different between the au, at, ax, ay, etc. the gain is different as is the torque [12ax7 have the most gain or hosepower; 12au7 have the least horsepower but much more torque] high torque means low plate impedance. it depends on the space between cathode and grid, grid and plate, overall distance between cathode and plate.

they are not sorted from one production run. maybe the early chinese ones were, but thats another story.

i love the internet and all the experts who post rubbish they heard.

Especially when posts are six-and-a-half years old!

Old but interesting.