Years ago when I built my analogue curve tracer I added a small, yet very effective valve leakage test circuit. Due to my laziness, I failed to test a transmitting tetrode which I bought on-line and despite being NOS it damaged my uTracer. I followed the repair and re-calibration process and got the tester back again running, however, I regretted not having used this simple one step test I normally did before. Lesson learned, now I do use it back again!
Here is the circuit in case you don’t have a proper tester and you want to build something similar yourself:
You can test for leakage current using a simple amplifier made out of a NPN transistor and an indicator. In this case I used a Russian Neon (80V/0.5mA) and the existing supply on my tester (+/-80V). You can replace all this with a simple LED and the supply you have at hand. The circuit is designed to turn on the bulb when 5 μA leakage current is provided on the base of Q1 thanks to grounding the valve element next to the one under test. So for example if we want to measure cathode to grid leakage, we simply ground the cathode and we connect the tester to the grid. Same process is repeated with the other valve elements.
When I asked for some help in the DIYAudio forum, someoone gently recommended this text. Unfortunately I don’t read German, but what I got out of this adivce was:
- valves with poor vacuum (i.e. failed the test described on the procedure in 18A)
- preamp valves with less than 4 μA are usable
- output valves whit less than 10 μA are usable
- Valves that are good and show little Gas on the gas test:
- should have less than 0,6-1μA for preamp valves
- And should have less than 1.5-2μA for output valves
So the 5μA threshold was good enough in my view. It does work well and the beauty is that when neon light is very dim is an indication that it may be a workable valve despite the tiny leakage in particular with output valves.
Hope this helps
Ale
Hi i like the simple design, but i still don’t know how i can make it actually work in my tester… Some things are unclear to me.
The +80v goes into the current limiting resistor thru the neon into the collector,and will light up if a current lager as 5uA is provided at the base..
That part is clear to me.
But what provides the current load to open the gate?
That is were i get confused.
You said a tube under test, so i guess that the heater must be on, voltage on the anode, grid voltage?
To test: measure cathode to grid leakage, we simply ground the cathode and we connect the tester to the grid.,
by ground you mean common ground like -80v? I guess so, otherwise there is no current to flow. then connect the tester to the grid, if there’s a short current will flow and the indicator will turn on
But is it okay to ground the Kathode under test? I have RAT tester which is Cathode biased, not that i’ll blow up my tester lol
So when the Cathode is grounded i can only test the elements, as Anode, Grid, Screen.
Since i have a dc heater i can connect it to the positive of the heater also to check for shorts?
Now i’m thinking but under test, hmmm this was a tester to prevent damage to the tube tester… Am i missing something?? Or do i need a supply voltage to run current through the elements of the tube.
I have a 120v AC isolation transformer and a 24v DC power supply for the heater, and i made the leakage tester on a breadboard with a led, but the only way to i could let the led burn is to touch it to the positive of the DC power supply, so i could tie the positive to the Cathode and the tester to the grid and if there’s a short or a leakage the led light up.
Is this the right way? I Don’t know if 24v is to low to test, mostly testers have a higher voltage to test for leakage & shorts, and the heater on or off can also have influence on leakage…
I really hope you can help me out with this.
Thnx
Hi Davie
The leak current flows here from 0V (as cathode is grounded) to -80V path via the transistor base. In fact, the circuit I published isn’t ideal as the gain of the transistor is variable. You want to add a resistor from collector to base (negative feedback) to predict the gain better of this stage and be consistent.
Cheers
Ale