DIY Valve tester

[retailer]

Reading past posts I've noticed a bit of discussion about a DIY valve tester. I've built one using the circuit designed by Steve Bench (RAT Tube Tester). It has regulated supplies for both anode and screen adjustable 0-300v. A regulated adjustable constant current source to control the amount of current through the valve. It was designed primarily to measure GM and Mu(triodes only) of audio valves. It does not test for shorts or grid current, for this I use a cheap tester I picked up on Ebay for $US20, a Sencore Mitey Mite. As descriibed by Steve Bench it will test a limited number of valves only so I built an adaptor with a 7pin 9pin and an octal socket, to allow testing any of these valves. Settings are taken directly out of any good valve data book. Rather than use an external DMM to make the measurements I built an internal Digital Millivoltmeter into the case to make it self contained, I also used some small moving coil meters to measure voltages and current rather than calibrated knobs as in the original article by Steve Bench. I used a junked medical equipment case that had small meters already mounted. I,ve included a few photos for those that are interested. The main power supplies are on a pc board with the f.e.t.s along the back. The millivoltmeter circuit is built onto veroboard and mounted on the right hand side. Transformer was removed from an old TV (240-110v). I removed the filament winding and wound my own with taps for 2.5, 5, 6.3 and 12.6 volts. I plan to build another with the external adaptor built in and also include a test for internal shorts and maybe grid current. The whole project took about 2.5 months with the pcb design and metalwork taking most of the time.

[Alan_Douglas]

That's certainly creative re-use of the old case.

[screengrid]

WOW!

Great work, retailer. For some years now I have read about the Steve Bench tester, both on RAT and on the Great Mans website. I know that many have built them, however, I have never seen any pics of the finished article.

Nice work.

[retailer]

I guess it is creative. I did'nt want to outlay a lot of money on the tester because I was'nt sure how well it would work. As many of you may know, with home built gear the cost of the hardware (knobs, chassis, sockets etc) is usually far greater than the cost of the compenents (active and passive). The only new parts were the semiconductors caps and resistors and the LCD panel meter. The most expensive items being the 450v electrolytic caps. As it turned out it works very well at measuring Gm and Mu, matching audio output valves, finding twin triodes with equally matched sides. A couple of aquaintances own AVO testers and now they come and visit me when they want to check their "standard" 12AT7's used for calibration. When testing a valve I set the anode voltage then the screen voltage if needed, then I increase the current through the valve whilst watching the anode voltage meter, if the meter dips at all it means the valve has an internal short and I go no further. I set the current at the value shown in my valve data book and wait for the reading on LCD panel meter to stabilise. I regard any valve that takes a long time to stabilise (more than about 1.5-2 min) as being servicable but comming to end of its useful life. I would love to have an AVO but for a hobby it's hard to justify the $750-$1000 that they bring. In any case my kids regard my vintage gear as junk and will probably chuck the whole lot into the dump when I'm gone anyway.

[retailer]

For those that are interested here is my PCB layout and the millivoltmeter details