screening germanium transistors for Neve 1061

[outoftune]

hi, im repairing some vintage neve 1061 preamps for a client.

some of the units have faulty germanium transistors. i've been able to track down a large batch of exact NOS replacements.

i'm wondering if anyone knows the process for selecting / screening the germanium transistors for noise...quite a few of the modules have issues when at higher gains when using the B100 board.

should i just use the geofx method listed here: http://www.geofex.com/Article_Folders/ffselect.htm

or does anyone know what they would have done at the factory?

any input would be great. thank you.

 

[RuudNL]

The link you mentioned only measures the Hfe.

To get an impression of the noise, I would build a simple common emitter gain stage.
The transistor under test should be part of this amplifier stage.
I would connect the output of this 'test amplifier' to an other amplifier (to bring the noise signal to a higher level), and measure the amount of the noise.
If you use a transistor socket in the test amplifier stage, it is very easy to swap the test transistor.
It might be a good idea to listen to the noise output, to make sure that you are actually measuring noise and not hum!
Try to use a source impedance at the input of the test amplifier stage that is in the same order as the impedance in the real circuit.
I have tested transistors for noise follwing this principle in the past and it worked!

 

[PRR]

Bodge a socket into one amplifier, rig amp and headphones, nice to have a meter across the phones. As said, terminate the input in the intended resistance (well shielded).

Try each transistor in the circuit it will be used in!! If you run any other current, load, impedances, the hiss will be different and you don't have enough information to transpose. A full hiss quatification is possible, but involved, and not necessary if you have the ONE application.

If the transistors are not rejects (lot of crap on the market), you will find that 50%-90% have all-the-same low hiss, and the others will hiss more (or crackle or popcorn). The "more" parts might be suitable for output stages.