Tube current tester

[gui_tarzan]

If a guy wanted to build a small tester that would allow seeing two or three measurements like this one, how would he go about it? In this video the guy is checking current draw on each of the tubes in this amp to make sure none are out of kilter.

This would allow for a quasi-matching scenario because you could use full B+ instead of low voltage like a normal tube tester uses.

It would seem logical to just build an output section (minus the OT) to get close to real-world operating specs or am I way off here?

[martin manning]

No problem. All you need is a voltage source that meets your need. You could tie the screen to the plate, and then set the grid bias using the usual sort of bias supply. If you are just comparing tubes, the exact voltages aren't critical. I'd put some resistors or inductors at the pins to damp oscillations, and it would be best to run them all at once so the power supply wouldn't need much regulation. Do a search on TAG for tube matcher; I have seen a couple of DIY versions in my time here. You can use any PT you have handy, and power it through a Variac to have adjustable plate voltage.

Here's one: http://priceamp.home.comcast.net/~price ... atcher.htm

[gui_tarzan]

So what else would he be measuring and what would that look like?

[martin manning]

I don't know... you mentioned a video, but there is no link.

[gui_tarzan]

He doesn't use it for anything else in this video, unfortunately. It's a video of him repairing an SVT CL, which I just got one of in the shop today that's cutting out after a few minutes and tripping the alarm light. That's what brought on the question though, as he checked the current draw of each of the six tubes to make sure there wasn't one totally out of whack.

I did search here and found one thread that looks interesting, but it is in a pretty large chassis with transformers and does four tubes instead of just one. I'd also like to have a built-in digital display but haven't gotten that far into the design part of it.

[M Fowler]

https://www.youtube.com/watch?v=h_U1GzbTDFY

[martin manning]

Thanks, Mark. So this is just a bias probe with the added capability of measuring screen current, and cathode as well as anode current. Measuring cathode and screen current would give the same information without the risk of opening the plate circuit accidentally. I don't like the mini toggles on the HV DC, and it'd be much safer to use current sense resistors instead of measuring current directly (like all the other bias probes out there). This device doesn't serve the same purpose as a tube tester, but it will let you see how closely the tube currents are matched in the circuit, with the bias voltage on each tube not necessarily being the same.

[TUBEDUDE]

Take a look at the bias rite on the Weber speakers website. It's a cheap tool for biasing tube in the actual amp they are used in. You'll be able to see how close the d.c. matching is with the transformer they are actually use with. I have an old Hickok 752 that will check for shorts, transconductance, etc, but I seldom use it. A maximatcher or one of the newer designed tube testers would be more useful, though somewhat expensive. I use several of my amp chassis to check out tubes before installing in a customers amp for biasing.

[gui_tarzan]

I have a couple of tube testers but they don't test at operating voltage as we all know. I thought his method made sense, that's why I was asking.

I bought one of the bias test sockets a few years ago and then made another one after I saw how easy it was, this is just an expansion of that idea and to see what else I can do to make testing easier and more dependable.

[Phil_S]

FWIW, I once built a Vibro Champ circuit with adjustable fixed bias. It was a pretty good tube tester, but just for one tube type. I had about 50 or so 6AQ5's of unknown quality. It worked well. I tested at 3 different bias voltages (of course plate and cathode voltage varied) and I was able to quantify how strong or weak each tube was. It was pretty obvious which ones were really NOS or ANOS, which were used, and which were nearly used up. I was also satisfied that I had several reasonably well matched pairs, although I'm not sure my matching technique would stand up to the demands of some of the guys who hang out here.

Unfortunately, the tremolo won't oscillate with fixed bias (wrong for the Vibro, d'oh <headsmack>) so I converted it to cathode bias at which time I lost the tester functionality. Love that VibroChamp. Tremolo to die for.

[gui_tarzan]

This is one I ran across, it seems to be fairly popular.

http://judyboxamp.blogspot.com/2011/05/ ... lding.html