tube test just for 12Ax7?

**mike_mccue** · 10-29-2010, 12:14 PM

I might as well start by clearly stating that I am aware that many people think the best way to test a tube is too put it in the circuit. I agree with this notion and consider it to be a wise way to work.

I am, however, very curious to continue learning about tubes. I have been collecting a bunch of 12AX7 tubes for my personal use with guitar amps and some rack gear.

I would like to somehow quantify some of their characteristics so That I may observe tendencies when I am listening.

I have an old tube tester that was a gift... I have never used it.

It is a Knight and it is in very clean condition... but I haven't powered it up.

I also have surfed the web and found some modern design for DIY tube testers, but they seem full featured and more complex.

I happen to own a CompuBias power tube bias measuring device and think it has been really useful for my personal education.

This has got me thinking... is there some sort of simplified design for a bench top rig that I might DIY assemble just to test small 9 pin tubes like a 12AX7? Maybe something that is very similar to a small tube amp that has some measurement points for attaching a DMM.

I imagine that if anyone has any interest in this they will ask me what I want to test for... so I'll admit that I really don't know. I've read about short tests, emissions tests, and transconductance measurement procedures but I need some experience before it all makes sense to me.

So in summary, I wonder if someone has adapted a small tube amp design to allow for measurement of preamp tubes?

Thank You.

best regards,
mike

**MWJB** · 10-29-2010, 12:55 PM

"So in summary, I wonder if someone has adapted a small tube amp design to allow for measurement of preamp tubes?"

Yes, it's called a 5F1 champ. I know you already seem to have ruled out an "in circuit" test methodology, but the criteria that people are most concerned with regarding preamp tubes are tone, gain & microphony - the things you have to listen for. If a preamp tube fails, it might make alot of noise but it's not going to damage anything, so testing for problems that you might in a power tube (that can blow screen resistors when it fails for example) isn't as necessary.

A voltmeter can easily tell you how the tubes differ in practical terms of voltage/current draw by measuring voltage drop accross plate & cathode resistors, hook up a signal generator and you will see amplification factor.

I seem to remember Bruce Collins stating that he took this a stage further & built a fixed bias, parallel SE champ style amp for tube testing/matching. Want to be able to AB different preamp tubes of the same type in an instant, go one further & build a fixed bias stereo champ & compare channels? If you can find a PT with a 300-0-300VAC to 330-0-330VAC B+ secondary, 5-6A 6.3VAC supply, 3A 5VAC this will let you test all the common octal power tube/5V rectifier types too (A BF Deluxe PT will work for 6V6/6L6/EL34/5881 & 5Y3/5R4/5U4/5V4/5AR4/SS etc). You can test tubes on it, play instruments through it, hell even play your I-Pod through it if you have decent speakers/hook up to hi fi speakers.

**mike_mccue** · 10-29-2010, 02:14 PM

I think maybe what I am asking for is some idea of how to adapt that 5F1 you are talking about to make some specific measurements that I can record and associate with that tube.

In other words... where might one set up some test points? procedures etc.

I assume I may also need to consider some protocol for using thoise measurements in an effort to calculate transconductance.

I really just want to focus on 12Axy types tubes at this point in my curiousity.

Thank You.

best regards,
mike

**MWJB** · 10-29-2010, 02:38 PM

Measure dc voltage either side of the 100K plate resistor (pins 1 & 6) divide voltage drop by resistor value, measure dc voltage at the tube side of the 1.5K cathode resistor (pin 3 & 8) again divide voltage by resistor value.

Connect a 4ohm dummy load to the speaker jack, apply a low AC voltage to the high gain input until you see say 5VAC at the dummy load (volume wide open) and see what AC it takes at the input to generate that (for differing speaker loads you will need to fine tune the VAC you look for - we're not really concerned with the amp's clean W RMS rating, we just want to see what impact the preamp tube has). Then either stick to that AC input voltage and mark the difference in tubes & output, or always hit the same dummy load voltage and mark what input it took to do that. Do one method, or the other, Don't mix. Record AC voltages at 12AX7 pin1 plate, 12AX7 pin 6 plate, 6V6/power tube pin 3 plate.

Remember that any tube specs are nominal, there will be small variations from brand to brand, even from tubes of the same brand.

I can already tell you that typically a 12AX7 tube will show ~68% of the dc voltage supplied to the 100K plate resistor from the B+ rail, at its plate (pin 1 or 6) in this type of circuit, ~50% for a 12AT (that was a JJ, they show more voltage that typical NOS), ~48/49% for a 12AY, ~30% for a 12AU7. Even tubes that show the same current draw/voltage still sound different...then you get bored measuring & just use the above figures as a rule of thumb, diagnostic tool! :-)

**J M Fahey** · 10-29-2010, 03:16 PM

Excellent suggestions, both the "real world" Champ and the test rig.
I might add, if you have a ton of 12AX7 (or similar pinout) to test, as in EBay reseller, bulk purchaser or whatever, you can build a specific designed tester, which will let you test, say, around 40 tubes an hour, the main delay being heating delay.
If you have to test , say, only 20 or so, go the Champ route, which has the added bonus of being playable.