New builder - question about directly heated cathodes with external filament supply

[Jennifer Edwards]

Hello,

I am new to the group and to the uTracer. I am into old pre war radio restorations, I also sell old valves, and am a life long HAM (ARS KQ4U). I Just finished a model 3+ Utracer kit. I also  have the version 6 kit on the shelf.

I am a bit confused when testing very old directly heated triodes using an external filament supply. In the manual (ill attach a screen grab) it says not to attach the cathode current from the uTracer. But then in illustration "D" it shows you can. 

While I am at it what about the grid connection?

Thank you for your help,

Jenny 

[Martin Manning]

The best way is to use an isolated (floating) DC supply to heat the filament, and make all other connections to the uTracer ASGK terminals as usual. I am using a small bench supply with the earth ground connection open. 

[Martin Manning]

PS the potential across the filament will vary using a DC source, so you will have to average readings with the two possible polarities.

[Jennifer Edwards]

Thank you for your reply. I got it about keeping the separate filament supply "floating". So it is ok then to use the cathode supply from my utracer on one of the filament pins as well? 

That is what I thought, I just did not want to mess up my new toy the first time I tested an old 1920's triode. For that matter I am not too keen on risking a relatively rare valve either. 

[Martin Manning]

Understood Re the old tubes. 

Yes, you can use the cathode terminal. It's a current sink. Just remember that it is sitting at the supply voltage (~20V). The current measurement in in the anode supply. Below is the setup for a 5U4. There is a post on testing rectifiers here (easy to find from the title) that has some info that might be useful. There you can see how I used AC on directly heated rectifiers and found that enough readings are taken and averaged to produce acceptable results. I have no directly heated triodes about, so I've never tried that. 

[Jennifer Edwards]

Great! Looks loke I have mothing to worry about except the fixed cathode voltage which is ok for my purpose.

Thank you once again for taking the time to reply to my query.

Jenny