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Line Stage Preamplifiers AX vs AU

Just learning a  few things about these components,,, seems to me best tube fora  linestage (PREamp term should only be applied to a  amp with phono section/ w/o phone, = Line Stage) anyway,, seems to me best circuitty for a  *passive line stage* would be the 12AU7 tube,  and not employ any 12AX7's. 
The AX ha s a mu of 100, so it really scrams and thus pushes the db level too much as it reaches the pwoer amp.
The AU has a  mu of only 15, so its alsmost passive, which is really whata  line stage should be , just passing the  signal from the source to the amp,  with the least involvement, IMHO alot of line stage amps, have too much coloration and muddiness due to the usage od 12AX7's. 
Leaving me with voting the only  line stage  employing all 12AU7 circuitry i know of under $2K as best Line stage on the market. 


https://en.cayin.cn/products_info?itemid=120
mozartfan
atmasphere's avatar
atmasphere
12,128 posts
 
@yogiboy  Yes, but you have to deal with the filament wiring, as the 12BH7 has a 12V center-tapped filament similar to a 12AU7. The 6CG7 can only be operated with 6.3V filament voltage. But the rest of the pin basing of the two tubes is the same, and BTW the 12BH7 is the same '9A' basing as the 12AU7, so if the circuit can operate with the increased filament current, the 12BH7 can operate the same socket. Due to minor differences between the 12AU7/12BH7 though, to really find out what they sound like the cathode circuit will need a slightly different value depending on which tube is in the socket.
yogiboy's avatar
yogiboy
5,875 posts
 
@atmasphere
Thanks for the Info! My amps use 12BH7 tubes so I am going to try some 12AU7 tubes that I have sittin’ around!
mozartfan OP
2,143 posts
 
You might want to hear what a 6SN7 does for a line stage

I’ve seen this pretube in a few of cayin/Line magentic integrated designs,,, nice looking tube for sure,,, 20 mu factor = niceeee.
Thing is I love the Tele AU,,and the Jadis Dpl has now 3 AU’s, as per Richard Gray mods.
He swaped out the AX for AU,, making some changes by eliminating a pair of .47 m evo oil caps,,, we just listened at his shop,, but he has Heathkit monos, and a pair of radio Shak vintage speakers, refurbished,,so hard to make a valid assessment vs my Defy7 modded with M caps + Seas Thor MTM..
Richard is anxious to see how i hear the AX swap to AU.
But yeah, the 6sn7 looks like a fine pretube.

mozartfan OP
2,143 posts
 
I was told that a 12BH7 can be used as a replacement for a 12AU7. Is that true?

I've read notes about this compare, So I boughta  bunch of 12BH7's, and none stood up to Tele 12AU7 as far as sonics, sound stage etc. 
atmasphere's avatar
atmasphere
12,128 posts
 
I was told that a 12BH7 can be used as a replacement for a 12AU7. Is that true?

I've read notes about this compare, So I boughta bunch of 12BH7's, and none stood up to Tele 12AU7 as far as sonics, sound stage etc.
As I mentioned above, there are slight differences between the two and while they will appear to sub each other, the simple fact is that they won't sound right if the circuit isn't optimized for them (if the two tubes were identical, then why it the 12BH7 taller and needing twice as much filament current?). So its no surprise that the 12AU7s sounded better in your case. But if the line state were optimized for the 12BH7 the tables may well be reversed.
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