what would be the most demanding tube preamp topology?

[capacitorless]

Hopefully selling I'll have a small number set aside for projects, and re-tubing equipment in the future. I have way more than anyone would likely need for their own use, unless they were building/selling gear and wanted to put good old stock tubes inside.

 

[Jen]

When I spent a lot of time sorting out low noise and low microphony double triodes I configured them as mu followers simply because then you know the gain is going to be very close to mu but it does not matter too much what topology you use.

The main problems you will have will be eliminating extraneous noise and interference from the test set up. You definitely need to build the circuit in a screened box with the tube fitted in a screened socket. To measure tube noise and microphony you should short the input to 0V. Make sure you use a large enough output capacitor to pass any 1/f noise. If you decide to run ac heaters you biggest problem will be hum and its elimination is as much a matter of layout as anything. I would recommend using dc heaters. I used batteries on my first test rig.

Cheers

Ian

Totally ditto on using DC heaters..

 

[capacitorless]

I'm using switchable AC/DC heaters, so that I know if a tube is more prone to hum (AC mode) and then able to eliminate any heater hum from noise measurements (DC mode). AC is via the normal filament supply from the transformer, DC is via a bench supply.

 

[Mr. Moscode]

I'm using switchable AC/DC heaters, so that I know if a tube is more prone to hum (AC mode) and then able to eliminate any heater hum from noise measurements (DC mode). AC is via the normal filament supply from the transformer, DC is via a bench supply.

Perfect.

Instead of measuring GM my SSTC read gain in DB and you could drive the tube until it distorted lighting a red led. Distortion was measured by attenuating the output and comparing it to the input in an op amp. People still ask me for them, but I didn't make enough $ to continue. If anyone wants to make them contact me.

Photo attached.

 

[capacitorless]

That's way awesome. Liking the method of determining the distortion threshold!

 

[Mr. Moscode]

That's way awesome. Liking the method of determining the distortion threshold!

It keeps the test in the linear region. Thanks

 

[CJ]

A Marshall 2000 amp is a noise generator supreme, I used the lead channel to check tubes for noise.

 

[Mr. Moscode]

Here's the manual for the SSTC: Small Signal Tube Checker