vacuum tube testing

I can only observe that you may be best served by spending your time fixing one of your proven tube testers rather than trying to build one from scratch.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

Call your local music store that advertises servicing guitar amps. Whoever does the work likely has a tester, or knows somebody who does.

I picked up an old tester recently, it'll tell you good/bad, but no good for gain matching, it's not a mutual conductance tester.

Or just buy one. Never heard of anybody making their own, even the hard-core geeks on AudioAsylum and AudioKarma don't do it, there's probably a reason for that...

I saw an on-line video about Manley Labs (tube amp manufacturer) in California... they've got a nifty computerized rig where a computer controls the inputs and monitors the outputs of tubes being tested... I am sure if tubes were still used in large quantities such software would probably be available for free on the internet.

Dave

Yeah, what he said.

Home brew CRT testers were very common in the service trade, the most difficult part was rigging the interchangeable bases for all the different CRT types.

Ok. A big thanks to everyone for the replies. I have done this before about twenty years ago but I had the help of an older gentleman who knew tubes very well. I had notes on how to test tubes using the mfr. specs and how to interpret the results but can't find them now. Once again I appreciate your comments and thanks for your help.

Don

The basic figure of merit for an amplifier tube is transconductance, aka mutual conductance. It's the change in plate current for a given change in grid voltage, and it's usually given in micromhos. The simplest way to measure it is to set up normal operating conditions and shift the bias voltage while measuring the plate current. You can get the values from a tube manual. For audio work, just build a resistance-coupled amplifier stage around the tube and probe the grid and plate with a scope. Knowing the plate load resistance, you can easily compute the plate current swing.

Thanks Stephen. This is what I'm attempting to do. I have been varying the control grid voltage from -2 volts to -3 volts while keeping screen grid at

200 volts. Supply voltage (B+) is 350 vice. Plate resistor is 15K and Grid is coupled to ground with 10K resistor. According to the curves for pate current vs. control grid volts I should be getting 10 ma plate current @ 200 volts plate voltage with -2.5 volts at control grid. What I'm getting is about 187 plate volts and 16 to 17 ma. plate current at this point. On some of the tubes the current goes quite high and the plate voltage drops to around 120 vdc with the control grid @ -2vdc. When I change the control grid voltage to -3 vdc the plate current sometimes drops off sharply to ua and the plate voltage climbs to around 260 and 280 vdc (I'm doing this from memory). To me it appears that the tubes are bad but I'm not quite sure how to interpret the measurements. Any thoughts on this?

Thanks,

Hi Don,

I found this which might be of help:

insula

Thanks Nick, what a great site. I'll have to take some time and look it over.

Donald Ennis Inscribed thus:

What a fascinating web site ! I would also like to offer my thanks to OP for bringing it to my attention.

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Best Reagrds:
                        Baron.

On Fri, 06 Jun 2008 22:07:03 -0400, Donald Ennis top-posted: [top-post repaired below]

It sounds like it's operating properly - do this with some graph paper and the curves will look very much like the ones in the book, depending on how they're arranged.

Good Luck! Rich