Anyone with electronics knowledge (12AX7 preamp circuit)?

[Damián]

Hi

I've been described a 'simple stupid' one-tube circuit on another board which I'd like to experiment with but can't really piece together the description.

Could anyone with some knowledge and 15 mins. spare time help me out?
From Memory the Circuit was a Simple Supply 250 volts or so with a
gyrator series fed Transistor smoothing the supply

Common Cathode Gain Stage
Anode Resistor 47kohm
Cathode resistor 1 k unbypassed(i think)
Output Cap 8 uF
and a 4k7 Ohm resistor on the output to gain dump
(from http://www.audioasylum.com/forums/tubediy/messages/35503.html)

Thanks a lot in advance, regards

Damián

[Drew]

Its difficult to build a preamp that uses one tube section with a 12AX7 because the tube has an MU (amplification factor) of 100. Thats a very high gain compared to a 12AU7 with an MU of 20. Most preamps using 12AX7's have 2 tube sections using a negative feedback resistor and then a cathode follower stage for impedance matching.

If your interested here's a web page with a semi-universal circuit using several different tubes:

http://www.users.globalnet.co.uk/~valveamp/LinePreProject.htm

Here's another page that compares Marantz 7C and Mcintosh C22 designs.

http://www.1388.com/articles/ampdesign-english/Page2__The_Search_for_an_Ultim/Page3__The_Search_for_an_Ultim/page3__the_search_for_an_ultim.html