Does any one have a circuit for building a valve tester.
I only need to test valves for amplifiers.
So testing valves like 12AX7 and 12AU7,
also common output valves like 6AU6.

To what extent do you want to test them?

Many of the basic testers only test the emmission.

Better ones, which is probably what you need, test the valve as an oscillator, or actually feed a sine wave into the grid to test the valve as a truly "functioning device"; giving a more realistic idea of its conditiion.

Marc

The usual range of tests available on a good quality tester include:

Shorts
Grid emission/gas current test
Mutual conductance (i.e. quality)
Life test

The cheaper testers usually do shorts and emission. Some would argue that's enough to sort the wheat from the chaff.

The first test is important when you have a batch of valves of unknown condition.

The third test is useful if you are trying to match tubes in an output stage.

The fourth runs the heater and plate at a lower voltage and gives an indication of how much life a tube may have left in it.

All these taken together give an idea of the condition of the valve, but by necessity all bench testers make compromises and none but the very expensive laboratory standard ones test the valves thoroughly under typical working conditions.

One saying goes that the best tube tester is the circuit it came from. Another is that valve testers helped the visiting TV serviceman sell new valves -- whether they were needed or not!

I'm not aware of a circuit for a simple DIY tester (old issues of "Radio, Television & Hobbies" would probably have them), however the emission type ones appear on eBay from time to time, but be warned they are now considered "collectables" and the bidding can be furious for some of them.

How often do you need to test valves, and how many at a time? If it's infrequent, then you should try to hook up with a local radio restorer who may do it for you for a beer and a chat.

"RTV&H Valve and Transistor Tester", p. 56, RT&H January 1961. This design tests for shorts and emission.

"There are now no Electronics Australia, Electronics Today International or Radio Television & Hobbies back issues available. However, we can supply project reprints of any article from EA, ETI (and even RTV&H and Wireless Weekly!) for $12.00 including GST and P&P per article/per issue (Overseas $AU15.00 each). Note that many of the projects in these older magazines (SILICON CHIP included) could now have major parts availability problems!"

http://www.siliconchip.com.au/cms/printversion/backissues.html

That last bit certainly applies to the RT&H tester, which needed a specially produced multi-tap transformer to supply every possible filament voltage!

If you're interested, I can scan the article for you.

Re the transformer, it's hard to know without seeing the schematic, but would a simple voltage divider suffice instead?

Maybe that would work if you added a voltmeter and a rheostat to fine tune the filament voltage; the current drain at the rated voltage of the filament would depend on the type of valve, wouldn't it?

Yes, I agree it's a crude solution and you would need a vernier to compensate for the various load currents, but given that Dmxtothmax has a limited range of valves in mind, he may get away with such a "hack" to an old design to overcome the multi tap transformer problem.

(And that's without considering the range of plate voltages required.)

If Sue can scan the articule for me that would be good.
I was only looking for a simple tester, testing mainly emission.
Shorts test would be handy.
I work on a lot of valve type guitar amplifiers,
And would like to test the output valves mainly.
The pre amp valves are usually 12AU7 or 12AX7.

OK. PM me with your address and I'll send the pages. If you're only testing 6.3V filament valves you could simplify the circuit so you wouldn't need the special components. What you would need is one known good example of each valve to calibrate the tester.

If you are only going to test the two tubes mentioned you do not need a special transformer and you can get the HT by reversing a 7VA transformer. These valves need little current so an LR8 could be used as a voltage regulator (limit 450VDC in for that reg)

If you are going the next step with the Power Amp tubes then things get interesting.

I did have an amp here that had been set up as a test bed for those and I think 6ca7's which need around 70mA

It had a mA meter in the plate circuit. and could test them running.

There is a similar idea in the radiotron manual where the tube is fed AC from a stepdown transformer, to test it running as an amp.

More commonly an audio feedback oscillator circuit is used (sine wave).

Marc