12AX7 ultra low noise selecting

[Martin.M]

hello friends,

my problem is not directly audio related, but I want to ask you this:

I need to select a 12AX7 as replace in my Tek "E" plugin,
this special amplifier goes down to 50µV/cm at a frequency range up to 60kc.

In the first stage is the 12AX7 placed.
I got the plugin with a broken tube, now it must have a new..

In my stock is a collection of various tubes, which of them is the lowest noise amplifier? , all is nos, ECC83 various brands like Valvo, Siemens, Telefunken, 12AX7 also, ECC803s Telefunken and "Jan",


what is the ideal application for testing the noise level, groundet Gate, 10k groundet Gate, 1M groundet gate?
In second stage of the test I have to calibrate the plugin, to screw it for the best rectangle response at very low level using 1kc from generator.

greetings
Martin

( Collector of warm test gear)

 

[audiodon]

Were Amperex bugle boys what came stock in the unit? If so, that's a selection you might be able to live with.

 

[jaymanaa]

The 7025 was low noise as is the modern Sovtek 12AX7LPS (spiral filament)

 

[miner]

Jim McShane (Citation amps) is a great source of tube knowledge and carries a good stock of tubes. He would be happy to help you.