Building tube tester help

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
Hi folks, I am working on building a vacuum tube tester. I am somewhat new to tubes but am gaining a lot of knowledge by reading books, forums, etc. I already have the short circuit tester designed and built. I have the high voltage power supply designed and most parts received. What I need a little help with is designing a circuit that will measure tube output power and comparing it to the tube data sheet values. I am working on the design of this circuit now and will post a schematic of it once I am done. I would like to know your thoughts, suggestions and any ideas to let me know if I am going down the right path or if I am heading into uncharted territory. I do not want to buy a vintage tube tester nor do I want to buy a current commercial tube tester. That won't educate me on any of the things I want to learn by building this unit. I don't want to plot power curves or anything fancy like that. I just would like to test audio preamp 12AX7, etc and power tubes 6L6GC, etc at their rated output and see if they are close +/- 20% to the published values in their data sheets.
 

crutschow

Joined Mar 14, 2008
23,480
I would think testing the emission and gm of the tube should be sufficient.
I see no reason to measure the power, as that is basically determined by the tube physical design.
 

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
Maybe I should elaborate. Here is what I would like to reproduce.
But I want to streamline it a little. He refers to tube tester roll charts but I would refer to tube data sheets. I have also attached a preliminary basic schematic of my design as it would apply to a 6L6GC power tube. I am just looking for a little info regarding selecting values for the load resistor and the screen resistor based on the tube data sheet information. I understand that I may need some high wattage resistors (>50W) for the load. That is ok. I can get those. I have the PS all designed and it will provide adjustable 0 to 700 VDC @ 150ma for full voltage testing. I am very familiar with HV safety requirements as I work with >1KV VFD DC bus voltages every day at work. So, I don't need safety hints. Im just trying to clear up some of the "secret voodoo" behind tube testing. As soon as I feel comfortable with my design, I will breadboard some of it and put up some results here to help others who may have similar questions.Screen Shot 10-31-19 at 02.18 PM.PNG
 

MaxHeadRoom

Joined Jul 18, 2013
19,146
I remember designing/building one back in the 60's, I had already picked up a manual published in the UK "Radio Valve Data' listed Characteristics of 7000 valves!
I re-wound a huge surplus transformer to cover all heater and working voltages!
Just don't ask me for the schematic! :p
Max.
 

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
I remember designing/building one back in the 60's, I had already picked up a manual published in the UK "Radio Valve Data' listed Characteristics of 7000 valves!
I re-wound a huge surplus transformer to cover all heater and working voltages!
Just don't ask me for the schematic! :p
Max.
That sounds like a lot of work. :) I will be using a 0-14.4 vdc 30 amp variable power supply for the heaters. I will also be using a variac connected to a home made 700 vdc max power supply for the B+ voltage. I also have a triple output, isolated power supply for the grid. And a pile of handheld meters to measure voltage and current. That's the easy part. The part I'm having trouble wrapping my head around is selection of load and screen resistor values based on tube data sheets. Here is an example for a 6L6GC power tube.Screen Shot 10-31-19 at 02.37 PM.PNG
So, for single tube operation, 250 volts is (Ua) B+. -14 volts is (Ug1) grid. It should produce 72 mA at (Ia) plate across the load. So, my calculations based on Ohm's law are 72mA at 250 volts should put out about 18 watts. The load resistor should be about 3472 ohms @ 18 watts following Ohm's law again. As for the screen resistor, it looks like it wants (Ig2) 5 mA @ 250 volts. That calculates out to 50k ohms at 1.2 watts. Am I correct? Or am I way off base?
 

Thread Starter

Jon Hoover

Joined Oct 10, 2019
11
Thanks Albert, after looking at the original design and doing a little thinking, I have revised my circuit to match his. I picked up a used 10 turn, 100k 5 watt potentiometer for <$20 US and will be using that to adjust the screen voltage. It doesn't look like he is using a load resistor but he has a 1 meg in the grid circuit for?? Current limiting?? Don't know but I will try it. Thoughts?


Screen Shot.PNG
 
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